Re: To make the Model WORK

but I can't see where it's going south and returning an array (or the string expression of an array). In other similar cases (outside the ifthenelse-decoder) sub-expression results are returned properly.

ifthenelse: if elsif(s?) else(?) endif
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Since there may be more than one elsif, and else may be optional, these corresponding subexpressions return arrays (see Subrules from P::RD's docs). For example, in this production, $item[2] is an array of the semantic values of all the elsif nonterminals that were found at this point.

Two points:

Use Data::Dumper instead of print statements to see what things are.
I find the way you've written your grammar slightly unintuitive (although I admit I'm not really sure what you're trying to accomplish in your semantic actions). I would write it more like this:

use Parse::RecDescent;
use Data::Dumper;

my $parser = Parse::RecDescent->new(<<'END_GRAMMAR');
    expr:
        /\d+/      { $item[1] }
      | ifthenelse { $item[1] }
  
    ifthenelse:
        "if" expr "then" expr elsif(s?) else(?) "endif"
           { [ if => [@item[2,4]], @{$item[5]},
                     ref $item[6] ? $item[6][0] : undef ] }

    elsif:
        "elsif" expr "then" expr { [ @item[2,4] ] }

    else:
        "else" expr { $item[2] }
END_GRAMMAR

for (<DATA>) {
   print $_;
   print Dumper $parser->expr($_);
}

__DATA__
if 1 then 2 else 3 endif
if 1 then 2 endif
if 1 then 2 elsif 3 then 4 elsif 5 then 6 endif
if 1 then 2 elsif 3 then 4 else 5 endif
[download]

This returns an array that is structured like this:

[ "if", [ cond1, result1 ],   # first "if" condition/result expression
+ pair
        [ cond2, result2 ],   # 0 or more "elsif" expression pairs
        ...
        otherwise ]           # else branch expression, or undef if no
+ne
[download]

This data structure is fairly uniform, and should be easy to evaluate for whatever it is you're doing.

blokhead

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Re^2: To make the Model WORK by ikegami (Patriarch) on Jul 13, 2005 at 16:48 UTC
The function evaluating the parse tree can be simplified if you eliminate `elsif` at compile time. Keep in mind that `if cond1 then action1 elsif cond2 then action2 elsif cond3 then action3 else action4 endif` [download] is equivalent to `if cond1 then action1 else if cond2 then action2 else if cond3 then action3 else action4 endif endif endif` [download] except the latter is easier to evaluate because the parse tree is simply `[ if => cond1, action1, [ if => cond2, action2, [ if => cond3, action3, action4 ] ] ]` [download] See my reply below (under the heading "The following returns a nice tree for expr to evaluate") for an implementation.	[reply] [d/l] [select]
Re^2: To make the Model WORK by samizdat (Vicar) on Jul 13, 2005 at 16:18 UTC
Ahhh... thanks, blokhead. Thanks, also, for your comments. Of necessity, I've compressed my example way down (expr has 16 alternate forms, for example), but gems of wisdom are always worth my time to consider.	[reply]