Higher-Order Perl : Transforming Programs with Programs by Mark-Jason Dominus

Author:Mark-Jason Dominus
Language: eng
Format: epub
Tags: Perl, Programming
ISBN: 1-55860-701-3
Publisher: Morgan Kaufmann
Published: 2005-08-14T16:00:00+00:00

(s|stail).(t | ttail) = s.t | s.ttail |stail.t|stail.ttail

The first of these contains only one string. The middle two are simple transformations of the tails of S and T. The last one is a recursive call to the concat() function itself. So the code is simple:

sub concat { my ($S, $T) = @_; return unless $S && $T; my ($s, $t) = (head($S), head($T)); node("$s$t", promise { union( postcat(tail($S), $t), precat(tail($T), $s), concat(tail($S), tail($T)), ) }); }

precat() and postcat() are simple utility functions that concatenate a string to the beginning or end of every element of a stream:

sub precat { my ($s, $c) = @_; transform {"$c$_[0]"} $s; } sub postcat { my ($s, $c) = @_; transform {"$_[0]$c"} $s; }

An example:

# I’m /ˆ(a|b)(c|d)$/ my $z = concat( union(literal("a"), literal("b")), union(literal("c"), literal("d")), ); show($z); acbc adbd

The behavior of concat() is illustrated in Figure 6.3.

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