This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE Replace => (stringifying comma) with => (pair constructor) =head1 VERSION Maintainer: Damian Conway <[EMAIL PROTECTED]> Date: 10 Aug 2000 Last Modified: 18 Sep 2000 Mailing List: [EMAIL PROTECTED] Number: 84 Version: 2 Status: Frozen =head1 ABSTRACT This RFC proposes the introduction of a new data type -- the I<pair> -- and the co-opting of the => operator to act as a pair constructor. Most existing uses of => would be preserved. =head1 DESCRIPTION It is proposed that a new data structure, to be known as a I<pair>, be added to the language. The => operator would cease to be a first-argument-stringifying comma operator, and becomes an anonymous pair constructor (just as [...] and {...} are anonymous list and hash constructors respectively). The => operator would return a reference to a pair containing the two scalar operands to =>. The first component of a pair would be called its I<key>, and the second, it's I<value>. Two new lvalue built-ins -- C<key> and C<value> -- would be introduced to provide read and write access the components of a pair. print key $pair_ref; value($pair_ref) = $newval; =head2 Pairs and hashes A hash could be constructed from a list of pair references. In fact a hash could be thought of (and perhaps implemented!) as a set of pairs. Thus: %hash = ( a=>1, b=>2, 'c', 3 ); does what it does in Perl 5, but works slightly differently. The list being assigned consists of four elements (not six): a pair reference, another pair reference, a string, and an integer. When a pair reference is assigned (in)to a hash, the pair's key becomes the hash entry's key, and the pair's value becomes the entry's value As the above example indicates, hashes could still be assigned "flat" lists. =head2 Pairs and arrays When a pair reference is assigned (in)to an array, it remains a single scalar (referential) value. So: @array = ( a=>1, b=>2, 'c', 3 ); assigns four elements (not six) to @array. =head2 Pairs and subroutines When a pair reference is used in the argument list of a subroutine with no parameter list, it is passed as a single scalar value (i.e it remains a pair reference). When a pair reference is passed to a subroutine with named parameters, it binds its value to the parameter of the same name, regardless of the order in which it is passed. Thus: use Data::Dumper; sub no_params { print "no_params:\n" print join "\n", map {ref||"scalar val"} @_; print Dumper @_; } sub params ( $name, $rank, $serial_num) { print "params:\n" print join "\n", map {ref||"scalar val"} @_; print Dumper @_; } no_params(serial_num=>1234, name=>'demo', rank=>'RFC'); params(serial_num=>1234, name=>'demo', rank=>'RFC'); prints: no_params: PAIR PAIR PAIR $VAR1 = ( 'serial_num' => 1234 ); $VAR2 = ( 'name' => 'demo' ); $VAR3 = ( 'rank' => 'RFC' ); params: scalar val scalar val scalar val $VAR1 = 'demo'; $VAR2 = 'RFC'; $VAR1 = 1234; Note that these semantics still support the popular: sub hash_like_args { my %args = @_; # etc. } hash_like_args(serial_num=>1234, name=>'demo', rank=>'RFC'); =head2 Pairs and multiway comparisons Pairs also provide a clean way of implementing multiway comparisons. It is proposed that when a pair is evaluated in a boolean context, it would evaluate to the truth value of its key. But when evaluated as the left operand of a comparison operator, it would evaluate to its value, I<but> would short-circuit if its key were false. Thus: 1 < 4 < 7 < 9 would be evaluated: (((1 < 4) < 7) < 9) (((true=>4) < 7) < 9) # true because 1 < 4 ((true=>7) < 9) # true because 4 < 7 (true=>9) # true because 7 < 9 1 # boolean context evals to truth of key On the other hand: 1 < 4 < 7 < 3 < 9 would be evaluated: ((((1 < 4) < 7) < 3) < 9) ((((true=>4) < 7) < 3) < 9) # true because 1 < 4 (((true=>7) < 3) < 9) # true because 4 < 7 ((''=>3) < 9) # '' (false) because !(7 < 3) '' # short circuits to false =head1 IMPLEMENTATION See Description. =head1 REFERENCES Forthcoming Conway RFC on subroutine parameter lists (including named parameters) RFC 25: Operators: Multiway comparisons
