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Dominant Value Expressions
Maintainer: Glenn Linderman <glenn@linderman.com> Date: 29 Sep 2000 Last Modified: 30 Sep 2000 Mailing List: perl6-language@perl.org Number: 356 Version: 2 Status: Frozen
An aid to determining if an input value has an impact on the result of an expression whole program. Can also be used for Perl poetry.
Version 2 adds output function, examples, and freezes.
This is an optional feature; to turn it on, "use domination;" is suggested.
When use domination is in scope, two new functions are available, and new rules for expression evaluation obtain. Each of these is described in a subsection below.
The "use domination 'output-function-name';" pragma enables the rest of the functionality. It should be scoped, affecting the current and nested blocks. domination;", but allows user specification of a method to use to convert dominant values to output strings. This is implicitly called only when an output stream is passed a dominant value. If an output-function-name is not supplied with "use domination;", the following function is implied:
sub output-dominant-value {
return sprintf "DOMINANT(%g)", dominant_weight($_[0]);
}
The "no domination;" pragma would turn off the effect of "use domination;" for the current and nested blocks. If a dominant value is encountered while "no domination;" is in effect, it is treated as "undef" by all scalar operators.
The dominant operation takes a scalar argument, which is considered to be a weight parameter. The scalar argument is converted to numeric, if possible, with the resultant positive weight parameter producing a dominant value with that given weight. A scalar argument that cannot be automatically converted to numeric, or that produces a negative or zero numeric value produces undef as a result.
dominant 47 # produces a dominant value of weight 47 dominant 0 # produces undef dominant -47 # produces undef dominant "ab" # produces undef dominant "14" # produces a dominant value of weight 14 dominant undef # produces undef (and a warning if "use warnings")
The dominant_weight operation takes a scalar argument. If the scalar argument is a dominant value, it returns its weight as a positive number. If the scalar argument is not a dominant value, the return of the dominant_weight operation is zero.
dominant_weight dominant 47 # produces 47 dominant_weight dominant 0 # produces 0 dominant_weight dominant -47 # produces 0 dominant_weight dominant "ab" # produces 0 dominant_weight dominant "14" # produces 14 dominant_weight 47 # produces 0 dominant_weight 0 # produces 0 dominant_weight "ab" # produces 0 dominant_weight "14" # produces 0 dominant_weight undef # produces 0 (and a warning if "use warnings")
All scalar operations are affected by the presence of dominant values. If a scalar operation other than the dominant_weight operation involves one or more dominant values, the result of the operation is the heaviest (by weight) dominant value among the operands. If no dominant values are supplied to the operation, the result of the operation is the same as it would be according to the usual definition of the operation.
use domination; $w = dominant 3; $x = dominant 47; $y = 33; $z = "abc"; $x + $y # produces dominant 47 $w . $z # produces dominant 3 $w - $x # produces dominant 47 "$z $x" # produces dominant 47 $z =~ m/$w/; # produces dominant 3 $x =~ m/$z/; # produces dominant 47 defined $w # produces dominant 3 $x == $x # produces dominant 47 $w eq $w # produces dominant 3 $w > 17 # produces dominant 3 $x > 17 # produces dominant 47 print "Show me: $w\n" # same result as: print "DOMINANT(3)" if ( $w ) # considered false if ( dominant_weight $w > 17 ) # false if ( dominant_weight $x > 17 ) # true
New functionality, no compatibility issues. The new functionality only obtains if the "use domination;" pragma is in effect.
The impact of dominant value expressions is pervasive, affecting all builtin scalar operators in a minor way. Any operators doing item by item operations on each scalar of a list or hash would be similarly affected (various RFCs exist for extending scalar operations to work on lists of values in "corresponding item" fashion).
RFC 263: Add null() keyword and fundamental data type