[Haskell-cafe] Expression parsing problem

[leledumbo]

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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