haskell - 为什么这个函数隐含一个单曲面约束，即使只有 SemiGroup 被列为约束？

Question

在以下堆栈脚本中，我无法应用我的 strangeHeadMay函数到 NonEmpty列表，但它在常规列表上工作正常。

#!/usr/bin/env stack
{- stack script --nix --resolver lts-13.6 -}

{-# LANGUAGE ScopedTypeVariables  #-}
{-# LANGUAGE PartialTypeSignatures #-}
{-# OPTIONS_GHC -Wno-partial-type-signatures #-} -- hide some scary types

import           Control.Arrow                    ((&&&))
import           Control.Monad                    (join)
import           Data.Foldable                    (toList)
import           Data.Function                    ((&))
import           Data.Functor                     ((<&>))
import           Data.List.NonEmpty               (NonEmpty(..))
import qualified Data.List.NonEmpty               as LNE
import           Data.Maybe                       (catMaybes)
import           Data.Tuple.Sequence              (sequenceT)


strangeHeadMay :: forall a t. (Semigroup (t a), _) => t a -> Maybe a
strangeHeadMay xs =
  let xsWrapped :: t (t a) = pure <$> xs -- <- Contrived example
      -- This is the functionality I actually need:
      xsMay :: Maybe (t a) = xsWrapped & (headMay &&& tailMay) & sequenceT
         <&> (\(h, t) -> foldr mappend h t)
   in do
    xs' <- xsMay
    xs' & toList & headMay

main :: IO ()
main = do
  let nexs :: NonEmpty (Maybe Double) = Nothing :| [Just (2.0)]
  let xs = LNE.toList nexs
  let xsHead = strangeHeadMay xs
  let nexsHead = strangeHeadMay nexs
  pure ()


headMay :: Foldable f => f a -> Maybe a
headMay = foldr (const . Just) Nothing

tailMay :: Foldable f => f a -> Maybe (f a)
tailMay = foldr (seq . Just) Nothing

错误是

/home/brandon/workspace/foldNEmaybes.hs:34:18: error:
    • No instance for (Monoid (NonEmpty (Maybe Double)))
        arising from a use of ‘strangeHeadMay’
    • In the expression: strangeHeadMay nexs
      In an equation for ‘nexsHead’: nexsHead = strangeHeadMay nexs
      In the expression:
        do let nexs :: NonEmpty (Maybe Double) = Nothing :| ...
           let xs = LNE.toList nexs
           let xsHead = strangeHeadMay xs
           let nexsHead = strangeHeadMay nexs
           ....
   |
34 |   let nexsHead = strangeHeadMay nexs
   |                  ^^^^^^^^^^^^^^^^^^^

另外，我必须承认我并不完全清楚为什么我需要部分键入的签名，而是更改 (Semigroup (t a), _)至 (Semigroup (t a))使坏事发生。

Answer 1

即使 <>和 mappend做同样的事情，如果你有 Monoid，你只能使用后者约束，所以一个被添加到你的 _在类型签名中。要修复它，请使用 foldr (<>)而不是 foldr mappend .

而且您不需要类型签名中的孔。你可以写出里面的内容，它仍然可以工作:Applicative t和 Foldable t .