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Syntax::Operator::In(3pm) User Contributed Perl Documentation Syntax::Operator::In(3pm)

NAME

"Syntax::Operator::In" - infix element-of-list meta-operator

SYNOPSIS

On a suitably-patched perl:

   use Syntax::Operator::In;
   if($x in:eq @some_strings) {
      say "x is one of the given strings";
   }

DESCRIPTION

This module provides an infix meta-operator that implements a element-of-list test on either strings or numbers.

Current versions of perl do not directly support custom infix operators. The documentation of XS::Parse::Infix describes the situation, with reference to a branch experimenting with this new feature. This module is therefore almost entirely useless on standard perl builds. While the regular parser does not support custom infix operators, they are supported via "XS::Parse::Infix" and hence XS::Parse::Keyword, and so custom keywords which attempt to parse operator syntax may be able to use it.

For operators that already specialize on string or numerical equality, see instead Syntax::Operator::Elem.

OPERATORS

in

   my $present = $lhs in:OP @rhs;
   my $present = $lhs in<OP> @rhs;

Yields true if the value on the lefhand side is equal to any of the values in the list on the right, according to some equality test operator "OP".

This test operator must be either "eq" for string match, or "==" for number match, or any other custom infix operator that is registered in the "XPI_CLS_EQUALITY" classification.

There are currently two accepted forms of the syntax for this operator, using either a prefix colon or a circumfix pair of angle-brackets. They are entirely identical in semantics, differing only in the surface-level syntax to notate them. This is because I'm still entirely undecided on which notation is better in terms of readable neatness, flexibility, parsing ambiguity and so on. This is somewhat of an experiment to see which will eventually win.

TODO

  • Improve runtime performance of compiletime-constant sets of strings, by detecting when the RHS contains string constants and convert it into a hash lookup.
  • Consider cross-module integration with Syntax::Keyword::Match, permitting 

       match($val : elem) {
      case(@arr_of_strings) { ... }
   }

    Or perhaps this would be too weird, and maybe "match/case" should have an "any-of" list/array matching ability itself. See also <https://rt.cpan.org/Ticket/Display.html?id=143482>.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

2023-05-25 perl v5.36.0