haskell - 为多态模式同义词编写完整的编译指示？

Question

我有以下代码，我不知道应该在??. 还是多态模式不能完成？

{-# 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 文件说，当所有的 conlikes 都是多态的时，你必须输入 conlike。

make 时?? ()，编译成功。但这是什么()意思？GHC 表示仍然没有详尽的使用情况。

{-# 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

我不是这方面PatternSynonyms的专家，但从表面上看，在多态模式的情况下，我们需要指定使它们完整的确切类型。

在这种情况下Only将是：

{-# COMPLETE Single :: Only #-}

为了举例，让我们添加另一个实例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 #-}

这让 GHC 不再抱怨：

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