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=head1 TITLE
hash slicing
=head1 VERSION
Maintainer: David Nicol <[EMAIL PROTECTED]>
Date: 7 September 2000
Mailing List: [EMAIL PROTECTED]
Number: 201
Version: 1
Status: Developing
=head1 ABSTRACT
a more concise syntax for producing subsets of hashes.
=head1 DESCRIPTION
=head2 details
Instead of
%subhash = map { f($_) ? ($_, $hash{$_}) : () } keys %hash; # lengthy
one may now write
%subhash = %hash{f($_)}; # code block f($_) will be evaluated for Truth
over all the keys
=head2 syntactically identifiable degenerate cases
Additionally, these special cases are recognized:
%subhash2 = %hash{@a}; # %subhash2 = map {($_, $hash{$_})} @a
%subhash3 = %hash{$s}; # $subhash3{$s} = $hash{$2}
Otherwise, C<%subhash2> and C<%subhash3> would become either empty or simple
copies, depending on the Truth of C<@a> and C<$s>, which would be misleading.
=head2 the function may be evaluated "with" the hash as well as "with" the key
In addition to the default scalar C<$_> being set to each key in the hash being
sliced, the "default hash" C<%_> may also be lexically aliased to the hash
being sliced. In the event that a "with" keyword is adopted which has a
different effect, that effect will occur so that the element selection function
is evaluated for truth for each key, with the hash.
this means that
%subhash = map { $hash{$_} =~ /string/ ? ($_, $hash{$_}) : () } keys %hash;
can be generalized to
%subhash = %hash{ $_{$_} =~ /string/ };
which is more general. All on the same line it looks unnecessary, but if the
filtering function was a big hairy monster and you wanted to create a dozen
subhashes all with different names it would same a lot of copying.
%subhashA = %hashA{ Criteria };
%subhashB = %hashB{ Criteria };
%subhashC = %hashC{ Criteria };
%subhashD = %hashD{ Criteria };
=head2 reasons
When I suggested a syntax extension that used
%hash_name{something in here}
it was roundly misread as a data access. Adoption of this proposal will fill
the curious apparent hole left by the validity of the other slicing operations
in what I hope is a reasonable way.
=head1 IMPLEMENTATION
The rewrites suggested above suffice as a reference implementation;
optimizations may be possible due to disappearance of various intermediate
temporary storages.
a framework for providing visibility of the magic C<$_> and C<%_> inside the
filtering functions (in threaded environments) will be required.
=head1 REFERENCES
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