RFC 285 (v2) Lazy Input / Context-sensitive Input

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=head1 TITLE

Lazy Input / Context-sensitive Input

=head1 VERSION

  Maintainer: Adam Turoff <[EMAIL PROTECTED]>
  Date: 24 Sep 2000
  Last Modified: 26 Sep 2000
  Mailing List: [EMAIL PROTECTED]
  Number: 285
  Version: 2
  Status: Frozen

=head1 ABSTRACT

Currently, file read operations work in either scalar context or list
context.  When the left-hand-side of an input operation contains a finite
list of scalars, a finite number of records should be returned, rather than
reading all of the records and losing most of them.

=head1 NOTES

Damian mentioned that this "tricky" bit of magic is trivial with his
proposed context sensitive want() function.

=head1 DESCRIPTION

Tom Christiansen proposed this in his perl6storm message:

        =item perl6storm #0000

        This:

                ($a,$b) = <FH>;  

        should not drain whole ahndle on known LHS count, to rescue

                my($x) = <FH>;

As Tom points out, this is an accidental misfeature, caused by enabling

        @lines = <FH>;

to be sensible.

=head1 IMPLEMENTATION

Not so tricky, if RFC 21 is implemented.  

In order for I/O operations (or any function) to emit the proper
number of return values in each of the following cases:

        $x = foo();

        ($x) = foo();

        ($x, $y) = foo();

        ($x, $y, ...) = foo();

        @values = foo();

The function in question needs to return values in a manner similar
to this RFC 21-inspired bit of pseudo code:

        if (want "SCALAR") {
                return next_singleton();
        } elsif ($count = want "COUNT") {
                my @list;
                push(@list, next_singleton()) for (1..$count);
                return @list;
        } elsif(want "LIST") {
                my @list;
                my $singleton;
                while (defined($singleton = next_singleton())) {
                        push(@list, $singleton);
                }
                return (@list)
        }

        ... ## More contexts to follow.  These three cover intelligent I/O.

=head1 REFERENCES

RFC 21: Subroutines: Replace C<wantarray> with a generic C<want> function

perl6storm