calculator-interpreter.hs

import Text.Parsec
import Text.Parsec.String
import Control.Applicative hiding (many, (<|>))

data Calc = Number Float
          | OpPlus Calc Calc
          | OpMult Calc Calc
          | OpSubt Calc Calc
          | OpDivd Calc Calc
          
instance Show Calc where
  show (Number a)    = show a
  show (OpPlus c c') = "(" ++ show c ++ " + " ++ show c' ++ ")"
  show (OpSubt c c') = "(" ++ show c ++ " - " ++ show c' ++ ")"
  show (OpMult c c') = "(" ++ show c ++ " * " ++ show c' ++ ")"
  show (OpDivd c c') = "(" ++ show c ++ " / " ++ show c' ++ ")"

-- INTERPRETING
intr :: String -> Either ParseError Float
intr str = eval <$> parse calc "(unknown)" str 
  
-- EVALUATING
eval :: Calc -> Float
eval (Number a)    = a
eval (OpPlus c c') = (eval c) + (eval c')
eval (OpSubt c c') = (eval c) - (eval c')
eval (OpMult c c') = (eval c) * (eval c')
eval (OpDivd c c') = (eval c) / (eval c') -- divide by zero doesn't error - that's 'inifinity'

-- PARSING
-- <expression> ::= <term> <plussubt>     | <term>
-- <plussubt>   ::= + <expression>        | - <expression>
-- <term>       ::= <factor> <multdivd>   | <factor>
-- <multdivd>   ::= * <factor>            | / <factor>
-- <factor>     ::= "(" <expression> ")"  | number

-- Parsers
calc = expression

expression = term   `chainl1` plusSubt <||> term
term       = factor `chainl1` multDivd <||> factor
factor     = parens expression <||> number

multDivd = withSpaces mult <||> withSpaces divd
plusSubt = withSpaces plus <||> withSpaces subt

-- minimal parsers
mult = char '*' >> return OpMult
plus = char '+' >> return OpPlus
subt = char '-' >> return OpSubt
divd = char '/' >> return OpDivd

number = Number <$> (read <$> many1 digit)

-- helpers
(<||>) :: Parser a -> Parser a -> Parser a
a <||> b = try a <|> try b

parens :: Parser a -> Parser a
parens = between (char '(') (char ')')

withSpaces :: Parser a -> Parser a
withSpaces = between spaces spaces