On Thu, Apr 05, 2007 at 03:19:15PM +0100, Joel Reymont wrote:
numExpr :: GenParser Char a NumExpr numExpr = choice [ integer >>= return . Int , float >>= return . Num ]
Parsec's choice operator works by parsing the first, and only parsing the second if the first fails immediately. So, given the input "123.456": - Parsec parses 'integer >>= return . Int' - this is successful - numExpr returns (Int 123, ".456") - we try to match . against ) and fail. The fix is to left-factor the grammar, or just use the existing factored choice operator:
numExpr :: GenParser Char a NumExpr numExpr = do sg <- lexeme sign nf <- natOrFloat return $ either (Int . sg) (Nat . sg) nf
It seems silly that there is no signed version of natOrFloat predefined, any Parsec experts? Stefan