logical.hs

import Text.Parsec

data Exp = And Exp Exp | Or Exp Exp | Not Exp | T | F deriving Show

expr :: Parsec String st Exp
expr = do
  t <- term
  Or t <$> (string "or" *> expr) <|> pure t

term :: Parsec String st Exp
term = do
  b <- inverse
  And b <$> (string "and" *> term) <|> pure b

inverse :: Parsec String st Exp
inverse = spaces *> (Not <$> (string "not" *> spaces *> bool) <|> bool)

bool :: Parsec String st Exp
bool = toBool <$> (oneOf ['t', 'f'] <* spaces)
  where
    toBool 't' = T
    toBool 'f' = F
  
parseE :: String -> Either ParseError Exp
parseE = parse expr "* ParseError *" 

calc :: String -> Either ParseError Bool
calc s = _calc <$> parseE s
  where
    _calc :: Exp -> Bool
    _calc (And a b) = (_calc a) && (_calc b)
    _calc (Or a b)  = (_calc a) || (_calc b)
    _calc (Not a)   = not (_calc a)
    _calc T         = True
    _calc F         = False