Haskell 手动遍历 HList - 异构列表

Question

根据我的问题：

根据回答：

import Data.HList (HList (HCons, HNil), hBuild, hEnd)

在 hello

的 HList 上

hello :: HList '[Integer, [Char]]
hello = hEnd $ hBuild 1 "2"

我可以用 print

遍历列表

class PrintEach ts where
  printEach :: HList ts -> IO ()

instance PrintEach '[] where
  printEach HNil = pure ()

instance (Show t, PrintEach ts) => PrintEach (t : ts) where
  printEach (HCons x xs) = print x *> printEach xs

main :: IO ()
main = printEach hello
-- 1 "2"

现在，我想traverse/mapiAiB

io :: a -> IO a
io = pure

iA :: IO Int
iA = io (1 :: Int)

iB :: IO [Char]
iB = io ("foo" :: [Char])

iAiB :: HList '[IO Int, IO [Char]]
iAiB = hEnd $ hBuild iA iB

到[IO Bool]

class MapToIObool ts where
  mapToIObool :: HList ts -> [IO Bool]

instance MapToIObool '[] where
  mapToIObool  HNil = []

instance ----------??

在这里，为了简单起见，[IO Bool]的每个元素都将无条件地io True。

result :: [IO Bool]
result = mapToIObool iAiB
-- expected result
result' :: [IO Bool]
result' = [io True, io True]

instance ----------??

的代码是什么

Answer 1

好吧，你可以这样做：

instance MapToIObool ts => MapToIObool (t : ts) where
  mapToIObool (HCons x xs) = io True : mapToIObool xs

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