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Apocalypse 4: Syntax
Larry Wall <[email protected]>
Maintainer: Larry Wall <[email protected]> Date: 18 Jan 2002 Last Modified: 18 May 2006 Number: 4 Version: 6
This Apocalypse is all about syntax in the large. The corresponding chapter in the Camel book is entitled "Statements and Declarations", but it could just as easily have been entitled, "All About Blocks". The basic underlying question is "What exactly do those curlies mean?"
For Perl 5 and earlier, the answer to that question was, "Too many things". Or rather, too many things with inconsistent rules. We'll continue to use curlies for much of what we've used them for up till now, but by making a few critical simplifications, the rules will be much more consistent. In particular, built-ins will parse with the same rules as user-defined constructs. It should be possible to make user-extensible syntax look just like built-in syntax. Perl 5 started down this road, but didn't get all the way there. In Perl 6, all blocks operate under the same rules. Effectively, every block is a kind of closure that can be run by user-defined constructs as well as built-ins.
Associated with block structure are the various constructs that make
use of block structure. Compound constructs like loops and conditionals
use blocks explicitly, whereas declarations refer to their enclosing
block implicitly. This latter feature was also inconsistently applied
in Perl 5. In Perl 6, the rule is simple: A lexically scoped
declaration is in effect from the declaration to the end of its
enclosing block. Since blocks are delimited only by curlies or by
the ends of the current compilation unit (file or string), that implies
that we can't allow multi-block constructs in which lexically scoped
variables "leak" or "tunnel" from the end of one block to the beginning
of the next. A right curly (without an intervening left curly)
absolutely stops the current lexical scope. This has direct bearing on
some of these RFCs. For instance, RFC 88 proposes to let lexical scope
leak from a try block into its corresponding finally block. This
will not be allowed. (We'll find a different way to solve that
particular issue.)
While lexical declarations may not leak out of a block, control flow
must be able to leak out of blocks in a controlled fashion. Obviously,
falling off the end of a block is the most "normal" way, but we need to
exit blocks in other "abnormal" ways as well. Perl 5 has several
different ways of exiting a block: return, next, last,
redo, and die, for instance. The problem is that these various
keywords are hard-wired to transfer control outward to a particular
built-in construct, such as a subroutine definition, a loop, or an
eval. That works against our unifying concept that every block is a
closure. In Perl 6, all these abnormal means of block exit are unified
under the concept of exceptions. A return is a funny kind of
exception that is trapped by a sub block. A next is an exception
that is trapped by a loop block. And of course die creates a
"normal" exception that is trapped by any block that chooses to trap
such exceptions. Perl 6 does not require that this block be an eval
or try block.
You may think that this generalization implies excessive overhead, since generally exception handling must work its way up the call stack looking for an appropriate handler. But any control flow exception can be optimized away to a "goto" internally when its target is obvious and there are no user-defined blocks to be exited in between. Most subroutine return and loop control operators will know which subroutine or loop they're exiting from because it'll be obvious from the surrounding lexical scope. However, if the current subroutine contains closures that are being interpreted elsewhere in user-defined functions, it's good to have the general exception mechanism so that all needed cleanup can be automatically accomplished and consistent semantics maintained. That is, we want user-defined closure handlers to stay out of the user's face in the same way that built-ins do. Control flow should pretend to work like the user expects, even when it doesn't.
Here are the RFCs covered in this Apocalypse. PSA stands for "problem,
solution, acceptance", my private rating of how this RFC will fit into
Perl 6. Interestingly, this time I've rejected more RFCs than I
accepted. I must be getting cruel and callous in my old age. :-)
RFC PSA Title
--- --- -----
006 acc Lexical variables made default
019 baa Rename the C<local> operator
022 abc Control flow: Builtin switch statement
063 rr Exception handling syntax
064 bdc New pragma 'scope' to change Perl's default scoping
083 aab Make constants look like variables
088 bbc Omnibus Structured Exception/Error Handling Mechanism
089 cdr Controllable Data Typing
106 dbr Yet another lexical variable proposal: lexical variables made default
113 rr Better constants and constant folding
119 bcr Object neutral error handling via exceptions
120 bcr Implicit counter in for statements, possibly $#
167 bcr Simplify do BLOCK Syntax
173 bcc Allow multiple loop variables in foreach statements
199 abb Short-circuiting built-in functions and user-defined subroutines
209 cdr Fuller integer support in Perl
262 cdr Index Attribute
279 cdr my() syntax extensions and attribute declarations
297 dcr Attributes for compiler hints
309 adr Allow keywords in sub prototypes
330 acc Global dynamic variables should remain the default
337 bcc Common attribute system to allow user-defined, extensible attributes
340 dcr with takes a context
342 bcr Pascal-like "with"
Note that, although these RFCs are in the "accepted" category, most are
accepted with major caveats (a "c" acceptance rating), or at least
some "buts" (a "b" rating). I'll try to list all those caveats here,
but where there are systematic changes, I may indicate these generally
in this document without attempting to rewrite the RFC in every detail.
Those who implement these features must be sensitive to these
systematic changes and not just uncritically implement everything the
RFC says.
I'd like to talk about exceptions first, but before that I have to deal with the switch statement, because I think it's silly not to unify exception handlers with switch statements.
Some OO purists say that any time you want to use a switch statement, you ought to make the discriminant of the switch statement into a type, and use method dispatch instead. Fortunately, we are not OO purists here, so forget that argument.
Another argument against having a switch statement in Perl 6 is that we never had it in the first five versions of Perl. But it would be incorrect to say that we didn't miss it. What actually happened was that every time we started discussing how to add a switch statement, it wasn't obvious how far to go. A switch statement in Perl ought to do more than a switch statement in C (or in most any other language, for that matter). So the fact that we haven't added a switch statement so far says more about how hard it is to design a good one than about how much we wanted a lousy one. Eventually the ever inventive Damian Conway came up with his famous design, with a Perl 5 module as proof of concept, and pretty much everyone agreed that he was on the right track, for some definition of "right" (and "track"). This RFC is essentially that design (not surprisingly, since Damian wrote it), so it will be accepted, albeit with several tweaks.
In the first place, as a quasi-linguist, I loathe the keywords
switch and case. I would prefer keywords that read better in
English. Much as I love verbing nouns, they don't work as well as real
verbs or real prepositions when topicalizers are called for. After
thrashing over several options with Damian and other folks, we've
settled on using given instead of switch, and when instead of
case:
given EXPR {
when EXPR { ... }
when EXPR { ... }
...
}
The other great advantage of using different words is that people won't expect it to work exactly like any other switch statement they may be familiar with.
That being said, I should point out that it is still called "the switch statement", and the individual components are still "cases". But you don't have to put "switch" or "case" into constant-width font, because they're not keywords.
Because curlies are so extremely overloaded in Perl 5, I was at first
convinced that we would need a separator of some sort between the
expression and the block, maybe a : or => or some such.
Otherwise it would be too ambigous to come upon a left curly when
expecting an operator--it would be interpreted as a hash subscript
instead. Damian's RFC proposes to require parentheses in certain
situations to disambiguate the expression.
But I've come to the conclusion that I'd rather screw around (a little) with the "insignificant whitespace" rule than to require an extra unnatural delimiter. If we observe current practice, we note that 99% of the time, when people write a hash subscript they do so without any whitespace before it. And 99% of the time, when they write a block, they do put some whitespace in front of it. So we'll just dwim it using the whitespace. (No, we're not going all the way to whole-hog whitespace dwimmery--Python will remain the best/worst example of that approach.)
Subscripts are the only valid use of curlies when an operator is
expected. (That is, subscripts are essentially postfix operators.) In
contrast, hash composers and blocks are terms, not operators.
Therefore, we will make the rule that a left curly that has whitespace
in front of it will never be interpreted as a subscript in Perl 6. (If
you think this is totally bizarre thing to do, consider that this new
approach is actually consistent with how Perl 5 already parses
variables within interpolated strings.) If there is any space before
the curly, we force it to start a term, not an operator, which means
that the curlies in question must delimit either a hash composer or a
block. And it's a hash composer only if it contains a => pair
constructor at the top level (or an explicit hash keyword on the
front.) Therefore it's possible to unambiguously terminate an
expression by following it with a block, as in the constructs above.
Interestingly, this one tweak to the whitespace rule also means that we'll be able to simplify the parentheses out of other similar built-in constructs:
if $foo { ... }
elsif $bar { ... }
else { ... }
while $more { ... }
for 1..10 { ... }
I think throwing out two required punctuation characters for one required whitespace is an excellent trade in terms of readability, particularly when it already matches common practice. (You can still put in the parens if you want them, of course, just for old times' sake.) This tweak also allows greater flexibility in how user-defined constructs are parsed. If you want to define your own constructs, they should be able to follow the same syntax rules as built-ins.
By a similar chain of logic (or illogic), I also want to tweak the
whitespace rules for the trailing curly. There are severe problems in
any C-derived language that allows user-defined constructs containing
curlies (as Perl does). Even C doesn't entirely escape the
head-scratching puzzle of "When do I put a semicolon after a curly?" A
struct definition requires a terminating semicolon, for instance,
while an if or a while doesn't.
In Perl, this problem comes up most often when people say "Why do I
have to put a semicolon after do {} or eval {} when it looks like
a complete statement?"
Well, in Perl 6, you don't, if the final curly is on a line by itself. That is, if you use an expression block as if it were a statement block, it behaves as one. The win is that these rules are consistent across all expression blocks, whether user-defined or built-in. Any expression block construct can be treated as either a statement or a component of an expression. Here's a block that is being treated as a term in an expression:
$x = do {
...
} + 1;
However, if you write
$x = do {
...
}
+ 1;
then the + will be taken erroneously as the start of a new statement. (So don't do that.)
Note that this special rule only applies to constructs that take a
block (that is, a closure) as their last (or only) argument. Operators
like sort and map are unaffected. However, certain constructs
that used to be in the statement class may become expression constructs
in Perl 6. For instance, if we change BEGIN to an expression
construct we can now use a BEGIN block inside an expression to force
compile-time evaluation of a non-static expression:
$value = BEGIN { call_me_once() } + call_me_again();
On the other hand, a one-line BEGIN would then have to have a
semicolon.
[Update: We ended up solving this by distinguishing statement BEGIN
from term BEGIN. Also, we can't really syntactically distinguish
the closures for constructs like sort or map if they are subject
to multiple dispatch, since we wouldn't know the signature. However,
you never have to end such a block on a line by itself, since you can
always put a comma, or even feed the list to the operator
using the <== pipe operator.]
Anyway, back to switch statements. Damian's RFC proposes various specific kinds of dwimmery, and while some of those dwims are spot on, others may need adjustment. In particular, there is an assumption that the programmer will know when they're dealing with an object reference and when they're not. But everything will be an object reference in Perl 6, at some level or other. The underlying characteristics of any object are most generally determined by the answer to the question, "What methods does this object respond to?"
Unfortunately, that's a run-time question in general. But in specific,
we'd like to be able to optimize many of these switch statements at
compile time. So it may be necessary to supply typological hints in
some cases to do the dwimmery efficiently. Fortunately, most cases are
still fairly straightforward. A 1 is obviously a number, and a
"foo" is obviously a string. But unary + can force anything to a
number, and unary _ can force anything to a string. Unary ? can
force a boolean, and unary . can force a method call. More
complicated thoughts can be represented with closure blocks.
[Update: String context is now forced by unary ~.]
Another thing that needs adjustment is that the concept of "isa" matching seems to be missing, or at least difficult to express. We need good "isa" matching to implement good exception handling in terms of the switch mechanism. This means that we need to be able to say something like:
given $! {
when Error::Overflow { ... }
when Error::Type { ... }
when Error::ENOTTY { ... }
when /divide by 0/ { ... }
...
}
and expect it to check $!.isa(Error::Overflow) and such, along with
more normal pattern matching. In the case of the actual exception
mechanism, we won't use the keyword given, but rather CATCH:
CATCH {
when Error::Overflow { ... }
when Error::Type { ... }
when Error::ENOTTY { ... }
when /divide by 0/ { ... }
...
}
CATCH is a BEGIN-like block that can turn any block into a "try"
block from the inside out. But the insides of the CATCH are an
ordinary switch statement, where the discriminant is simply the current
exception object, $!. More on that later--see RFC 88 below.
Some of you may recall that I've stated that Perl 6 will have no
barewords. That's still the case. A token like Error::Overflow is
not a bareword because it's a declared class. Perl 6 recognizes package
names as symbolic tokens. So when you call a class method as
Class::Name.method(), the Class::Name is actually a class object
(that just happens to stringify to "Class::Name"). But the class
method can be called without a symbolic lookup on the package name at
run time, unlike in Perl 5.
Since Error::Overflow is just such a class object, it can be
distinguished from other kinds of objects in a switch statement, and an
"isa" can be inferred. It would be nice if we could go as far as to say
that any object can be called with any class name as a method name to
determine whether it "isa" member of that class, but that could
interfere with use of class name methods to implement casting or
construction. So instead, since switch statements are into heavy
dwimmery anyway, I think the switch statement will have to recognize
any Class::Name known at compile time, and force it to call
$!.isa(Class::Name).
[Update: Actually, it just follows from how smart matching works. But
in fact, smart match calls the .does method, which is more
general than the .isa method, insofar as .does can check both
roles and inheritance. See A12 for more.]
Another possible adjustment will involve the use of switch statements as a means of parallelizing regular expression evaluation. Specifically, we want to be able to write parsers easily in Perl, which means that we need some way of matching a token stream against something like a set of regular expressions. You can think of a token stream as a funny kind of string. So if the "given" of a switch statement is a token stream, the regular expressions matched against it may have special abilities relating to the current parse's data structure. All the regular expressions of such a switch statement will likely be implicitly anchored to the current parse location, for instance. There may be special tokens referring to terminals and non-terminals. Basically, think of something like a yacc grammar, where alternative pattern/action grammar rules are most naturally expressed via switch statement cases. More on that in the next Apocalypse.
[Update: Parsing rules tend to use the :p modifier, which anchors the
next token automatically to the start of the rule. That's all that
needs to be user visible. Everything else is the domain of the optimizer.]
Another possible adjustment is that the proposed else block could be
considered unnecessary. The code following the final when is
automatically an "else". Here's a duodecimal digit converter:
$result = given $digit {
when "T" { 10 }
when "E" { 11 }
$digit;
}
Nevertheless, it's probably good documentation to line up all the
blocks, which means it would be good to have a keyword. However, for
reasons that will become clearer when we talk about exception handlers,
I don't want to use else. Also, because of the identification of
when and if, it would not be clear whether an else should
automatically supply a break at the end of its block as the ordinary
when case does.
So instead of else, I'd like to borrow a bit more from C and use
default:
$result = given $digit {
when "T" { 10 }
when "E" { 11 }
default { $digit }
}
Unlike in C, the default case must come last, since Perl's cases are
evaluated (or at least pretend to be evaluated) in order. The optimizer
can often determine which cases can be jumped to directly, but in cases
where that can't be determined, the cases are evaluated in order much
like cascaded if/elsif/else conditions. Also, it's allowed to
intersperse ordinary code between the cases, in which case the code
must be executed only if the cases above it fail to match. For example,
this should work as indicated by the print statements:
given $given {
print "about to check $first";
when $first { ... }
print "didn't match $first; let's try $next";
when $next { ... }
print "giving up";
default { ... }
die "panic: shouldn't see this";
}
We can still define when as a variant of if, which makes it
possible to intermix the two constructs when (or if) that is desirable.
So we'll leave that identity in--it always helps people think about it
when you can define a less familiar construct in terms of a more
familiar one. However, the default isn't quite the same as an
else, since else can't stand on its own. A default is more
like an if that's always true. So the above code is equivalent to:
given $given {
print "about to check $first";
if $given =~ $first { ...; break }
print "didn't match $first; let's try $next";
if $given =~ $next { ...; break }
print "giving up";
if 1 { ...; break; }
die "panic: shouldn't see this";
}
We do need to rewrite the relationship table in the RFC to handle some
of the tweaks and simplifications we've mentioned. The comparison of
bare refs goes away. It wasn't terribly useful in the first place,
since it only worked for scalar refs. (To match identities we'll need
an explicit .id method in any event. We won't be relying on the
default numify or stringify methods to produce unique representations.)
[Update: Instead of an .id method, which would be unclear how to compare,
there's a =:= operator for comparing identities. Though you can always
compare two id's with the === operator.]
I've rearranged the table to be applied in order, so that default interpretations come later. Also, the "Matching Code" column in the RFC gave alternatives that aren't resolved. In these cases I've chosen the "true" definition rather than the "exists" or "defined" definition. (Except for certain set manipulations with hashes, people really shouldn't be using the defined/undefined distinction to represent true and false, since both true and false are considered defined concepts in Perl.)
Some of the table entries distinguish an array from a list. Arrays look like this:
when [1, 3, 5, 7, 9] { "odd digit intersection" }
when @array { "array intersection" }
while a list looks like this:
when 1, 3, 5, 7, 9 { "odd digit" }
when @foo, @bar, @baz { "intersection with at least one array" }
Ordinarily lists and arrays would mean the same thing in scalar
context, but when is special in differentiating explicit arrays from
lists. Within a when, a list is a recursive disjunction. That is,
the comma-separated values are treated as individual cases OR-ed
together. We could use some other explicit notation for disjunction
such as:
when any(1, 3, 5, 7, 9) { "odd" }
But that seems a lot of trouble for a very common case of case, as it were. We could use vertical bars as some languages do, but I think the comma reads better.
[Update: since we ended up adding junctions, you do use vertical bars now.
If you like commas you can always use any().]
Anyway, here's another simplification. The following table will also
define how the Perl 6 =~ operator works! That allows us to use a
recursive definition to handle matching against a disjunctive list of
cases. (See the first entry in the table below.) Of course, for
precedence reasons, to match a list of things using =~ you'll have
to use parens:
$digit =~ (1, 3, 5, 7, 9) and print "That's odd!";
Alternatively, you can look at this table as the definition of the
=~ operator, and then say that the switch statement is defined in
terms of =~. That is, for any switch statement of the form
given EXPR1 {
when EXPR2 { ... }
}
it's equivalent to saying this:
for (scalar(EXPR1)) {
if ($_ =~ (EXPR2)) { ... }
}
[Update: the =~ operator has been renamed ~~.]
$a $b Type of Match Implied Matching Code
====== ===== ===================== =============
expr list recursive disjunction match if $a =~ any($b)
list list recursive disjunction* match if any($a) =~ any($b)
hash sub(%) hash sub truth match if $b(%$a)
array sub(@) array sub truth match if $b(@$a)
expr sub($) scalar sub truth match if $b($a)
expr sub() simple closure truth* match if $b()
hash hash hash key intersection* match if grep exists $a{$_}, $b.keys
hash array hash value slice truth match if grep {$a{$_}} @$b
hash regex hash key grep match if grep /$b/, keys %$a
hash scalar hash entry truth match if $a{$b}
array array array intersection* match if any(@$a) =~ any(@$b)
array regex array grep match if grep /$b/, @$a
array number array entry truth match if $a[$b]
array expr array as list match if any($a) =~ $b
object class class membership match if $a.isa($b)
object method method truth match if $a.$b()
expr regex pattern match match if $a =~ /$b/
expr subst substitution match match if $a =~ subst
expr number numeric equality match if $a == $b
expr string string equality match if $a eq $b
expr boolean simple expression truth* match if $b
expr undef undefined match unless defined $a
expr expr run-time guessing match if ($a =~ $b) at runtime
[Update: This is inaccurate in several ways; see the most recent table in S03.]
In order to facilitate optimizations, these distinctions are made
syntactically at compile time whenever possible. For each comparison,
the reverse comparison is also implied, so $a/$b can be thought
of as either given/when or when/given. (We don't reverse the matches
marked with * are because it doesn't make sense in those casees.)
If type of match cannot be determined at compile time, the default is to try to apply the very same rules in the very same order at run time, using the actual types of the arguments, not their compile-time type appearance. Note that there are no run-time types corresponding to "method" or "boolean". Either of those notions can be expressed at runtime as a closure, of course.
In fact, whenever the default behavior is not what you intend, there are ways to force the arguments to be treated as you intend:
Intent Natural Forced
====== ======= ======
array @foo [list] or @{expr}
hash %bar {pairlist} or %{expr}
sub(%) { %^foo.aaa } sub (%foo) { ... }
sub(@) { @^bar.bbb } sub (@bar) { ... }
sub($) { $^baz.ccc } sub ($baz) { ... }
number numeric literal +expr int(expr) num(expr)
string string literal _expr str(expr)
regex //, m//, qr// /$(expr)/
method .foo(args) { $_.$method(args) }
boolean $a == $b ?expr or true expr or { expr }
[Update: qr// is now spelled rx//. String context is forced ~.
A regex is "interpolated" with /<$expr/>, but it's not interpolated
as a separate pass as in Perl 5.]
A method must be written with a unary dot to distinguish it from other forms. The method may have arguments. In essence, when you write
.foo(1,2,3)
it is treated as if you wrote
{ $_.foo(1,2,3) }
and then the closure is evaluated for its truth.
A class match works only if the class name is known at compile time.
Use .isa("Class") for more complicated situations.
[Update: You can specify a run-time class name via ::($expr) without
explicitly using .does (or .isa, for that matter).]
Boolean expressions are recognized at compile time by the presence of a
top-level operator that is a comparison or logical operator. As the
table shows, an argumentless closure (a sub (), that is) also
functions as a boolean. However, it's probably better documentation to
use the true function, which does the opposite of not. (Or the
unary ? operator, which does the opposite of unary !.)
It might be argued that boolean expressions have no place here at all,
and that you should use if if that's what you mean. (Or use a
sub() closure to force it to ignore the given.) However, the "comb"
structure of a switch is an extremely readable way to write even
ordinary boolean expressions, and rather than forcing people to write:
anyblock {
when { $a == 1 } { ... }
when { $b == 2 } { ... }
when { $c == 3 } { ... }
default { ... }
}
I'd rather they be able to write:
anyblock {
when $a == 1 { ... }
when $b == 2 { ... }
when $c == 3 { ... }
default { ... }
}
This also fits better into the use of "when" within CATCH blocks:
CATCH {
when $!.tag eq "foo" { ... }
when $!.tag eq "bar" { ... }
default { die }
}
To force all the when clauses to be interpreted as booleans without
using a boolean operator on every case, simply provide an empty given,
to be read as "given nothing...":
given () {
when $a.isa(Ant) { ... }
when $b.isa(Bat) { ... }
when $c.isa(Cat) { ... }
default { ... }
}
[Update: you can just say given {.true} to get the same effect without
a special case.]
A when can be used by other topicalizers than just given. Just as
CATCH will imply a given of $!, a for loop (the foreach
variety) will also imply a given of the loop variable:
for @foo {
when 1 { ... }
when 2 { ... }
when "x" { ... }
default { ... }
}
By symmetry, a given will by default alias $_ to the "given".
Basically, the only difference between a given and a for is that
a given takes a scalar expression, while a for takes a
pre-flattened list and iterates over it.
[Update: for takes a lazily flattened list, not a pre-flattened list.
Use the eager listop to pre-flatten a list.]
Suppose you want to preserve $_ and alias $g to the value
instead. You can say that like this:
given $value -> $g {
when 1 { /foo/ }
when 2 { /bar/ }
when 3 { /baz/ }
}
In the same way, a loop's values can be aliased to one or more loop variables.
for @foo -> $a, $b { # two at a time
...
}
That works a lot like the definition of a subroutine call with two
formal parameters, $a and $b. (In fact, that's precisely what it
is.) You can use modifiers on the formal paramaters just as you would
in a subroutine type signature. This implies that the aliases are
automatically declared as my variables. It also implies that you can
modify the formal parameter with an rw property, which allows you to
modify the original elements of the array through the variable. The
default loop:
for @foo { ... }
is really compiled down to this:
for @foo -> $_ is rw { ... }
Since for and given work by passing arguments to a closure, it's
a small step to generalize that in the other direction. Any method
definition is a topicalizer within the body of the method, and will
assume a "given" of its $self object (or whatever you have named
it). Bare closures topicalize their first argument, implicitly aliasing
it to $_ unless $^a or some such is used. That is, if you say
this:
grep { $_ eq 3 } @list
it's equivalent to this more explicit use of a curried function:
grep { $^a eq 3 } @list
But even a grep can use the aliasing syntax above:
grep -> $x { $x eq 3 } @list
Outside the scope of any topicalizer, a when will assume that its
given was stored in $_ and will test implicitly against that
variable. This allows you to use when in your main loop, for
instance, even if that main loop was supplied by Perl's -n or -p
switch. Whenever a loop is functioning as a switch, the break implied
by finishing a case functions as a next, not a last. Use last
if that's what you mean.
A when is the only defaulting construct that pays attention to the
current topicalizer regardless of which variable it is associated with.
All other defaulting constructs pay attention to a fixed variable,
typically $_. So be careful what you're matching against if the
given is aliased to something other than $_:
$_ = "foo";
given "bar" -> $f {
if /foo/ { ... } # true, matches against $_
when /bar/ { ... } # true, matches against $f
}
[Update: The first argument is always aliased to $_ now.]
Oh, one other tweak. The RFC proposes to overload next to mean "fall
through to the next case". I don't think this is wise, since we'll
often want to use loop controls within a switch statement. Instead, I
think we should use skip to do that. (To be read as "Skip to the
next statement.")
[Update: That's now spelled continue.]
Similarly, if we make a word to mean to explicitly break out of a
topicalizer, it should not be last. I'd suggest break! It will,
of course, be unnecessary to break out of the end of a when case
because the break is implied. However, there are times when you
might want to break out of a when block early. Also, since we're
allowing when modifiers that do not implicitly break, we could use
an explicit break for that situation. You might see cases like this:
given $x {
warn("Odd value") when !/xxx/;
warn("No value"), break when undef;
when /aaa/ { break when 1; ... }
when /bbb/ { break when 2; ... }
when /ccc/ { break when 3; ... }
}
So it looks to me like we need a break.
This RFC posits some requirements for exception handling (all of which I agree with), but I do have some additional requirements of my own:
try-less try blocks must also be able to specify
mandatory cleanup on exit.return,
next, and last on exception handling.BEGIN and
END blocks are to be considered prior art.RFC 88 is massive, weighing in at more than 2400 lines. Annotating the entire RFC would make this Apocalypse far too big. ("Too late!" says Damian.) Nonetheless, I will take the approach of quoting various bits of the RFC and recasting those bits to work with my additional requirements. Hopefully this will convey my tweaks most succinctly.
Here's what the RFC gives as its first example:
exception 'Alarm';
try {
throw Alarm "a message", tag => "ABC.1234", ... ;
}
catch Alarm => { ... }
catch Error::DB, Error::IO => { ... }
catch [email protected] =~ /divide by 0/ => { ... }
catch { ... }
finally { ... }
Here's how I see that being written in Perl 6:
my class X::Alarm is Exception { } # inner class syntax?
try {
throw X::Alarm "a message", tag => "ABC.1234", ... ;
CATCH {
when X::Alarm { ... }
when Error::DB, Error::IO { ... }
when /divide by 0/ { ... }
default { ... }
}
POST { ... }
}
[Update: the comma in the second case should be a |.]
The outer block does not have to be a try block. It could be a
subroutine, a loop, or any other kind of block, including an eval
string or an entire file. We will call such an outer block a try block,
whether or not there is an explicit try keyword.
The biggest change is that the various handlers are moved inside of the
try block. In fact, the try keyword itself is mere documentation in
our example, since the presence of a CATCH or POST block is
sufficient to signal the need for trapping. Note that the POST block
is completely independent of the CATCH block. (The POST block has
a corresponding PRE block for design-by-contract programmers.) Any
of these blocks may be placed anywhere in the surrounding block--they
are independent of the surrounding control flow. (They do have to
follow any declarations they refer to, of course.) Only one CATCH is
allowed, but any number of PRE and POST blocks. (In fact, we may
well encourage ourselves to place POST blocks near the constructors
to be cleaned up after.) PRE blocks within a particular try block
are evaluated in order before anything else in the block. POST
blocks will be evaluated in reverse order, though order dependencies
between POST blocks are discouraged. POST blocks are evaluated
after everything else in the block, including any CATCH.
[Update: PRE and POST are now spelled ENTER and LEAVE.
(PRE and POST are reserved for Design By Contract assertions).]
A try {} without a CATCH is equivalent to Perl 5's eval {}.
(In fact, eval will go back to evaluating only strings in Perl 6,
and try will evaluate only blocks.)
The CATCH and POST blocks are naturally in the lexical scope of
the try block. They may safely refer to lexically scoped variables
declared earlier in the try block, even if the exception is thrown
during the elaboration sequence. (The run-time system will guarantee
that individual variables test as undefined (and hence false) before
they are elaborated.)
The inside of the CATCH block is precisely the syntax of a switch
statement. The discriminant of the switch statement is the exception
object, $!. Since the exception object stringifies to the error
message, the when /divide by 0/ case need not be explicitly compared
against $!. Likewise, explicit mention of a declared class implies
an "isa" lookup, another built-in feature of the new switch statement.
In fact, a CATCH of the form:
CATCH {
when xxx { ... } # 1st case
when yyy { ... } # 2nd case
... # other cases, maybe a default
}
means something vaguely like:
BEGIN {
%MY.catcher = {
given current_exception() -> $! {
when xxx { ... } # 1st case from above
when yyy { ... } # 2nd case from above
... # other cases, maybe a default
die; # rethrow $! as implicit default
}
$!.markclean; # handled cleanly, in theory
}
}
The unified "current exception" is $!. Everywhere this RFC uses
[email protected], it should be read as $! instead. (And the too-precious @@
goes away entirely in favor of an array stored internally to the $!
object that can be accessed as @$! or $![-1].) (For the legacy
Perl 5 parser, [email protected] and $? will be emulated, but that will not be
available to the Perl 6 parser.)
Also note that the CATCH block implicitly supplies a rethrow (the
die above) after the cases of the switch statement. This will not be
reached if the user has supplied an explicit default case, since the
break of that default case will always bypass the implicit die.
And if the switch rethrows the exception (either explicitly or
implicitly), $! is not marked as clean, since the die will bypass
the code that marks the exception as "cleanly caught". It should be
considered an invariant that any $! in the normal control flow
outside of a CATCH is considered "cleanly caught", according to the
definition in the RFC. Unclean exceptions should only be seen inside
CATCH blocks, or inside any POST blocks that have to execute
while an exception is propagating to an outer block because the current
try block didn't handle it. (If the current try block does successfully
handle the exception in its CATCH, any POST blocks at the same
level see a $! that is already marked clean.)
RFC:
eval {die "Can't foo."}; print [email protected]; continues to work as
before.That will instead look like
try { die "Can't foo" }; print $!;
in Perl 6. A try with no CATCH:
try { ... }
is equivalent to:
try { ... CATCH { default { } } }
(And that's another reason I didn't want to use else for the default
case of a switch statement--an else without an if looks really
bizarre...)
Just as an aside, what I'm trying to do here is untangle the exception
trapping semantics of eval from its code parsing and running
semantics. In Perl 6, there is no eval {}. And eval $string
really means something like this:
try { $string.parse.run }
RFC:
However, Perl core functions will by default signal failure using
unthrown proto-exceptions (that is, interesting values of undef) that
can easily be turned into thrown exceptions via die. By "interesting
values of undef", I don't mean undef with properties. I mean
full-fledged exception objects that just happen to return false from
their .defined and .true methods. However, the .str method
successfully returns the error message, and the .int method returns
the error code (if any). That is, they do stringify and numify like
$! ought to. An exception becomes defined and true when it is
thrown. (Control exceptions become false when cleanly caught, to avoid
spoofing old-style exception handlers.)
RFC:
try { fragile(); } catch { } # Go on no matter what.
This will simply be:
try { fragile; }
But it means the same thing, and it's still the case that all
exceptions propagate unless they are cleanly caught. In this case, the
caught exception lives on in $! as a new proto-exception that could
be rethrown by a new die, much as we used to use [email protected]. Whether an
exception is currently considered "cleanly caught" can be reflected in
the state of the $! object itself. When $! passes through the end
of a CATCH, it is marked as clean, so that subsequent attempts to
establish a new $! know that they can clear out the old @$!
stack. (If the current $! is not clean, it should just add its
information without deleting the old information--otherwise an error in
a CATCH could delete the exception information you will soon be
wanting to print out.)
RFC:
try { ... } catch <test> => { ... } finally { ... }
Now:
{ ... CATCH { when <test> { ... } } POST { ... } }
(The angle brackets aren't really there--I'm just copying the RFC's metasyntax here.)
Note that we're assuming a test that matches the "boolean" entry from the switch dwimmery matrix. If not, you can always wrap closure curlies around the test:
{ ... CATCH { when { <test> } { ... } } POST { ... } }
That will force the test to be called as a subroutine that ignores its
argument, which happens to be $!, the exception object. (Recall that
the implied "given" of a CATCH statement sets $! as the given
value. That given value is automatically passed to any "when" cases
that look like subroutines or closures, which are free either to ignore
the passed value, or access it as $_ or $^a.)
Or you might just prefer to use the unary true operator:
{ ... CATCH { when true <test> { ... } } POST { ... } }
I personally find that more readable than the closure.
[Update: POST is now LEAVE.]
RFC:
The test argument of a when clause is NOT optional, since it would
be impossible to distinguish a conditional closure from the following
block. Use default for the default case.
RFC:
try, catch, and finally blocks should share the same
lexical scope, in the way that while and continue do.Actually, this is not so--the while and continue blocks don't
share the same lexical scope even in Perl 5. But we'll solve this issue
without "tunneling" in any case. (And we'll change the continue
block into a NEXT block that goes inside, so we can refer to lexical
variables from within it.)
RFC:
try is a keyword, not a function. This is so that
a ; is not needed at the end of the last block. This is because a
try/catch/finally now looks more like an
if/elsif/else, which does not require such a ;, than like
an eval, which does).Again, this entire distinction goes away in Perl 6. Any expression
block that terminates with a right curly on its own line will be
interpreted as a statement block. And try is such an expression
block.
RFC:
[email protected] contains the current exception, and @@ contains the
current exception stack, as defined above under die. The unshift
rule guarantees that [email protected] == [email protected][0].Why an unshift? A stack is most naturally represented in the other
direction, and I can easily imagine some kinds of handlers that might
well treat it like a stack, stripping off some entries and pushing
others.
Also, @@ is a non-starter because everything about the current
exception should all be in a single data structure. Keeping the info
all in one place makes it easy to rethrow an exception without losing
data, even if the exception was marked as cleanly caught. Furthermore I
don't think that the exception stack needs to be Huffman coded that
badly.
So $! contains the current exception, and $!.stack accesses the
current exception stack. Through the magic of overloading, the $!
object can likely be used as an array even though it isn't one, in
which case @$! refers to that stack member. The push rule
guarantees that $!.id == $![-1].id.
RFC (speaking of the exception declaration):
@MyError::App::DB::Foo::ISA = 'MyError::App::DB';
and all non-existent parent classes are automatically created as
inheriting from their parent, or Exception in the tail case. If a
parent class is found to exist and not inherit from Exception, a
run-time error exception is raised.
If I understand this, I think I disagree. A package ought to able to contain exceptions without being an exception class itself. There certainly ought to be a shorthand for exceptions within the current package. I suspect they're inner classes of some sort, or inner classes of an inner package, or some such.
RFC:
@Alarm::ISA = 'Exception';
This means that every exception class isa Exception, even if
Exception:: is not used at the beginning of the class name.
Ack! This could be really bad. What if two different modules declare an
Alarm exception with different derivations?
I think we need to say that unqualified exceptions are created within the current package, or maybe within the X subpackage of the current package. If we have inner classes, they could even be lexically scoped (and hence anonymous exceptions outside the current module). That might or might not be a feature.
I also happen to think that Exception is too long a name to prefix
most common exceptions, even though they're derived from that class. I
think exceptions will be better accepted if they have pithier names
like X::Errno that are derived from Exception:
our class X::Control is Exception;
our class X::Errno is Exception;
our class X::NumericError is Exception;
our class C::NEXT is X::Control;
our class E::NOSPC is X::Errno;
our class X::FloatingUnderflow is X::NumericError;
Or maybe those could be:
c::NEXT
e::NOSPC
x::FloatingUnderflow
if we decide uppercase names are too much like user-defined package
names. But that looks strange. Maybe we just reserve single letter
top-level package names for Perl. Heck, let's just reserve all
top-level package names for Perl. Er, no, wait... :-)
RFC 80 suggests that exception objects numerify to the system's errno number when those are available. That's a possibility, though by the current switch rules we might have to write
CATCH {
when +$ENOSPC { ... }
}
to force $ENOSPC to do a numeric comparison. It may well be better
to go ahead and make the errno numbers into exception classes, even if
we have to write something like this:
CATCH {
when X::ENOSPC { ... }
}
That's longer, but I think it's clearer. Possibly that's E::NOSPC
instead. But in any event, I can't imagine getting people to prefix
every exception with "Exception::". That's just gonna discourage
people from using exceptions. I'm quite willing to at least reserve the
X top-level class for exceptions. I think X:: is quite
sufficiently distinctive.
RFC:
try { my $f = open "foo"; ... } finally { $f and close $f; }
Now:
{
my $f = open "foo"; ...
POST { $f and close $f }
}
Note that $f is naturally in scope and guaranteed to have a boolean
value, even if the exception is thrown before the declaration statement
is elaborated! (An implementation need not allocate an actual variable
before the my. The code of the POST block could always be
compiled to know that $f is to be assumed undefined if the
allocating code has not yet been reached.)
We could go as far as to make
POST { close $f }
do something reasonable even without the guard. Maybe an undefined
object could "emulate" any method for you within a POST. Maybe
try is really a unary operator:
POST { try close $f }
Or some such. I dunno. This needs more thought along transactional lines...
Time passes...
Actually, now that I've thought on it, it would be pretty easy to put
wrappers around POST blocks that could do commit or rollback
depending on whether the block exits normally. I'd like to call them
KEEP and UNDO. KEEP blocks would only be executed if the block
succeeded. UNDO blocks would only be executed if the block failed.
One could even envision a syntax that ties the block to particular
variable:
UNDO $f { close $f }
After all, like the CATCH block, all of these blocks are just fancy
BEGIN blocks that attach some meaning to some predefined property of
the block.
It's tempting to make the execution of UNDO contingent upon whether
the block itself was passed during execution, but I'm afraid that might
leave a window in which a variable could already be set, but subsequent
processing might raise an exception before enabling the rollback in
question. So it's probably better to tie it to a particular variable's
state more directly than just by placing the block at some point after
the declaration. In fact, it could be associated directly with the
variable in question at declaration time via a property:
my $f is undo { close $f } = open $file or die;
[Update: All of these should use will rather than is, or the
closure has to be an explicit argument to undo with no intervening
space.]
Note that the block is truly a closure because it relies on the lexical
scoping of $f. (This form of lexical scoping works in Perl 6 because
the name $f is introduced immediately within the statement. This
differs from the Perl 5 approach where the name is not introduced till
the end of the current statement.)
Actually, if the close function defaults to $_, we can say
my $f is undo { close } = open $file;
presuming the managing code is smart enough to pass $f as a
parameter to the closure. Likewise one could attach a POST block to
a variable with:
my $f is post { close } = open $file;
Since properties can be combined, you can set multiple handlers on a variable:
my $f is post { close } is undo { unlink $file } = open ">$file" or die;
There is, however, no catch property to go with the CATCH block.
I suppose we could allow a pre property to set a PRE block on a
variable.
RFC:
sub attempt_closure_after_successful_candidate_file_open
{
my ($closure, @fileList) = @_; local (*F);
foreach my $file (@fileList) {
try { open F, $file; } catch { next; }
try { &$closure(*F); } finally { close F; }
return;
}
throw Exception "Can't open any file.",
debug => @fileList . " tried.";
}
Now:
sub attempt_closure_after_successful_candidate_file_open
(&closure, @fileList)
{
foreach my $file (@fileList) {
my $f is post { close }
= try { open $file or die; CATCH { next } }
&closure($f);
return;
}
throw Exception "Can't open any file.",
debug => @fileList . " tried.";
}
[Update: That "foreach" should now be for @fileList - $file>. And
the is post should be is leave. Also these days I'd leave the
ampersand off the call to closure() (though it's still necessary
in the declaration of the parameter).]
Note that the next within the CATCH refers to the loop, not the
CATCH block. It is legal to next out of CATCH blocks, since we
won't use next to fall through switch cases.
However, X::Control exceptions (such as X::NEXT) are a subset of
Exceptions, so
CATCH {
when Exception { ... } # catch any exception
}
will stop returns and loop exits. This could be construed as a feature. When it's considered a bug, you could maybe say something like
CATCH {
when X::Control { die } # propagate control exceptions
when Exception { ... } # catch all others
}
to force such control exceptions to propagate outward. Actually, it would be nice to have a name for non-control exceptions. Then we could say (with a tip of the hat to Maxwell Smart):
CATCH {
when X::Chaos { ... } # catch non-control exceptions
}
And any control exceptions will then pass unimpeded (since by default
uncaught exceptions are rethrown implicitly by the CATCH).
Fortunately or unfortunately, an explicit default case will not
automatically rethrow control exceptions.
[Update: We now distinguish a CONTROL block from a CATCH block.
Control exceptions are invisible to the CATCH block.]
Following are some more examples of how the expression evaluation of
when can be used. The RFC versions sometimes look more concise, but
recall that the "try" is any block in Perl 6, whereas in the RFC form
there would have to be an extra, explicit try block inside many
subroutines, for instance. I'd rather establish a culture in which it
is expected that subroutines handle their own exceptions.
RFC:
try { ... } catch [email protected]>{message} =~ /.../ => { ... }
Now:
try {
...
CATCH {
when $!.message =~ /.../ { ... }
}
}
This works because =~ is considered a boolean operator.
[Update: The operator is ~~ now.]
RFC:
catch not &TooSevere => { ... }
Now:
when not &TooSevere { ... }
The unary not is also a boolean operator.
RFC:
try { ... } catch ref [email protected] =~ /.../ => { ... }
Now:
try { ... CATCH { when $!.ref =~ /.../ { ... } } }
RFC:
try { ... } catch grep { $_->isa("Foo") } @@ => { ... }
Now:
try {
...
CATCH {
when grep { $_.isa(Foo) } @$! { ... }
}
}
I suppose we could also assume grep to be a boolean operator in a scalar context. But that's kind of klunky. If we accept Damian's superposition RFC, it could be written this way:
try {
...
CATCH {
when true any(@$!).isa(Foo) { ... }
}
}
Actually, by the "any" rules of the =~ table, we can just say:
try {
...
CATCH {
when @$! =~ Foo { ... }
}
}
[Update: Lists must be explicitly made into junctions, so now we'd
write that as any($![]) ~~ Foo.]
The RFC proposes the following syntax for finalization:
try { my $p = P->new; my $q = Q->new; ... }
finally { $p and $p->Done; }
finally { $q and $q->Done; }
A world of hurt is covered over by that "...", which could move the
finally clauses far, far away from what they're trying to clean up
after. I think the intent is much clearer with POST. And note also
that we avoid the "lexical tunneling" perpetrated by finally:
{
my $p = P.new; POST { $p and $p.Done; }
my $q = Q.new; POST { $q and $q.Done; }
...
}
More concisely, we can say:
{
my $p is post { .Done } = P.new;
my $q is post { .Done } = Q.new;
...
}
RFC:
try { TryToFoo; }
catch { TryToHandle; }
finally { TryToCleanUp; }
catch { throw Exception "Can't cleanly Foo."; }
How I'd write that:
try {
try {
TryToFoo;
POST { TryToCleanUp; }
CATCH { TryToHandle; }
}
CATCH { throw Exception "Can't cleanly Foo."; }
}
That also more clearly indicates to the reader that the final CATCH
governs the inner try completely, rather than just relying on ordering.
RFC:
Exception class
itself are used for simple exceptions, for those cases in which one
more or less just wants to say throw Exception "My message.",
without a lot of extra tokens, and without getting into higher levels
of the taxonomy of exceptions.die "My message." has much the same effect. I think fail "My
message." will also default similarly, though with return-or-throw
semantics that depend on the caller's use fatal settings.
RFC (regarding on_raise):
on_raise throws or returns true the exception is raised, otherwise
it is not. An exception can be manipulated or replaced and then
propagated in modified form simply by re-raising it in on_raise.Offhand, I don't see this one. Not only does it seem to be making the
$SIG{__DIE__} mistake all over again, it also makes little sense to
me to use "throw" to do something that doesn't throw. A throw should
guarantee termination of control, or you're just going to run user code
that wasn't expected to be run. It'd be like return suddenly not
returning! Let's please use a different method to generate an unthrown
exception. I think a fail method is the right approach--it
terminates the control flow one way or another, even if just returning
the exception as a funny-looking undef.
The on_catch might be a bit more useful.
RFC:
else and continue with unwind semantics not traditionally
associated with else and continue can be confusing, especially
when intermixed with local flow-control forms of else and
continue (which may be present in any { ... } block), or when an
else die [email protected] is forgotten on a switch that needs to re-throw.CATCH will rethrow by default (unless there is a user-specified
default).
RFC:
{ } else { } to
indicate non-local flow-control at work. Yikes!The try is not for Perl's sake. It's for the developer's sake. It
says, watch out, some sort of non-local flow control is going on here.
It signals intent to deal with action at a distance (unwinding
semantics). It satisfies the first requirement listed under
MOTIVATION.
try {} is the new spelling of eval {}, so it can still be used
when self-documentation is desired. It's often redundant, however,
since I think the all-caps CATCH and POST also serve the purpose
of telling the developer to "watch out". I expect that developers will
get used to the notion that many subroutines will end with a CATCH
block. And I'm always in favor of reducing the bracket count of
ordinary code where practical. (That's why the package declaration
has always had a bracketless syntax. I hope to do the same for classes
and modules in Perl 6.)
RFC:
= in a conditional catch clause is required so
the expression can be parsed from the block, in the fashion of Perl
5's parsing of: map <expression>, <list>; Without the comma,
the form catch $foo { ... } could be a test for $foo or a test
for $foo{...} (the hash element).We now require whitespace before a non-subscript block, so this is not much of a problem.
RFC:
Exception and control the class
namespace? For example, if the core can use any Exception::Foo,
where does one connect non-core Exceptions into the taxonomy?
Possibly the core exceptions can derive from Exception::CORE, and
everyone else can use the Exception::MyPackage convention.I don't think defining things as core vs non-core is very useful--"core" is not a fundamental type of exception. I do think the standard exception taxonomy should be extensible, so that non-standard exceptions can migrate toward being standard over time. I also think that modules and classes should have their own subpackage in which to store exceptions.
RFC:
Exception and control those namespaces? Perhaps this
will be covered by some new Perl 6 object technology. Otherwise, we
will need yet another naming scheme convention.Instance variables and methods in a derived class will not interfere with base classes (except by normal hiding of duplicate method names).
RFC:
Exception object
instance variables not specified to the constructor? For example,
tag could default to file + line number.Depends on the constructor, I suspect.
RFC:
Probably depends on the class.
RFC:
I lean towards just the message, with a different method for more info. But this is somewhat dependent on which representational methods we define for all Objects. And that has not been entirely thunk through.
RFC:
Some of the reference texts, when discussing exception handling,
refer to the matter that it may be difficult to implement a go to
across an unwinding semantics block, as in:
try { open F, $f } catch { next; }
This matter will have to be referred to the internals experts. It's ok if this functionality is not possible, it can always be simulated with lexical state variables instead.
However, the authors would very much prefer that gotos across
unwinding boundaries would dwim. If that is not possible, hopefully
some sort of compile-time warning could be produced.
We can do this with special control exceptions that aren't caught until
it makes sense to catch them. (Where exactly control exceptions fit in
the class hierarchy is still open to debate.) In any event, there's no
problem throwing a control exception from a CATCH, since any
exception thrown in a CATCH or POST would propagate outside the
current try block in any event.
Ordinary goto should work as long as it's leaving the current try
scope. Reentering the try somewhere in the middle via goto is likely
not possible, or even desirable. A failed try should be re-entered from
the top, once things have been cleared up. (If the try is a loop block,
going to the next iteration out of its CATCH will probably be
considered safe, just as if there had been an explicit try block
within the loop. But I could be wrong on that.)
RFC:
%@ for Errors from BuiltinsRFC 151 proposes a mechanism for consolidating the information
provided by of [email protected], $!, $?, and $^E. In the opinion of the
author of RFC 88, merging [email protected] and $! should not be undertaken,
because [email protected] should only be set if an exception is raised.
The RFC appears to give no justification for this last assertion. If we
unify the error variables, die with no arguments can simply raise
the current value of $!, and we stay object oriented all the way
down. Then $! indicates the current error whether or not it's being
thrown. It keeps track of its own state, as to whether it is currently
in an "unclean" state, and refuses to throw away information unless
it's clean.
[email protected] current exception @@ current exception stack %@ current core fault information [email protected][0] same as [email protected] [email protected]{type} "IO::File::NotFound" [email protected]{message} "can't find file" [email protected]{param} "/foo/bar/baz.dat" [email protected]{child} $? [email protected]{errno} $! [email protected]{os_err} $^E [email protected]{chunk} That chunk thingy in some msgs. [email protected]{file} Source file name of caller. [email protected]{line} Source line number of caller.
%@ should not contain a severity or fatality classification.
Every call to a core API function should clear %@ if it returns
successfully.
Internally, Perl can use a simple structured data type to hold the
whole canonical %@. The code that handles reading from %@ will
construct it out of the internal data on the fly.
If use fatal; is in scope, then just before returning, each core
API function should do something like: %@ and internal_die %@;
The internal_die becomes the one place where a canonical
Exception can be generated to encapsulate %@ just before raising
an exception, whether or not the use of such canonical Exceptions is
controlled by a pragma such as use exceptions;.
This %@ proposal just looks like a bunch of unnecessary complication
to me. A proto-exception object with methods can be just as easily (and
lazily) constructed, and will map straight into a real exception,
unlike this hash. And an object can always be used as a hash to access
parameterless methods such as instance variable accessors.
RFC:
The semantics of eval are, "clear [email protected] and don't unwind unless
the user re-dies after the eval". The semantics of try are
"unwind after try, unless any raised exception was cleanly and
completely handled, in which case clear [email protected]".
In the author's opinion, both eval and try should exist in Perl
6. This would also mean that the legacy of examples of how to use
eval in Perl will still work.
And, of course, we still need eval $string.
Discussions on perl6-language-errors have shown that some would
prefer the eval { ... } form to be removed from Perl 6, because
having two exception handling methods in Perl could be confusing to
developers. This would in fact be possible, since the same effect can
be achieved with:
try { } catch { } # Clears [email protected]
my $e;
try { ... } catch { $e = [email protected]; }
# now process $e instead of [email protected]
On the other hand, eval is a convenient synonym for all that,
given that it already works that way.
I don't think the exact semantics of eval {...} are worth
preserving. I think having bare try {...} assume a CATCH { default
{} } will be close enough. Very few Perl 5 programs actually care
whether [email protected] is set within the eval. Given that and the way we've
defined $!, the translation from Perl 5 to Perl 6 involves simply
changing eval {...} to try {...} and [email protected] to $! (which lives
on as a "clean" exception after being caught by the try). Perhaps
some attempt can be made to pull an external handler into an internal
CATCH block.
RFC:
catch v/s else + switchSome participants in discussions on perl6-language-errors have
expressed the opinion that not only should eval be used instead of
try, but else should be used instead of multiple catch blocks.
They are of the opinion that an else { switch ... } should be
used to handle multiple catch clauses, as in:
eval { ... }
else {
switch ([email protected]) {
case [email protected]>isa("Exception::IO") { ... }
case [email protected]>my_method { ... }
}
}
This problem with else { switch ... } is: how should the code
implicitly rethrow uncaught exceptions? Many proponents of this model
think that uncaught exceptions should not be implicitly rethrown; one
suggests that the programmer should undef [email protected] at the end of *every*
successful case block, so that Perl re-raises any [email protected] still extant at
the end of the else.
This RFC allows a switch to be used in a catch { ... }
clause, for cases where that approach would minimize redundant code
in catch <expr> { ... } clauses, but with the mechanism
proposed in this RFC, the switch functionality shown above can be
written like this, while still maintaining the automatic exception
propagation when no cases match:
try { ... }
catch Exception::IO => { ... }
catch [email protected]>my_method => { ... }
The switch construct works fine, because the implied break of each
handled case jumps over the default rethrow supplied by the CATCH.
There's no reason to invent a parallel mechanism, and lots of reason
not to.
RFC:
In the name of extensibility and debugging, there should be hooks for
callbacks to be invoked when a try, catch, or finally block is
entered or exited, and when a conditional catch is evaluated. The
callbacks would be passed information about what is happening in the
context they are being called from.
In order to scope the effect of the callbacks (rather than making them global), it is proposed that the callbacks be specified as options to the try statement, something like this:
try on_catch_enter => sub { ... },
on_catch_exit => sub { ... },
{
...
}
The (dynamic, not lexical) scope of these callbacks is from their try
down through all trys nested under it (until overridden at a lower
level). Nested callbacks should have a way of chaining to callbacks
that were in scope when they come into scope, perhaps by including a
reference to the outer-scope callback as a parameter to the callback.
Basically, they could be kept in "global" variables overridden with
local.
Yuck. I dislike cluttering up the try syntax with what are
essentially temp assignments to dynamically scoped globals. It
should be sufficient to say something like:
{
temp &*on_catch_enter = sub { ... };
temp &*on_catch_exit = sub { ... };
...
}
provided, of course, the implementation is smart enough to look for those hooks when it needs them.
RFC:
Authors of modules who wish to provide a public API that respects the
current state of use fatal; if such a mechanism is available, can do
so as follows.
Internal to their modules, authors can use lexically scoped use
fatal; to explicitly control whether or not they want builtins to
raise exceptions to signal errors.
Then, if and only if they want to support the other style, and only for public API subroutines, they do something like one of these:
sub Foo
{
my $err_code = ... ; # real code goes here
# Replace the old return $err_code with this:
return $err_code unless $FATAL_MODE && $error_code != $ok;
throw Error::Code "Couldn't Foo.", code => $err_code;
}
sub Foo
{
try {
# real code goes here, may execute:
throw Exception "Couldn't foo.", code => $err_code;
}
catch !$FATAL_MODE => { return [email protected]>{code}; }
return $ok;
}
Yow. Too much mechanism. Why not just:
return proto Exception "Couldn't foo.", code => $err_code;
The proto method can implement the standard use fatal semantics
when that is desired by the calling module, and otherwise set things up
so that
Foo() or die;
ends up throwing the proto-exception. (The current proto-exception can
be kept in $! for use in messages, provided it's in thread-local
storage.)
Actually, this is really important to make simple. I'd be in favor of a built-in that clearly says what's going on, regardless of whether it ends in a throw or a return of undef:
fail "Couldn't foo", errno => 2;
Just as an aside, it could be argued that all such "built-ins" are
really methods on an implicit class or object. In this case, the
Exception class...
RFC:
The try, catch, and finally clauses localize and undef
$SIG{__DIE__} before entering their blocks. This behavior can be
removed if $SIG{__DIE__} is removed.
$SIG{__DIE__} must die. At least, that name must die--we may install
a similar global hook for debugging purposes.
RFC:
The only changes in respect of Perl 5 behaviour implied by this RFC
are that (1) [email protected] is now always an Exception object (which
stringifies reasonably), it is now read-only, and it can only be set
via die, and (2) the @@ array is now special, and it is now
read-only too.
Perhaps $! could be implicitly declared to have a type of
Exception. But I see little reason to make $! readonly by
default. All that does is prevent clever people from doing clever
things that we haven't thought of yet. And it won't stop stupid people
from doing stupid things. In any event, $! is just a reference to an
object, and access to the object will controlled by the class, not by
Perl.
First I should note in passing that it is likely that
my ($found) = grep { $_ == 1 } (1..1_000_000);
will be smart enough to stop on the first one without additional hints, since the left side will only demand one value of the right side.
However, we do need to unify the behaviors of built-ins with user-defined control structures. From an internal point of view, all of these various ways of exiting a block will be unified as exceptions.
It will be easy enough for a user-defined subroutine to catch the appropriate exceptions and do the right thing. For instance, to implement a loop wrapper (ignoring parser issues), you might write something like this:
sub mywhile ($keyword, &condition, &block) {
my $l = $keyword.label;
while (&condition()) {
&block();
CATCH {
my $t = $!.tag;
when X::Control::next { die if $t && $t ne $l); next }
when X::Control::last { die if $t && $t ne $l); last }
when X::Control::redo { die if $t && $t ne $l); redo }
}
}
}
Remember that those die calls are just rethrows of the current
exception to get past the current try scope (the while in this
case).
How a block gets a label in general is an interesting question. It's
all very well to say that the keyword is the label, but that doesn't
help if you have two nested constructs with the same name. In Perl 5,
labels are restricted to being at the beginning of the statement, but
then how do you label a grep? Should there be some way of specifying
a label on a keyword rather than on a statement? We could end up with
something like this:
my $found = grep:NUM { $_ == 1 and last NUM: $_ } (1..1_000_000);
On the other hand, considering how often this feature is (not) going to used, I think we can stick with the tried-and-true statement label:
my $found = do { NUM: grep { $_ == 1 and last NUM: $_ } (1..1_000_000) };
This has the advantage of matching the label syntax with a colon on the end in both places. I like that.
I don't think every block should implicitly have a way to return, or we'll have difficulty optimizing away blocks that don't do anything blockish. That's because setting up a try environment is always a bit blockish, and does in fact impose some overhead that we'd just as soon avoid when it's unnecessary.
However, it's probably okay if certain constructs that would know how to deal with a label are implicitly labelled by their keyword name when they don't happen to have an explicit label. So I think we can allow something like:
last grep: $_
Despite its appearance, that is not a method call, because grep is
not a predefined class. What we have is a unary operator last that
is taking an adverbial modifier specifying what to return from the
loop.
[Update: there's now a general leave verb that can exit from an inner
block.]
The interesting policy question as we go on will be whether a given
construct responds to a given exception or not. Some exceptions will
have to be restricted in their use. For instance, we should probably
say that only explicit sub declarations may respond to a return.
People will expect return to exit the subroutine they think they're
in, even if there are blocks floating around that are actually closures
being interpreted elsewhere. It might be considered antisocial for
closure interpreters like grep or map or sort to trap
X::Control::return sooner than the user expects.
As for using numbers instead of labels to indicate how many levels to break out of, that would be fine, except that I don't believe in breaking out by levels. If the problem is complex enough that you need to break out more than one level, you need a name, not a number. Then it doesn't matter if you refactor your code to have more block levels or less. I find I frequently have to refactor my code that way.
It's possible to get carried away and retrofit grep and map with
every conceivable variety of abort, retry, accept, reject, reduce,
reuse, recycle, or whatever exception. I don't think that's necessary.
There has to be some reason for writing your own code occasionally. If
we get rid of all the reasons for writing user-defined subroutines, we
might as well pack our bags and go home. But it's okay at minimum to
treat a looping construct like a loop.
This RFC proposes that strict vars should be on by default. This is
motivated by the desire that Perl better support (or cajole, in this
case) the disciplines that enable successful programming in the large.
This goal is laudable.
However, the programming-in-the-small advocates also have a valid point: they don't want to have to go to all the trouble of turning off strictures merely to write a succinct one-liner, since keystrokes are at a premium in such programming, and in fact the very strictures that increase clarity in large programs tend to decrease clarity in small programs.
So this is one of those areas where we desire to have it both ways, and
in fact, we pretty much can. The only question is where to draw the
line. Some discussion suggested that only programs specified on the
command line via the -e switch should be exempt from stricture. But
I don't want to force every little file-based script into the large
model of programming. And we don't need to.
Large programming requires the definition of modules and classes. The
typical large program will (or should) consist mostly of modules and
classes. So modules and classes will assume strict vars. Small
programming does not generally require the definition of modules and
classes, though it may depend on existing modules and classes. But even
small programs that use a lot of external modules and classes may be
considered throw-away code. The very fact that the main code of a
program is not typically reused (in the sense that modules and classes
are reused) means that there is where we should draw the line. So in
Perl 6, the main program will not assume strict vars, unless you
explicitly do something to turn it on, such as to declare "class Main".
This is fine for the main program, but modules and classes should be
held to the higher standard of use strict.
It's important to keep in mind the distinction between variables and values. In a pure OO environment, variables are merely references to values, and have no properties of their own--only the value itself would be able to say whether it is constant. Some values are naturally constant, such as a literal string, while other values could be marked constant, or created without methods that can modify the object, or some such mechanism. In such an environment, there is little use for properties on variables. Any time you put a property on a variable, it's potentially lying about its value.
However, Perl does not aspire to be a pure OO environment. In Perl-think, a variable is not merely a container for a value. Rather, a variable provides a "view" of a value. Sometimes that view could even be construed as a lie. That's okay. Lying to yourself is a useful survival skill (except when it's not). We find it necessary to repeat "I think I can" to ourselves precisely when we think we can't. Conversely, it's often valuable psychologically to treat possible activities as forbidden. Abstinence is easier to practice if you don't have to decide anew every time there's a possible assignation, er, I mean, assignment.
Constant declarations on variables fall into this category. The value itself may or may not naturally be constant, but we will pretend that it is. We could in theory go farther than that. We could check the associated object to make sure that it is constant, and blow up if it's not, but that's not necessary in this case for consistent semantics. Other properties may be stricter about this. If you have a variable property that asserts a particular shape of multidimensional array, for instance, the object in question had better be able to supply semantics consistent with that view, and it's probably a good idea to blow up sooner rather than later if it can't. This is something like strong typing, except that it's optional, because the variable property itself is optional.
Nevertheless, the purpose of these variable properties is to allow the compiler to deduce things about the program that it could not otherwise deduce, and based on those deductions, produce both a more robust and more efficient compile-time interpretation of the semantics of the program. That is to say, you can do more optimizations without compromising safety. This is obviously true in the case of inlining constants, but the principle extends to other variable properties as well.
The proposed syntax is fine, except that we'll be using is instead
of : for properties, as discussed in Apocalypse 2. (And it's
constant, not const.)
[Update: compile-time properties are now called "traits".]
As already revealed in Apocalypse 2, attributes will be known as
"properties" in Perl 6, to avoid confusion with existing OO
nomenclature for instance variables. Also, we'll use the is keyword
instead of the colon.
[Update: run-time properties are set with but, while traits
are set with is.]
Setting properties on array and hash elements bothers me, particularly
when those properties have names like "public" and "private". This
seems to me to be an attempt to paper over the gap of some missing OO
functionality. So instead, I'd rather keep arrays and hashes mostly for
homogenous data structures, and encourage people to use objects to
store data of differing types. Then public and private can be
properties of object attributes, which will look more like real
variables in how they are declared. And we won't have to worry about
the meaning of my @foo[2], because that still won't be allowed.
Again, we need to be very clear that the object representing the variable is different than any objects contained by the variable. When we say
my Dog @dogpound is loud;
we mean that the individual elements of @dogpound are of type
Dog, not that the array variable is of type Dog. But the loud
property applies to the array, not to the dogs in the array. If the
array variable needs to have a type, it can be supplied as if it were a
property:
my Dog @dogpound is DogPound is loud;
That is, if a property is the name of a known package/class, it is
taken to be a kind of tie. Given the declaration above, the
following is always true:
@dogpound.is.loud
[Update: The .is is no longer needed, since properties are really
mixin methods now.]
since the loud is a property of the array object, even if it
contains no dogs. It turns out that
@dogpound.is.DogPound
is also true. This does not do an isa lookup. For that, say:
@dogpound.isa(Pound)
Note that you can use:
@dogpound =~ Dog
to test the individual elements for Doghood.
[Update: We use ~~ instead of =~ now. And @dogpound.DogPound
throws an exception if @dogpound doesn't have a DogPound method.]
Unfortunately, the proposed syntax could also be interpreted as parallel traversal:
foreach ($a, $b) (@a, @b)
Also the RFC assumes pairs will be passed as two elements, which is no longer necessarily the case. A hash by itself in list context will return a list of pair objects. We'll need to say something like:
%hash.kv
to get a flattened list of keys alternating with values. (The same method on arrays produces alternating indices and values.)
I like the idea of this RFC, but the proposed syntax is not what I'd like. There are various possible syntaxes that could also potentially fulfill the intent of RFC 120:
for [$i => $elem] (@array) { }
for {$i => $elem} (@array) { }
for ($i, $elem) = (@array.kv) { }
But I like the idea of something that feels like repeated binding. We
could use the := binding operator, but since binding is actually the
operation performed by formal parameters of subroutines, and since we'd
like to keep the list near the for and the formals near the closure,
we'll use a variant of subroutine declaration to declare for loops:
for @list -> $x { ... } # one value at a time
for @list -> $a, $b { ... } # two values at a time
You can un-interleave an array by saying:
for @xyxyxy -> $x, $y { ... }
Iterating over multiple lists in parallel needs a syntax much like a multi-dimensional slice. That is, something like a comma that binds looser than a comma. Since we'll be using semicolon for that purpose to delimit the dimensions of multi-dimensional slices, we'll use similar semicolons to delimit a parallel traversal of multiple lists: So parallel arrays could be stepped through like this:
for @xxx; @yyy; @zzz -> $x; $y; $z { ... }
If there are semicolons on the right, there must be the same number as on the left.
Each "stream" is considered separately, so you can traverse two arrays each two elements at a time like this:
for @ababab; @cdcdcd -> $a, $b; $c, $d { ... }
If there are no semicolons on the right, the values are taken sequentially across the streams. So you can say
for @aaaa; @bbbb -> $a, $b { ... }
and it ends up meaning the same thing as if the comma were a semicolon, but only because the number of variables on the right happens to be the same as the number of streams on the right. That doesn't have to be the case. To get values one at a time across three streams, you can say
for @a; @b; @c -> $x { ... }
Each semicolon delimited expression on the left is considered to be a list of generated values, so it's perfectly legal to use commas or "infinite" ranges on the left. The following prints "a0", "b2", "c3", and so on forever (or at least for a very long time):
for 0 .. Inf; "a" .. "z" x 1000 -> $i; $a {
print "$a$i";
}
[Update: This has been simplified. The arguments to a closure are
always separated by commas, and you can use the each function to
interleave two or more arrays on the left.]
We'll go with temp for the temporizing operator.
In addition, we're going to be storing more global state in objects (such as file objects). So it ought to be possible to temporize (that is, checkpoint/restore) an attribute of an object, or at least any attributes that can be treated as an lvalue.
I can't stop people from experimenting, but I'm not terribly interested
in performing this experiment myself. I made my short for a reason.
So I'm accepting this RFC in principle, but only in principle. Standard
Perl declarations will be plainly marked with my or our.
Just because I've rejected these RFCs doesn't mean that they weren't addressing at a valid need. Usually an RFC gets rejected simply because I think there's a better way to do it. Often there's little difference between a rejected RFC that I've borrowed ideas from and an RFC accepted with major caveats.
We're already running long, so these descriptions will be terse. Please read the RFC if you don't understand the commentary.
This is pretty close to what we've been planning for Perl for a long time. However, a number of the specifics are suboptimal.
If you declare a constant, it's a constant. There's no point in allowing warnings on that by default. It should be fatal to modify a constant. Otherwise you lose all your optimization possibilities.
For historical reasons, the assignment in
my ($a, $b) = new Foo;
will not distribute automatically over $a and $b. If you want
that, use the ^= hyperassignment instead, maybe.
Constraint lists are vaguely interesting, but seem to be too much mechanism for the possible benefits. If you really want a data type that can be polymorphic, why not just define a polymorphic type?
In general, there seems to be a lot of confusion in this RFC between constraints on variables and constraints on values. For constraints to be useful to the compiler, they have to be on the variable, and you can't be "pushing" constraints at runtime.
On aliasing via subroutine calls, note that declared parameters will be constant by default.
So anyway, although I'm rejecting this RFC, we'll certainly have a declaration syntax resembling some of the tables in the RFC.
Yes, it's true that other widely-admired languages like Ruby do implicit declaration of lexicals, but I think it's a mistake, the results of which don't show up until things start getting complicated. (It's a sign of this weakness that in Ruby you see the workaround of faking up an assignment to force declaration of a variable.)
I dislike the implicit declaration of lexicals because it tends to defeat the primary use of them, namely, catching typos. It's just too easy to declare additional variable names by accident. It's also too easy to broaden the scope of a variable by accident. You might have a bunch of separate subroutines each with their own lexical, and suddenly find that they're all the same variable because you accidentally used the same variable name in the module initialization code.
When you think about it, requiring my on declaration is a form of
orthogonality. Otherwise you find your default scoping rules
arbitrarily tied to an inner scope, or an outer scope, or a subroutine
scope. All of these are suboptimal choices. And I don't buy the notion
of using my optionally to disambiguate when you feel like it. Perl
gives you a lot of rope to hang yourself with, but this is the wrong
kind of rope, because it obscures a needful visual distinction.
Declarations should look like declarations, not just to the programmer,
but also to whoever has to read the program after them, whether
carbon-based or silicon-based.
And when it comes down to it, I believe that declarations with my
are properly Huffman encoded. Declaring a lexical ought to be harder
than assigning to one. And declaring a global ought to be harder than
declaring a lexical (at least within classes and modules).
Good goals, but I don't want yet another independent system of exception handling. Simplicity comes through unification. Also, the proposed syntax is all just a little too intertwingled for my tastes. Let's see, how can I explain what I mean?
The out-of-band stuff doesn't stand out visually enough to me, and I
don't like thinking about it as control flow. Nevertheless, I think
that what we've ended up with solves a number of the problems pointed
out in this RFC. The RFC essentially asks for the functionality of
POST, KEEP and UNDO at a statement level. Although POST,
KEEP, and UNDO blocks cannot be attached to any statement, I
believe that allowing post, keep, and undo properties in
scoped declarations is powerful enough, and gives the compiler
something tangible to attach the actions to. There is a kind of
precision in attaching these actions to a specific variable--the state
is bound to the variable in a transactionally instantaneous way. I'm
afraid if we attach transactional actions to statements as the RFC
proposes, it won't be clear exactly when the statement's state change
is to be considered successful, since the transaction can't "know"
which operation is the crucial one.
Nonetheless, some ideas from this RFC will live on in the post,
keep, and undo property blocks.
I am prejudiced against this one, simply because I've been burned too many times by implicit variables that mandate implicit overhead. I think if you need an index, you should declare one, so that if you don't declare one, the compiler knows not to bother setting up for it.
Another problem is that people will keep asking what
for (@foo,@bar) { print $# }
is supposed to mean.
I expect that we'll end up with something more like what we discussed earlier:
for @array.kv -> $i, $elem { ... }
Everyone has a use for : these days...
This one seems not to be of very high utility, suffering from similar problems as the RFC 120 proposal. I don't think it's possible to efficiently track the container of a value within each contained object unless we know at compile time what a looping construct is, which is problematic with user-defined control structures.
And what if an item is a member of more than one list?
Again, I'd rather have something declared so we know whether to take the overhead. Then we don't have to pessimize whenever we can't do a complete static analysis.
I think the "do" on a do block is useful to emphasize that the
closure in the braces is to be executed immediately. Otherwise Perl (or
the user (or both)) might be confused as to whether someone was trying
to write a closure that is to be executed later, particularly if the
block is the last item in a subroutine that might be wanting to return
a closure. In fact, we'll probably outlaw bare blocks at the statement
level as too ambiguous. Use for 1 {} or some such when you want a
one-time loop, and use return or sub when you want to return a
closure.
We'll solve the ; problem by jiggering the definition of {...},
not by fiddling with do.
[Update: Bare blocks are still legal, and always execute immediately
as expected. To return a closure you should use an explicit return.]
The old use integer pragma was a hack. I think I'd rather use types
and representation specs on individual declarations for compile-time
selection, or alternate object constructors for run-time selection,
particularly when infinite precision is desired. I'm not against using
pragmas to alter the defaults, but I think it's generally better to be
more specific when you have the capability. You can force your programs
to be lexically scoped with pragmas, but data wants to flow wherever it
likes to go, so your lexically scoped module had better be able to deal
rationally with any data thrown at it, even if it isn't in the exact
form that you prefer.
By the way, the RFC is misleading when it asserts that 32-bit integer precision is lost when represented in floating point. That's only true if you use 32-bit floats. Perl has always used 64-bit doubles, which give approximately 15 digits of integer precision. (The issue does arise with 64-bit integers, of course.)
All that being said, Perl 6 will certainly have better support for integer types of various sorts. I just don't think that a pragma redefining what an "integer" is will provide good documentation to whoever is trying to understand the program. Better to declare things of type MagicNum, or whatever.
I could be wrong, of course. If so, write your pragma, and have the appropriate amount of fun.
my() syntax extensions and attribute declarationsWe already treated this in Apocalypse 2.
The RFC assumes that the type always distributes over a my list.
This is not what is necessary for function signatures, which need
individual types for each formal argument.
And again, it doesn't make much sense to me to put properties on a variable at run-time.
It makes even less sense to me to be able to declare the type of an array element lexically. This is the province of objects, not arrays pretending to be structs.
Sorry, we can't have the semantics suddenly varying drastically merely because the user decided to run the program through a different translator. I think there's a happy medium in there somewhere where we can have the same semantics for both interpreter and compiler.
This RFC is rejected only because it doesn't go far enough. What we'll eventually need is to allow a regex-ish syntax notation for parsing that may be separate from the argument declarations. (Then again, maybe not.) In any event, I think some kind of explicit regex notation is called for, not the promotion of identifiers to token matchers. We may want identifiers in signatures for something else later, so we'll hold them in reserve.
[Update: This functionality is provided by macros using the is parsed
trait. See A06.]
This seems like a solution in search of a problem. Even if we end up
with a context stack as explicit as Perl 5's, I don't think the amount
we'll deal with it warrants a keyword. (And I dislike "return with;"
as a needlessly opaque linguistic construct.)
That being said, if someone implements (as user-defined code) the
Pascalish with as proposed in RFC 342 (and rejected), and if the
caller function (or something similar) returns sufficient
information to build references to the lexical scope associated with
the call frame in question, then something like this could also be
implemented as user code. I can't decide whether it's not clear that
this is a good idea, or it's clear that this is not a good idea. In any
event, I would warn anyone doing this that it's likely to be extremely
confusing, akin to goto-considered-harmful, and for similar reasons,
though in this case by displacing scopes rather than control flow.
Note that some mechanism resembling this will be necessary for modules
to do exportation to a lexical scope (see %MY in Apocalypse 2).
However, lexical scope modification will be allowed only during the
compile time of the lexical scope in question, since we need to be
careful to preserve the encapsulation that lexical scoping provides.
Turning lexical variables back into dynamic variables will tend to
destroy that security.
So I think we'll stick with closures and continuations that don't transport lexical scopes at runtime.
I expect Perl's parsing to be powerful enough that you could write a "with" if you wanted one.
Due to syntactic ambiguities with the new for syntax of Perl 6, the
generalized C-style for loop is going to get its keyword changed to
loop. And for will now always mean "foreach". The expression
"pill" is now optional, so instead of writing an infinite loop like
this:
for (;;) {
...
}
you can now write it like this:
loop {
...
}
In Perl 5, when you used a while statement modifier on a statement
consisting of nothing but a do {}, something magical happened, and
the block would be evaluated once before the condition was evaluated.
This special-cased construct, seldom used and often misunderstood, will
no longer be in Perl 6, and in fact will produce a compile-time error
to prevent people from trying to use it. Where Perl 5 code has this:
do {
...
} while CONDITION;
Perl 6 code will use a construct in which the control flow is more explicit:
loop {
...
last unless CONDITION;
}
[Update: We'll also allow the <loop {...} while CONDITION;> form to mean
execute once first. Loop until also works, of course.]
In Perl 5, bare blocks (blocks used as statements) are once-through
loops. In Perl 6, blocks are closures. It would be possible to
automatically execute any closure in void context, but unfortunately,
when a closure is used as the final statement in an outer block, it's
ambiguous as to whether you wanted to return or execute the closure.
Therefore the use of a closure at the statement level will be
considered an error, whether or not it's in a void context. Use do
{} for a "once" block, and an explicit return or sub when you
want to return a reference to the closure.
[Update: Bare blocks are legal again and work as expected.]
The continue block changes its name to NEXT and moves inside the
block it modifies, to work like POST blocks. Among other things,
this allows NEXT blocks to refer to lexical variables declared
within the loop, provided the NEXT block is placed after them. The
generalized loop:
loop (EXPR1; EXPR2; EXPR3) { ... }
can now be defined as equivalent to:
EXPR1;
while EXPR2 {
NEXT { EXPR3 }
...
}
(except that any variable declared in EXPR3 would have different
lexical scope). The NEXT block is called only before attempting the
next iteration of the loop. It is not called when the loop is done and
about to exit. Use a POST for that.
[Update: s/POST/LEAVE/]
Well, that about wraps it up for now. You might be interesting to know that I'm posting this from the second sesquiannual Perl Whirl cruise, on board the Veendam, somewhere in the Carribean. If the ship disappears in the Bermuda Triangle, you won't have to worry about the upcoming Exegesis, since Damian is also on board. But for now, Perl 6 is cruising along, the weather's wonderful, wish you were here.