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Ideally, you'd want:

expr : bin_op_2 { $item[1] } bin_op_2 : bin_op_2 /[+-]/ bin_op_1 { [ @item[2, 1, 3] ] } | bin_op_1 { $item[1] } bin_op_1 : bin_op_1 /[*\\\/%]/ term { [ @item[2, 1, 3] ] } | term { $item[1] }

but the module doesn't allow left-recursive grammars. Here's how left-recursion is removed:

bin_op_2 : bin_op_1 bin_op_2_(s?) { treeify($item[1], map { @$_ } @{$item[2]}); } bin_op_1 : term bin_op_1_(s?) { treeify($item[1], map { @$_ } @{$item[2]}); } bin_op_2_ : /[+-]/ bin_op_1 { [ $item[1], $item[2] ] } bin_op_1_ : /[*\\\/%]/ term { [ $item[1], $item[2] ] }

Since it returns a list, we need to spend extra effort making it into a tree again. treeify is defined as:

sub treeify { my $t = shift; $t = [ shift, $t, shift ] while @_; return $t; }

Parse::RecDescent provides a shortcut in the form of <leftop>:

# Lowest precedence. bin_op_2 : <leftop: bin_op_1 SUM bin_op_1 > { treeify(@{$item[1]}); } bin_op_1 : <leftop: term PROD term > { treeify(@{$item[1]}); } # Highest precedence. SUM : '+' | '-' PROD : '*' | '/' | '\\' | '%'

Here's the final product (with a couple of operators added for demonstration purposes):

use strict; use warnings; use Data::Dumper (); use Parse::RecDescent (); my $grammar = <<'__EOI__'; { use strict; use warnings; sub treeify { my $t = shift; $t = [ shift, $t, shift ] while @_; return $t; } } parse : expr EOF { $item[1] } expr : assign { $item[1] } # Lowest precedence. assign : IDENT '=' assign { [ $item[2], $item[1], $item[3] ] } | log_or { $item[1] } log_or : <leftop: log_and LOG_OR log_and > {treeify(@{$item[1]})} log_and : <leftop: sum LOG_AND sum > {treeify(@{$item[1]})} sum : <leftop: prod SUM prod > {treeify(@{$item[1]})} prod : <leftop: term PROD term > {treeify(@{$item[1]})} # Highest precedence. term : '(' expr ')' { $item[2] } | NUMBER { [ $item[0], $item[1] ] } # Tokens NUMBER : /\d+/ IDENT : /\w+/ LOG_OR : '||' LOG_AND : '&&' SUM : '+' | '-' PROD : '*' | '/' | '\\' | '%' EOF : /^\Z/ __EOI__ # $::RD_HINT = 1; # $::RD_TRACE = 1; my $parser = Parse::RecDescent->new($grammar) or die("Bad grammar\n"); foreach ( '1*2*3', '1%2*3', '1+2*3', 'a=b=c=1', ) { print(Data::Dumper->Dump([ $parser->parse($_) ], [ $_ ])); print("\n"); }

You could use the following for assignment, but I think it's slower:

sub treeify_r { my $t = pop; $t = [ pop, pop, $t ] while @_; return $t; } assign : <rightop: IDENT ASSIGN assign > { treeify_r(@{$item[1]}); } ASSIGN : '='

Update: Changed
expr : bin_op_5 { $item[1] }
to
expr : assign { $item[1] }

Changed
'\'
to
'\\'


In reply to Re: Order of Precedence in Parse::RecDescent grammar by ikegami
in thread Order of Precedence in Parse::RecDescent grammar by suaveant

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