Question

这是haskell教程的一个例子：

type Bindings = Map.Map String Int

isCountCorrect :: Bindings -> Bool
isCountCorrect bindings = runReader calc_isCountCorrect bindings

calc_isCountCorrect :: Reader Bindings Bool
calc_isCountCorrect = do
    count <- asks (lookupVar "count")
    bindings <- ask
    return (count == (Map.size bindings))

lookupVar :: String -> Bindings -> Int
lookupVar name bindings = fromJust (Map.lookup name bindings)

sampleBindings = Map.fromList [("count", 4), ("1", 1), ("b", 2)]

main = do
    putStr $ "Count is correct for bindings " ++ (show  sampleBindings) ++ ": "
    putStrLn $ show (isCountCorrect sampleBindings)

我创建了函数来获取“2”的值。我不知道如何使它成为任意String的通用。你能帮帮我吗？

get_valueM :: Reader Bindings Int
get_valueM = asks (lookupVar "b") >>= (\value -> ask >>= (\_ -> return value))

get_value :: Bindings -> Int
get_value bindings = runReader get_valueM bindings

Answer 1

我不确定我完全理解这个问题。为什么不为"b"添加参数？

get_valueM :: String -> Reader Bindings Int
get_valueM x = asks (lookupVar "b") >>= (\value -> ask >>= (\_ -> return value))

顺便问一下，最后ask是什么？你不使用它的结果，它没有副作用，所以它看起来可以删除。

get_valueM :: String -> Reader Bindings Int
get_valueM x = asks (lookupVar "b") >>= (\value -> return value)

可以简化为

get_valueM :: String -> Reader Bindings Int
get_valueM x = asks (lookupVar "b") >>= return

可以简化为

get_valueM :: String -> Reader Bindings Int
get_valueM x = asks (lookupVar "b")

最后，让我补充一点，我认为这些部分功能很危险。找不到绑定时，lookupVar不应该崩溃。不过，这些在玩弄时很方便。

Haskell，从绑定中获取价值

1 个答案: