=begin pod

=head1 NAME

Raku port of Perl's defined() / undef() built-ins

=head1 SYNOPSIS

=begin code :lang<raku>
use P5defined;

my $foo = 42;
given $foo {
    say defined();  # True
}

say defined($foo);  # True

$foo = undef();
undef($foo);
=end code

=head1 DESCRIPTION

This module tries to mimic the behaviour of Perl's C<defined> and C<undef>
built-ins as closely as possible in the Raku Programming Language.

=head1 ORIGINAL PERL 5 DOCUMENTATION

=begin output
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other
        than the undefined value "undef". If EXPR is not present, $_ is
        checked.

        Many operations return "undef" to indicate failure, end of file,
        system error, uninitialized variable, and other exceptional
        conditions. This function allows you to distinguish "undef" from
        other values. (A simple Boolean test will not distinguish among
        "undef", zero, the empty string, and "0", which are all equally
        false.) Note that since "undef" is a valid scalar, its presence
        doesn't necessarily indicate an exceptional condition: "pop"
        returns "undef" when its argument is an empty array, or when the
        element to return happens to be "undef".

        You may also use "defined(&func)" to check whether subroutine
        &func has ever been defined. The return value is unaffected by any
        forward declarations of &func. A subroutine that is not defined
        may still be callable: its package may have an "AUTOLOAD" method
        that makes it spring into existence the first time that it is
        called; see perlsub.

        Use of "defined" on aggregates (hashes and arrays) is deprecated.
        It used to report whether memory for that aggregate had ever been
        allocated. This behavior may disappear in future versions of Perl.
        You should instead use a simple test for size:

            if (@an_array) { print "has array elements\n" }
            if (%a_hash)   { print "has hash members\n"   }

        When used on a hash element, it tells you whether the value is
        defined, not whether the key exists in the hash. Use "exists" for
        the latter purpose.

        Examples:

            print if defined $switch{D};
            print "$val\n" while defined($val = pop(@ary));
            die "Can't readlink $sym: $!"
                unless defined($value = readlink $sym);
            sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
            $debugging = 0 unless defined $debugging;

        Note: Many folks tend to overuse "defined" and are then surprised
        to discover that the number 0 and "" (the zero-length string) are,
        in fact, defined values. For example, if you say

            "ab" =~ /a(.*)b/;

        The pattern match succeeds and $1 is defined, although it matched
        "nothing". It didn't really fail to match anything. Rather, it
        matched something that happened to be zero characters long. This
        is all very above-board and honest. When a function returns an
        undefined value, it's an admission that it couldn't give you an
        honest answer. So you should use "defined" only when questioning
        the integrity of what you're trying to do. At other times, a
        simple comparison to 0 or "" is what you want.

        See also "undef", "exists", "ref".

undef EXPR
undef   Undefines the value of EXPR, which must be an lvalue. Use only on
        a scalar value, an array (using "@"), a hash (using "%"), a
        subroutine (using "&"), or a typeglob (using "*"). Saying "undef
        $hash{$key}" will probably not do what you expect on most
        predefined variables or DBM list values, so don't do that; see
        "delete". Always returns the undefined value. You can omit the
        EXPR, in which case nothing is undefined, but you still get an
        undefined value that you could, for instance, return from a
        subroutine, assign to a variable, or pass as a parameter.
        Examples:

            undef $foo;
            undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
            undef @ary;
            undef %hash;
            undef &mysub;
            undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
            return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
            select undef, undef, undef, 0.25;
            ($a, $b, undef, $c) = &foo;       # Ignore third value returned

        Note that this is a unary operator, not a list operator.
=end output

=head1 PORTING CAVEATS

=head2 Parentheses

Because of some overzealous checks for Perl 5isms, it is necessary to put
parentheses when using C<undef> as a value. This is possible to use the
C<isms> pragma:

=begin code :lang<raku>
use isms <Perl5>;
say undef;    # Nil
=end code

=head2 $_ no longer accessible from caller's scope

With the arrival of RakuAST, it has become impossible to access the
topic variable (C<$_>) of the caller's scope.  So Perl's idiom of
calling C<defined> without arguments has become impossible.  Code
such as:

=begin code :lang<raku>
say defined;
=end code

should be changed to either:

=begin code :lang<raku>
say defined($_);
=end code

or, using the subroutine as a method syntax, with the prefix C<.> shortcut
to use that scope's C<$_> as the invocant:

=begin code :lang<raku>
say .defined;
=end code

=head1 AUTHOR

Elizabeth Mattijsen <liz@raku.rocks>

Source can be located at: https://github.com/lizmat/P5defined . Comments and
Pull Requests are welcome.

If you like this module, or what I’m doing more generally, committing to a
L<small sponsorship|https://github.com/sponsors/lizmat/>  would mean a great
deal to me!

=head1 COPYRIGHT AND LICENSE

Copyright 2018, 2019, 2020, 2021, 2023, 2026 Elizabeth Mattijsen

Re-imagined from Perl as part of the CPAN Butterfly Plan.

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.

=end pod

# vim: expandtab shiftwidth=4
