expr.hs

import Text.Parsec

-- simple datatype for expressions
data BinOp = Add | Sub | Mul | Div deriving (Show,Eq)

data ArithExpr =
      Compound ArithExpr BinOp ArithExpr
    | Value Int          
    deriving Show


numberParser:: Parsec String st ArithExpr
numberParser = do
  digs <- many1 digit
  let num = read digs
  return $ Value num

operatorParser:: Parsec String st BinOp
operatorParser = do
  op <- (oneOf "+-*/")
  return (selectOp op)
    where selectOp '+' = Add
          selectOp '-' = Sub
          selectOp '*' = Mul
          selectOp '/' = Div

expressionParser:: Parsec String st ArithExpr
expressionParser = (between (char '(') (char ')') binaryExpressionParser) <|>
                   numberParser

binaryExpressionParser:: Parsec String st ArithExpr
binaryExpressionParser = do
  e1 <- expressionParser
  op <- operatorParser
  e2 <- expressionParser
  return (Compound e1 op e2)




-- now parseTest expressionParser "(1+1)"
-- or parse expressionParser "error" "(1+(2*3))"