module Expr2 where

import Parser 
import Char

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

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

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

pFactor :: Parse Char Expr
pFactor = 
     some (spot isDigit) `build` Number . read
     `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 "Ambiguous input."