calc with parsec

calc.hs

import           Text.Parsec
import           Text.Parsec.Expr
import           Text.Parsec.Language (emptyDef)
import           Text.Parsec.String   (Parser)
import qualified Text.Parsec.Token    as P

lexer = P.makeTokenParser emptyDef
parens = P.parens lexer

number :: Parser Double
number = do res <- P.naturalOrFloat lexer
            case res of
                Left n -> return $ fromInteger n
                Right f -> return f

table = [[op "*" (*) AssocLeft, op "/" (/) AssocLeft]
        ,[op "+" (+) AssocLeft, op "-" (-) AssocLeft]]
      where op s f assoc = Infix (do string s; return f) assoc

expr :: Parser Double
expr = buildExpressionParser table factor
    where factor = parens expr <|> number

eval = parse expr ""

main = interact (unlines . map (show . eval) . lines)