I'm trying to work out how to use Parse::RecDescent and am a little overwhelmed.

I can see how to use it as a recognizer, but to do a simple syntax-directed translation, all of the *items, $return, $text, real return value, etc, stuff has thrown me off.

Let's say you want a translator for languages over ('a','b','c','d','e') such that any balanced pairs of equal length sequences of a's and b's are replaced by c's and d's. 

my $start = "eeeeaaaabbbeeee";
my $end = translate($start);
# $end eq "eeeeacccdddeeee"  (aaabbb -> cccddd)

[download]

(Side note: This is the "classic example" of something that can't be done with Update: regular grammars, and needs a context-free grammar. This is because within the regexp /(a*)(b*)/ there is no way to assert that length($1) == length($2).)

The syntax-directed translation (in pseudo-code) would be: 

start -> part(s)    { start.t := join ('', part(s).t) }

part -> AnB         { part.t := AnB.t }
part -> 'a'         { part.t := 'a' }
part -> /[^a]+/     { part.t := /[^a]+/.t }

AnB -> 'a' AnB 'b'  { AnB.t := 'c' . AnB.t . 'd' }
AnB -> 'ab'         { AnB.t := 'cd' }

[download]

Any ideas on how this translates into Parse::RecDescent? Is there a more appropriate parsing module to use for cases where the input language is very similiar to the output language?

-Andrew.

In reply to Parse::RecDescent for simple syntax-directed translation by tomazos

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