[Haskell-cafe] Parsec : Problems with operator precedence

[Daniel Fischer]

Am Montag, 29. Dezember 2008 10:27 schrieb Erik de Castro Lopo:
> Hi all,
>
> I'm using Text.ParserCombinators.Parsec.Expr to parse expressions for
> a Javascript like language. This language has C-like logical operators
> ('&&' and '||') and bitwise operators ('&' and '|'). Furthermore, the
> language definition states that bitwise operators have a higher precedence
> than the logical ones.
>
> I therefore have the following (trimmed):
>
>     import qualified Text.ParserCombinators.Parsec.Expr as E
>
>     opTable :: [[ E.Operator Char st Expression ]]
>     opTable = [
>         -- Operators listed from highest precedence to  lowest precedence.
>
>         {- snip, snip -}
>
>         [    binaryOp "&" BinOpBinAnd E.AssocLeft ],
>         [    binaryOp "^"  BinOpBinXor E.AssocLeft ],
>         [    binaryOp "|"  BinOpBinOr E.AssocLeft ],
>
>         [    binaryOp "&&" BinOpLogAnd E.AssocLeft ],
>         [    binaryOp "||" BinOpLogOr E.AssocLeft ]
>         ]
>
>     binaryOp :: String -> (SourcePos -> a -> a -> a) -> E.Assoc ->
> E.Operator Char st a binaryOp name con assoc =
>         E.Infix (reservedOp name >>
>             getPosition >>=
>             return . con) assoc
>
> but I still get the following parse error:
>
>     unexpected "|"
>     expecting end of "|" or term
>
> on the line:
>
>     if (name == null || value == null)
>
> If I change the above from a logical to a bitwise OR, the parser
> accepts it quite happily.

The problem is that "|" is a prefix of "||" and it gets the first bite. So 
when the parser gets to "||" it first tries to parse a bitwise or. That 
succeeds. Then the parser is looking for an operand, but it finds the second 
- unexpected - "|". I don't remember how Parsec's expression parsers work, 
maybe you can add a "try" some parser(s) to make it work.

>
> Any clues as to what I'm doing wrong here?
>
> Cheers,
> Erik