Re: Left-associative binary operators in Parse::RecDescent

[jryan]

Precedence discussions aside (it didn't seem like you need precedence levels here), your problem is that you're only trying to match two things in a binary_op. Binary ops are more like "chains of one or more expressions of equal precedence", and since you only seem to have one level of precedence here, we can just do a loop:

binary_op     : '(' subexpression (op subexpression {[\@item[1..2]]} )
+(s?) ')'
                { [  \$item[2], map { \@\$_ } \@{\$item[3]} ] }

If you want to be able to have an alternative that doesn't need parens, just mirror the same rule without parens. Let me know if you still have trouble...

P.S.: When writing PRD grammars, its useful to use a non-interpolating heredoc (e.g. my $grammar = << '__ENDG__') so you don't have to backslash anything. :)

binary_op     : '(' subexpression (op subexpression {[@item[1..2]]})(s
+?) ')'
                { [ $item[2], map { @$_ } @{$item[3]} ] }

[samtregar]

Thanks for the help! I've tried just dropping the '(' and ')' but then I get errors from PRD about the grammar being left-associative.

-sam

[jryan]

Yeah, that's a good point. You'll need to get rid of the left-recursion. To do that, you'd need to get rid of binary_op and subexpression in your subexpression rule. Next, you'll need to factor out all of the paren stuff into a single rule, and then use that rule within the binary_op rule instead of subexpression. Here's an example:

paren:    '(' binary_op ')' { \$item [2] } # parens belong here, and o
+nly here!
         | subexpression
        
subexpression: function_call
              | var
              | literal
              | <error>

binary_op     : paren (op paren { [ \@item[1..2] ] })(s?)       # any 
+parenned expression will sink down here
                { [  \$item[1], map { \@\$_ } \@{\$item[2]} ] }

[demerphq]

I wrote an article on eliminating left recursion in P::RD a while back: Eliminating Left Recursion in Parse::RecDescent

BTW, got a copy of your book recently. Nice one. ++ to you for sure. :-)