{-
    Title: Expression Parser Using Recursive Descent
    Author; Robin Cockett
    Date 10 Jan 2006
-}

 module Main where
 import Text.ParserCombinators.Parsec
 import Exlexer

 -- output type from parser
 data Exp =  Add Exp Exp
          |  Mul Exp Exp
          |  Num Integer
          deriving (Show,Eq)

{-  Grammar:
          expr -> term expr1
          expr1 -> ADD expr 
              |
          term -> factor term1
          term1 -> MUL term 
              | 
          factor -> '(' expr ')' 
              | NUM
 -}

-- recursive descent parser beside grammar
-- expr -> term expr1 
 expr input = expr1 (term input)
-- expr1 -> ADD expr 
 expr1 ((_,ADD):input) = expr input
-- expr1 ->
 expr1 input = input
-- term -> factor term1
 term input = term1 (factor input)
-- term1 -> MUL term 
 term1 ((_,MUL):input) = term input
-- term1 ->
 term1 input = input
-- factor -> '(' expr ')' 
 factor ((_,LPAR):input) = case (expr input) of (_,RPAR):rest -> rest 
-- factor -> NUM
 factor ((_,NUM n):input) = input

 -- function for running a parsers
 rn:: GenParser a () b -> [a] ->b
 rn p input
        = case (parse p "" input) of 
            Left err -> error ("parse error at "++(show err))
            Right x -> x

 parse input = expr (rn lexer input)