Question

我有以下代码，但我不知道在??应该输入什么。还是无法完成多态模式？

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns    #-}

module Data.Tuple.Single.Class
  ( Single (..)
  , pattern Single
  ) where

class Single t where
  wrap :: a -> t a
  unwrap :: t a -> a

pattern Single :: Single t => a -> t a
pattern Single a <- (unwrap -> a) where
  Single a = wrap a

{-# COMPLETE Single :: ?? #-}

GHC document说，当所有赞叹词都是多态的时，您必须键入conlike。

制作?? ()时，编译成功。但是()是什么意思？ GHC表示使用情况还不完全。

{-# LANGUAGE PatternSynonyms #-}

{-# OPTIONS_GHC -Wno-orphans #-}

module Data.Tuple.Single.Only
  ( Single (..)
  , pattern Single
  ) where

import           Data.Tuple.Only         (Only (Only, fromOnly))
import           Data.Tuple.Single.Class (Single (unwrap, wrap), pattern Single)

instance Single Only where
  wrap = Only
  unwrap = fromOnly

ghci> Single a = wrap 1 :: Only Int

<interactive>:2:1: warning: [-Wincomplete-uni-patterns]
    Pattern match(es) are non-exhaustive
    In a pattern binding: Patterns not matched: _

GHC 8.6.5

Answer 1

我不是PatternSynonyms的专家，但是从它的外观来看，如果是多态模式，我们需要指定准确的类型以使其完整。

对于Only，这将是：

{-# COMPLETE Single :: Only #-}

为了示例，我们将另一个实例添加到Single：


instance Single Identity where
  wrap = Identity
  unwrap (Identity a) = a

pattern Single :: Single t => a -> t a
pattern Single a <- (unwrap -> a) where
  Single a = wrap a

{-# COMPLETE Single :: Only #-}
{-# COMPLETE Single :: Identity #-}

这使GHC停止抱怨：

λ> Single a = wrap 1 :: Identity Int
λ> Single a = wrap 1 :: Only Int

为多态模式同义词写一个COMPLETE编译指示吗？

1 个答案: