[% setvar title Subroutines: Co-routines %]
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Subroutines: Co-routines
Maintainer: Damian Conway <damian@conway.org> Date: 4 Aug 2000 Last Modified: 18 Sep 2000 Number: 31 Version: 2 Mailing List: perl6-language-subs@perl.org Status: Frozen
This RFC proposes the addition of a new function return command:
yield
. Unlike return
, yield
preserves the execution state of
the subroutine in which it's called, allowing the execution to be
resumed at the following statement, next time the subroutine is called.
It is also proposed that yields
may nest, to simplify the
construction of recursive co-routines and iterators.
Normally, when a subroutine returns, its execution terminates and it final context is completely lost. The next time the subroutine is invoked, it recommences executing from its first statement.
In a coroutine, a value may be returned in such a way that the execution of the routine is suspended, along with all its local context. The next time the routine in invoked, its execution resumes from the statement after the previous point of return.
It is proposed to add a new control statement to Perl: yield
.
A yield
acts very much like a return
in that it terminates
execution of the enclosing subroutine and returns a value to its calling
context. However, when a value is yield
'ed, the subroutine's execution
is suspended in such a way that it resumes from the following statement
next time the subroutine is invoked.
Note that any subroutine containing a yield
is implicitly a co-routine.
There is no explicit keyword or attribute proposed.
Coroutines make it very easy to implement generic parameteric closures and iterators:
package Tree; sub next_inorder ($self) { yield $self->{left}->next_inorder if $self->{left}; yield $self; yield $self->{right}->next_inorder if $self->{right}; return undef; } # and later... while (my $node = $root->next_inorder()) { print $node->{data}; }
Note that the above example implies (correctly) that yielding a result
which itself was yielded leaves the suspended execution of the
subroutine at the same yield
statement (not the following
statement). Furthermore, yielding an undef
is a no-op (i.e. it
doesn't cause the subroutine to return, but passes control to the next
statement).
Note that the arguments of a co-routine are ignored when it is resumed. Hence:
sub every_second ( @vals ) { yield (splice @vals, 0, 2)[0] while @vals; return; }
Another interesting application is to make a map
block a coroutine,
to allow it to process hashes in a natural manner. For example, instead
of writing:
@newhash{map {transform_key($_)} keys %oldhash} = map {transform_val($_)} values %oldhash;
one could write:
%newhash = map {yield transform_key($_); transform_val($_)} %oldhash;
This flattens the %oldhash to a sequence of key/value pairs. Then the
first time the map block is called (i.e. on a key) it transforms the
key and immediately yields it. On the second iteration of the map,
the block resumes after the yield
and transforms the value.
That normal return resets the block so that for the next
iteration (another key) it applies the key transform and yields,
then tranforms the second value, etc., etc.
Tricky.
None.