NAME

Syntax::Keyword::Match - a match/case syntax for perl

SYNOPSIS

   use v5.14;
   use Syntax::Keyword::Match;

   my $n = ...;

   match($n : ==) {
      case(1) { say "It's one" }
      case(2) { say "It's two" }
      case(3) { say "It's three" }
      default { say "It's something else" }
   }

DESCRIPTION

This module provides a syntax plugin that implements a control-flow block called match/case, which executes at most one of a choice of different blocks depending on the value of its controlling expression.

This is similar to C's switch/case syntax (copied into many other languages), or syntax provided by Switch::Plain.

This is an initial, experimental implementation. Furthermore, it is built as a non-trivial example use-case on top of XS::Parse::Keyword, which is also experimental. No API or compatbility guarantees are made at this time.

KEYWORDS

match

   match( EXPR : OP ) {
      ...
   }

A match statement provides the controlling expression, comparison operator, and sequence of case statements for a match operation. The expression is evaluated to yield a scalar value, which is then compared, using the comparison operator, against each of the case labels in the order they are written, topmost first. If a match is found then the body of the labelled block is executed. If no label matches but a default block is present, that will be executed instead. After a single inner block has been executed, no further tests are performed and execution continues from the statement following the match statement.

The braces following the match block must only contain case or default statements. Arbitrary code is not supported here.

Even though a match statement is a full statement and not an expression, it can still yield a value if it appears as the final statment in its containing sub or do block. For example:

   my $result = do {
      match( $topic : == ) {
         case(1) { ... }
      }
   };

Comparison Operators

The comparison operator must be either eq (to compare cases as strings) or == (to compare them as numbers), or =~ (to compare cases using regexps).

On Perl versions 5.32 onwards, the isa operator is also supported, allowing dispatch based on what type of object the controlling expression gives.

   match( $obj : isa ) {
      case(A::Package)       { ... }
      case(Another::Package) { ... }
   }

Remember that comparisons are made in the order they are written, from the top downwards. Therefore, if you list a derived class as well as a base class, make sure to put the derived class before the base class, or instances of that type will also match the base class case block and the derived one will never match.

   class TheBase {}
   class Derived extends TheBase {}

   match( $obj : isa ) {
      case(TheBase) { ... }
      case(Derived) {
         # This case will never match as the one above will always happen first
      }
   }

case

   case(VAL) { STATEMENTS... }

A case statement must only appear inside the braces of a match. It provides a block of code to run if the controlling expression's value matches the value given in the case statement, according to the comparison operator.

If the value is a non-constant expression, such as a variable or function call, it will be evaluated as part of performing the comparison every time the match statement is executed. For best performance it is advised to extract values that won't need computing again into a variable that can be calculated just once at program startup; for example:

   my $condition = a_function("with", "arguments");

   match( $var : eq ) {
      case($condition) { ... }
      ...
   }

default

A default statement must only appear inside the braces of a match. If present, it must be the final choice, and there must only be one of them. It provides a block of code to run if the controlling expression's value did not match any of the given case labels.

TODO

This is clearly an early experimental work. There are many features to add, and design decisions to make. Rather than attempt to list them all here it would be best to check the RT bug queue at

https://rt.cpan.org/Dist/Display.html?Name=Syntax-Keyword-Match

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

cpanm Syntax::Keyword::Match

perl -MCPAN -e shell
install Syntax::Keyword::Match