Question

我有一个Map (Int,Int) Char，我正在尝试在密钥中包含的位置绘制每个Char。我的职责是：

import qualified Data.Map.Strict as SM
data Position = Position !GLint !GLint

drawMirrors :: SM.Map (Int,Int) Char -> IO()
drawMirrors mirrors = do
    mapM_ (\(x,y) c -> drawMirror c (Position x y)) mirrors

drawMirror :: Char -> Position -> IO()
drawMirror orientation (Position x y) = do
    -- Some irrelevant stuff

在drawMirrors mirrors = do mapM_ (\(x,y) c -> drawMirror c (Position x y)) mirrors行，我收到错误：

src\Main.hs:200:33:

Couldn't match expected type `Char -> IO ()'
            with actual type `IO b0'
The lambda expression `\ (x, y) c -> drawMirror c (Position y)'
has two arguments,
but its type `(t0, GLint) -> IO b0' has only one
In the first argument of `mapM_', namely
  `(\ (x, y) c -> drawMirror c (Position y))'
In a stmt of a 'do' block:
  mapM_ (\ (x, y) c -> drawMirror c (Position y)) mirrors

我如何在drawMirrors中获取字典中的所有键和值，并将drawMirror函数应用于这些键和值？

Answer 1

你的lambda \(x,y) c -> drawMirror c (Position x y)有两个参数。但是，它使用(key, value)形式的单个参数调用（在您的情况下为((x, y), c)。

(\((x,y), c) -> drawMirror c (Position x y))

此外，mapM_（我相信你的情况来自Data.Foldable）只会遍历密钥，所以你可能想要调用SM.toList来获得{{1}的列表}}

最终结果是：

(key, value)

Answer 2

我一直在玩这个，并找到了一种方法来使用mapM_代替mapWithKeyM_。具体地，

{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleInstances #-}

data UnCurry :: (* -> * -> *) -> * -> * where
  UnCurry :: f k v -> UnCurry f (k, v)

instance Foldable (UnCurry Map) where
  foldr c n (UnCurry m) = M.foldrWithKey go n m
    where
      go k a b = c (k, a) b

现在，您可以使用mapM_ f (UnCurry m)为Map中的每个键值对执行操作。

Haskell：对于Map中的每个（k，v），用k和v做IO（）

2 个答案: