This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE Control flow: Builtin switch statement =head1 VERSION Maintainer: Damian Conway <[EMAIL PROTECTED]> Date: 4 Aug 2000 Last Modified: 18 Sep 2000 Mailing List: [EMAIL PROTECTED] Number: 22 Version: 2 Status: Frozen =head1 ABSTRACT This RFC proposes a syntax and semantics for an explicit case mechanism for Perl. The syntax is minimal, introducing only the keywords C<switch> and C<case> and conforming to the general pattern of existing Perl control structures. The semantics are particularly rich, allowing any one (or more) of nearly 30 forms of matching to be used when comparing a switch value with its various cases. =head1 BACKGROUND In seeking to devise a "Swiss Army" case mechanism suitable for Perl, it is useful to generalize the switch/case notion of distributed conditional testing as far as possible. Specifically, the concept of "matching" between the switch value and the various case values need not be restricted to numeric (or string or referential) equality, as it is in other languages. Indeed, as Table 1 illustrates, Perl offers at least seventeen different ways in which two values could generate a match. =head2 Table 1: Matching a switch value (C<$s>) with a case value (<$c>) Switch Case Type of Match Implied Matching Code Value Value ====== ===== ===================== ============= number I<same> numeric or referential C<match if $s == $c;> or ref equality object string result of method call <match if $s->$c();> ref or sub ref other other string equality C<match if $s eq $c;> non-ref non-ref scalar scalar string regexp pattern match C<match if $s =~ /$c/;> array scalar array entry existence C<match if 0E<lt>=$c && $cE<lt>@$s;> ref array entry definition C<match if defined $s-E<gt>[$c];> array entry truth C<match if $s-E<gt>[$c];> array array array intersection C<match if intersects(@$s, @$c);> ref ref (apply this table to all pairs of elements C<$s-E<gt>[$i]> and C<$c-E<gt>[$j]>) array regexp array grep C<match if grep /$c/, @$s;> ref hash scalar hash entry existence C<match if exists $s-E<gt>{$c};> ref hash entry definition C<match if defined $s-E<gt>{$c};> hash entry truth C<match if $s-E<gt>{$c};> hash regexp hash grep C<match if grep /$c/, keys %$s;> ref sub scalar return value definition C<match if defined $s-E<gt>($c);> ref return value truth C<match if $s-E<gt>($c);> sub array return value definition C<match if defined $s-E<gt>(@$c);> ref ref return value truth C<match if $s-E<gt>(@$c);> In reality, Table 1 covers 31 alternatives, because only the equality and intersection tests are commutative; in all other cases, the roles of the C<$s> and C<$c> variables could be reversed to produce a different test. For example, instead of testing a single hash for the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), one could test for the existence of a single key in a series of hashes (C<match if exists $c-E<gt>{$s}>). As L<perltodo> observes, a Perl case mechanism must support all these "ways to do it". =head2 DESCRIPTION Adding a switch statement would require two new control structures: C<switch> and C<case>. The C<switch> statement takes a single parenthesized argument (the "switch value") of any scalar type, including any reference type. A C</.../> or C<m{...}> pattern may also be specified and is automagically converted to a C<qr/.../>. A hash or array may also be specified and is automatically enreferenced. Regardless of the type of argument, its value is stored as the current switch value in an inaccessible localized control variable. The switch value is followed by a block, which may contain any statements, including zero or more C<case> statements. The C<case> statement takes a single scalar argument (which need be parenthesized only if it's a variable), followed by a block. The C<case> statement selects the appropriate type of matching between that argument and the current switch value, as determined by the respective types of the switch value and the C<case> argument (see Table 1). If the match is successful, the block associated with the C<case> statement is executed. In most other respects, the C<case> statement is semantically identical to an C<if> statement. For example, a C<case> statament can be followed by an C<else> clause, and can be used as a postfix statement qualifier. However, once the block of any C<case> statement finishes executing, control is immediately transferred to the end of the enclosing C<switch> statement. The same effect may be obtained by executing a C<last> within the block of the C<switch>. This means that, normally, there is no C-like "fall-through" from a Perl C<case> statement. For example: my $val = 7; switch ($val) { case [0..9] { print "single digit\n" } case [10..] { print "digits\n"; last } case 7 { print "lucky seven!\n" } } would never print "lucky seven", since the first C<case> will always succeed and transfer control out of the C<switch>. However it I<is> possible to cause a C<case> statement to "fall-through". If a C<next> is executed within a C<case> block, control is immediately transferred to the statement following the C<case> block. For example: switch ($count) { case 0 { print "nothing\n"; next } print "fell through!\n"; case /^\d$/ { print "single\n"; next } print "fell through again!\n"; case sub{$_[0]%2} { print "odd\n" } else { print "even\n" } } =head2 Larger examples %special = ( woohoo => 1, d'oh => 1 ); while (<>) { switch ($_) { case (%special) { print "homer\n"; last } # if $special{$_} case /a-z/i { print "alpha\n"; last } # if $_ =~ /a-z/i case [1..9] { print "small num\n"; last } # if $_ in [1..9] case sub { $_[0] >= 10 } { # if $_ >= 10 my $age = <>; switch (sub { $_[0] < $age } ) { case 20 { print "teens\n"; next } case 30 { print "twenties\n"; next } else { print "history\n"; next } } } } print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ } The use of intersection tests against an array reference is particularly useful for aggregating integral cases: sub classify_digit { switch ($_[0]) { case 0 { return 'zero' } case [2,4,6,8] { return 'even' } case [1,3,4,7,9] { return 'odd' } case /[A-F]/i { return 'hex' } } } =head2 C<switch> as an rvalue It is also proposed that a C<switch> statement in a non-void context might return the value of the last evaluated statement in the last C<case> or C<else> it executes. For example: my $descr = switch ($num) { case 0 { 'zero' } case [2,4,6,8] { 'even' } case [1,3,4,7,9] { 'odd' } case /[A-F]/i { 'hex' } } my $index = switch ($index) { case ^_ < 0 { $length + $index } case [0..9] { $index } else { 9 } } my %args = switch (@_) { case 0 { %defaults } case ^_%2 { warn "Usage: foo(%args)"; (%args, undef) } else { %args } } =head1 IMPLEMENTATION A sample (pure Perl) implementation is available on the CPAN as Switch.pm. There is probably also good opportunity for compile-time optimization for constant (or inferrable) switch/case values. =head1 REFERENCES RFC 23 : Higher order functions RFC 24 : Semi-finite (lazy) lists perltodo perlsyn perlfaq7 Christiansen, T., Message posted to comp.lang.perl.misc, September 1998, archived at http://language.perl.com/misc/fmswitch Torkington, N., Message posted to perl5-porters mailing list, February 1997, archived at http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/1997-02/msg01558.html Barr, G., Message posted to perl5-porters mailing list, October 1997, archived at http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/1997-10/msg00057.html
