type Function = (String, [String], Expression)
type Functions = [Function]

data Interpreter = Interpreter
data Operator = Plus | Minus | Times | Div | Mod
data Expression = Constant Double |
                  Variable String |
                  Operation Expression Operator Expression |
                  Assignment String Expression |
                  Funcall String [Expression]

evalExpression :: Expression -> Variables -> Functions -> Result
evalExpression (Constant d) _ _ = Just d
evalExpression (Variable s) vars _ = snd <$> find ((==s) . fst) vars
evalExpression (Assignment _ expr) vars funs = evalExpression expr vars funs
evalExpression (Operation lhs Plus rhs) vars funs = (+) <$> evalExpression lhs vars funs <*> evalExpression rhs vars funs
evalExpression (Operation lhs Minus rhs) vars funs = (-) <$> evalExpression lhs vars funs <*> evalExpression rhs vars funs
evalExpression (Operation lhs Times rhs) vars funs = (*) <$> evalExpression lhs vars funs <*> evalExpression rhs vars funs
evalExpression (Operation lhs Div rhs) vars funs = (/) <$> evalExpression lhs vars funs <*> evalExpression rhs vars funs

evalExpression (Operation lhs Mod rhs) vars funs = fmap fromIntegral $ mod <$> intLhs <*> intRhs
  where intLhs = fmap floor (evalExpression lhs vars funs)
        intRhs = fmap floor (evalExpression rhs vars funs)

evalExpression (Funcall funName paramExprs) vars funs = funcResult
  where evaluatedParams = map (\expr -> evalExpression expr vars funs) paramExprs
        matchingFunction = find ((==funName) . (^._1)) funs
        okToCall (Just (_, fparams, _)) = length fparams == length paramExprs && notElem Nothing evaluatedParams
        okToCall (Nothing) = False
        funcResult = if okToCall matchingFunction
                     then evalExpression (fromJust matchingFunction^._3) (zip (fromJust matchingFunction^._2) (map fromJust evaluatedParams)) funs
                     else Nothing