module ParserExpr2 where

import Parser 
import Char


data Expr   = Plus  Expr Expr
            | Minus Expr Expr
            | Times Expr Expr
            | Div   Expr Expr
            | Number Int
            | Var    String
            deriving (Eq, Show)      

pExpr :: Parse Char Expr
pExpr = pTerm >* token '+' >*> pExpr `use` uncurry Plus
  `alt` pTerm >* token '-' >*> pExpr `use` uncurry Minus
  `alt` pTerm 

pTerm :: Parse Char Expr
pTerm = pFactor >* token '*' >*> pTerm `use` uncurry Times
  `alt` pFactor >* token '/' >*> pTerm `use` uncurry Div         
  `alt` pFactor

pFactor :: Parse Char Expr
pFactor = some (spot isDigit) `use` Number. read
    `alt` some (spot isAlpha) `use` Var
    `alt` token '(' *> pExpr >* token ')'

parse :: String-> Expr
parse i = 
  case filter (null. snd) 
         (pExpr (filter (not.isSpace) i)) of 
     [] -> error "Input does not parse." 
     [(e, _)] -> e
     _  -> error "Input is ambiguous."