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