对于项目,我们被赋予编写命令式语言并通过Haskell执行的任务。解析器(这里省略)和部分评估完成。现在只剩下编码效果了。操纵一个小机器人。执行IO操作,但返回其他类型
考虑下面的代码:
data Env = Env [Binding]
instance Show Env where
show (Env (x:xs)) = show x ++ ", " ++ show (Env xs)
show (Env []) = ""
data Binding = Binding (String,Int)
instance Show Binding where
show (Binding x) = fst x ++ " : " ++ show (snd x)
lookup' :: String -> Env -> Int
lookup' zoek (Env env) = case elemIndex zoek [fst x | Binding x <- env] of
Just y -> y
Nothing -> error "Not found"
eval :: Stmt -> Env -> Env
eval (Seq s) env = foldl (flip eval) env s
eval (Assign varName aexpr) env = evalAssign varName aexpr env
eval (If bool stmt1 stmt2) env = evalIf bool stmt1 stmt2 env
eval (While bool stmt) env = undefined
eval (MotorInstruct string aExpr) env = undefined
eval (SensorRead string) env = undefined
eval Skip env = env
evalAExpr :: AExpr -> Env -> Int
evalAExpr (IntConst int) _ = fromInteger int
evalAExpr (Neg a) env = - evalAExpr a env
evalAExpr (ABinary Add a b) env = evalAExpr a env + evalAExpr b env
evalAExpr (ABinary Subtract a b) env = evalAExpr a env - evalAExpr b env
evalAExpr (ABinary Multiply a b) env = evalAExpr a env * evalAExpr b env
evalAExpr (ABinary Divide a b) env = evalAExpr a env `div` evalAExpr b env
evalAExpr (Var x) env = getElementAtEnv env (lookup' x env)
where
getElementAtEnv (Env env) index = getSndFromBinding (env !! index)
getSndFromBinding (Binding (_,t)) = t
evalBExpr :: BExpr -> Env -> Bool
evalBExpr (BoolConst bool) _ = bool
evalBExpr (Not expr) env = not $ evalBExpr expr env
-- Boolean operators
evalBExpr (BBinary And a b) env = evalBExpr a env && evalBExpr b env
evalBExpr (BBinary Or a b) env = evalBExpr a env || evalBExpr b env
-- Relational operators
evalBExpr (RBinary Greater a b) env = evalAExpr a env > evalAExpr b env
evalBExpr (RBinary Less a b) env = evalAExpr a env < evalAExpr b env
evalBExpr (RBinary Equal a b) env = evalAExpr a env == evalAExpr b env
evalIf :: BExpr -> Stmt -> Stmt -> Env -> Env
evalIf expr s1 s2 env = if evalBExpr expr env
then
eval s1 env
else
eval s2 env
evalAssign :: String -> AExpr -> Env -> Env
evalAssign term s (Env env)= if term `elem` transform
then
Env (take (lookup' term (Env env)) env ++ [Binding (term, evalAExpr s (Env env))]++ drop (lookup' term (Env env) + 1) env)
else
Env (env ++ [Binding (term, evalAExpr s (Env env))])
where transform = [ fst ele | Binding ele <- env]
zoekMotor :: String -> Int
zoekMotor "left" = 0x9
zoekMotor "right" = 0xa
zoekMotor _ = error "No such motor"
sendToMotor :: String -> Int -> IO()
sendToMotor m s = do
bot <- openMBot
sendCommand bot $ setMotor (zoekMotor m) s s
closeMBot bot
evalMotorInstruct :: String -> AExpr -> Env -> Env
evalMotorInstruct welke waarde env = do
sendToMotor welke (evalAExpr waarde env)
return env
我怎么会去执行函数sendToMotor
(返回IO()
)在我的评价功能evalMotorInstruct
应该返回Env
?我对于如何执行我的'动作'功能有点不知所措,只会让我的Env
退出评估功能。
请注意,evalMotorInstruct
的当前代码不正确。该函数应该返回一个Env
,但实际上返回IO Env
谢谢
您已经发现Haskell的一个有趣属性 - 它的纯度。你不能执行一个IO操作并返回一个结果*不包含在IO中。这是因为Haskell函数不能执行任意的副作用;他们必须是纯粹的。根据你对'do'和'return'的使用,你可以*返回一个'IO Env',这似乎是你在这里寻找的东西。 –
@AlexisKing这是否意味着我应该转换所有其他的“eval”函数以返回IO Env的呢?在各种情况下“解开”它们? – MrKickkiller
你可能想用一个免费的monad来避免提交'IO'。 http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.html – user2297560