I'm writing a paper as a replacement for writing exam and decided to write a simple compiler (got a little experience with it). However, I got trouble in parsing expression. The grammar: expression = "get" | [ "+" | "-" ] term { ( "+" | "-" ) term } term = factor { ( "*" | "/" ) factor } factor = IDENTIFIER | VALUE | "(" expression ")" I can't make term parse, for instance "1 * 2 / 3" (the number is not important, identifier is also accepted). It stops after parsing 2, i.e. only the first multiplication is parsed. Interchanging * and / gives the same result, only differs in operation. Whichever got encountered first will be parsed. The same problem also arises from expression, where it can't parse "1 + 2 - 3". Both problems are identical, but I can't figure out what's wrong (don't count the optional +/- before term in expression, I haven't done it yet). Sorry, but I'm lack of knowledge about Monad. I know it can be done better with it, but I need to learn a lot about it, while I don't have enough time (only 2 weeks). Below are necessary definitions for the parser (some taken from the scanner). For testing purpose, please try: expression [("1",Value),("+",Plus),("2",Value),("-",Minus),("3",Value),("EOF",EOF)] term [("1",Value),("*",Times),("2",Value),("/",Slash),("3",Value),("EOF",EOF)] expression [("1",Value),("-",Minus),("2",Value),("+",Plus),("3",Value),("EOF",EOF)] term [("1",Value),("/",Slash),("2",Value),("*",Times),("3",Value),("EOF",EOF)]
data Token = Identifier | OpenBlock | CloseBlock | SemiColon | Slash | Equals | OpenBrace | CloseBrace | Minus | Times | Plus | Nil | Value | Var | Const | Put | Get | Comma | EOF deriving (Show,Eq)
type Symbol = (String,Token) type ASL = [Symbol]
type ParseFunc = ASL -> (ASL,[String])
expression :: ParseFunc expression (h:s) | snd h == Get = (s,["IN"]) | op `elem` [Plus,Minus] = (s2,r1 ++ r2 ++ [operation op]) | otherwise = (s1,r1) where (s1,r1) = term (h:s) (s2,r2) = term $ tail s1 op = if s1 /= [] then snd $ head s1 else Nil expression s = (s,[])
term :: ParseFunc term s = if op `elem` [Times,Slash] then (s2,r1 ++ r2 ++ [operation op]) else (s1,r1) where (s1,r1) = factor s (s2,r2) = factor $ tail s1 op = if s1 /= [] then snd $ head s1 else Nil
factor :: ParseFunc factor ((id,Identifier):s) = (s,["LOAD " ++ id]) factor ((val,Value):s) = (s,["PUSH " ++ val]) factor (("(",OpenBrace):s) = if head s1 == (")",CloseBrace) then (tail s1,r1) else error $ "\")\" expected, got" ++ (show $ fst $ head s1) where (s1,r1) = expression s factor s = (s,[])
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