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
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