txt: ## dist_zef-lizmat-P5defined.md
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  # NAME
  
  Raku port of Perl's defined() / undef() built-ins
  
  # SYNOPSIS
  
  ```
  use P5defined;
  
  my $foo = 42;
  given $foo {
      say defined();  # True
  }
  
  say defined($foo);  # True
  
  $foo = undef();
  undef($foo);
  
  ```
  
  # DESCRIPTION
  
  This module tries to mimic the behaviour of Perl's `defined` and `undef` built-ins as closely as possible in the Raku Programming Language.
  
  # ORIGINAL PERL 5 DOCUMENTATION
  
  ```
  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.
  
  ```
  
  # PORTING CAVEATS
  
  ## Parentheses
  
  Because of some overzealous checks for Perl 5isms, it is necessary to put parentheses when using `undef` as a value. Since the 2018.09 Rakudo compiler release, it is possible to use the `isms` pragma:
  
  ```
  use isms <Perl5>;
  say undef;    # Nil
  
  ```
  
  ## $\_ no longer accessible from caller's scope
  
  In future language versions of Raku, it will become impossible to access the `$_` variable of the caller's scope, because it will not have been marked as a dynamic variable. So please consider changing:
  
  ```
  defined;
  
  ```
  
  to either:
  
  ```
  defined($_);
  
  ```
  
  or, using the subroutine as a method syntax, with the prefix `.` shortcut to use that scope's `$_` as the invocant:
  
  ```
  .&defined;
  
  ```
  
  # AUTHOR
  
  Elizabeth Mattijsen [liz@raku.rocks](mailto:liz@raku.rocks)
  
  If you like this module, or what I’m doing more generally, committing to a [small sponsorship](https://github.com/sponsors/lizmat/) would mean a great deal to me!
  
  Source can be located at: <https://github.com/lizmat/P5defined> . Comments and Pull Requests are welcome.
  
  # COPYRIGHT AND LICENSE
  
  Copyright 2018, 2019, 2020, 2021, 2023 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.
  