perl-Switch

A switch statement for Perl, do not use if you can use given/when

The Switch.pm module implements a generalized case mechanism that covers most (but not all) of the numerous possible combinations of switch and case values described above. The module augments the standard Perl syntax with two new control statements: 'switch' and 'case'. The 'switch' statement takes a single scalar argument of any type, specified in parentheses. 'switch' stores this value as the current switch value in a (localized) control variable. The value is followed by a block which may contain one or more Perl statements (including the 'case' statement described below). The block is unconditionally executed once the switch value has been cached. A 'case' statement takes a single scalar argument (in mandatory parentheses if it's a variable; otherwise the parens are optional) and selects the appropriate type of matching between that argument and the current switch value. The type of matching used is determined by the respective types of the switch value and the 'case' argument, as specified in Table 1. If the match is successful, the mandatory block associated with the 'case' statement is executed. In most other respects, the 'case' statement is semantically identical to an 'if' statement. For example, it can be followed by an 'else' clause, and can be used as a postfix statement qualifier. However, when a 'case' block has been executed control is automatically transferred to the statement after the immediately enclosing 'switch' block, rather than to the next statement within the block. In other words, the success of any 'case' statement prevents other cases in the same scope from executing. But see the "Allowing fall-through" manpage below. Together these two new statements provide a fully generalized case mechanism: use Switch; %special = ( woohoo => 1, d'oh => 1 ); while (<>) { chomp; switch ($_) { case (%special) { print "homer\n"; } # if $special{$_} case /[a-z]/i { print "alpha\n"; } # if $_ =~ /a-z/i case [1..9] { print "small num\n"; } # if $_ in [1..9] case { $_[0] >= 10 } { print "big num\n"; } # if $_ >= 10 print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ } } Note that 'switch'es can be nested within 'case' (or any other) blocks, and a series of 'case' statements can try different types of matches -- hash membership, pattern match, array intersection, simple equality, etc. -- against the same switch value. The use of intersection tests against an array reference is particularly useful for aggregating integral cases: sub classify_digit { switch ($_[0]) { case 0 { return 'zero' } case [2,4,6,8] { return 'even' } case [1,3,5,7,9] { return 'odd' } case /[A-F]/i { return 'hex' } } }

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