Left-associative binary operators in Parse::RecDescent

samtregar has asked for the wisdom of the Perl Monks concerning the following question:

Hello all. I'm working on a hairy parsing problem in HTML::Template::Expr and I'm not making much progress. In case you're not familiar, HTML::Template::Expr is an add-on to HTML::Template which adds basic expression support to the templating language. Stuff like:

  <tmpl_if expr="color eq 'blue'">BLUE</tmpl_if>
  <tmpl_var expr="my_func(color, 'foo')">
  <tmpl_if expr="(color eq 'blue') || (color eq 'red')">
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All the above expressions (and a lot more) work just fine. I parse them with a Parse::RecDescent grammar which produces a tree. Executing the tree is a piece of cake.

Now I want to support:

  <tmpl_if expr="(color eq 'blue') || (color eq 'red') || (color eq 'b
+lack')">
[download]

But that won't parse with my grammar. The way I have it setup each || requires exactly two items and must be enclosed in parens unless it's on the outer-most scope. The outer-most scope is special because my code will add an enclosing () if none is present. To do three conditions this is required:

  <tmpl_if expr="((color eq 'blue') || (color eq 'red')) || (color eq 
+'black')">
[download]

I've been hacking on the grammar all morning but nothing's working. I'm sure it's got something to do with leftop but what exactly is beyond me. If anyone could help I'd be greatly appreciative!

Here's the grammar, which you can also find in the latest version of the module:

expression    : subexpression /^\$/  { \$return = \$item[1]; } 

subexpression : binary_op             { \$item[1] }
              | function_call         { \$item[1] }
              | var                   { \$item[1] }
              | literal               { \$item[1] }
              | '(' subexpression ')' { \$item[2] }
              | <error>

binary_op     : '(' subexpression op subexpression ')'
                { [ \$item[3][0], \$item[3][1], \$item[2], \$item[4] ]
+ }

op            : />=?|<=?|!=|==/      { [ ${\BIN_OP},  \$item[1] ] }
              | /le|ge|eq|ne|lt|gt/  { [ ${\BIN_OP},  \$item[1] ] }
              | /\\|\\||or|&&|and/   { [ ${\BIN_OP},  \$item[1] ] }
              | /[-+*\\/\%]/         { [ ${\BIN_OP},  \$item[1] ] }

function_call : function_name '(' args ')'  
                { [ ${\FUNCTION_CALL}, \$item[1], \$item[3] ] }
              | function_name ...'(' subexpression
                { [ ${\FUNCTION_CALL}, \$item[1], [ \$item[3] ] ] }
              | function_name '(' ')'
                { [ ${\FUNCTION_CALL}, \$item[1] ] }

function_name : /[A-Za-z_][A-Za-z0-9_]*/
                { \$item[1] }

args          : <leftop: subexpression ',' subexpression>

var           : /[A-Za-z_][A-Za-z0-9_]*/  { \\\$item[1] }

literal       : /-?\\d*\\.\\d+/           { \$item[1] }
              | /-?\\d+/                  { \$item[1] }
              | <perl_quotelike>          { \$item[1][2] }
[download]

Thanks!
-sam

PS: I'd also be interested in a way to remove the need for parens entirely ("n == 10 || n == 50") which seems like it could be the same problem...

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