如何解决重叠实例

Question

我有以下代码(转换类似于转换)

instance {-# OVERLAPS #-} Transformable a a where
  transform x = x

instance {-# OVERLAPPABLE #-} (Transformable l l',   Transformable r r' )
         => Transformable (Either l r) (Either l' r')
  where
    transform = bimap transform transform

当然，在我试图将Either a b转换为Either a b并获得以下错误消息时，这些实例会重叠(ParsingError是Either something somethingElse的类型别名)

    Overlapping instances for Transformable
                                (parsingerror text) (parsingerror text)
      arising from a use of ‘makereceipt’
    matching instances:
Matching instances:    Overlapping instances for Transformable
                            (ParsingError Text) (ParsingError Text)
  arising from a use of ‘makeReceipt’
Matching instances:
  instance [overlappable] (Transformable l l', Transformable r r') =>
                          Transformable (Either l r) (Either l' r')
      instance [overlappable] (Transformable l l', Transformable r r') =>
                              Transformable (Either l r) (Either l' r')
        -- Defined at Handler/GLEnterReceiptSheet/ReceiptRow.hs:154:31
      instance [overlap ok] Transformable a a
        -- Defined at Handler/GLEnterReceiptSheet/ReceiptRow.hs:151:27

我尝试了OVERLAPS、OVERLAPPING和OVERLAPPABLE的不同组合，但都没有效果。我怎么才能解决这个问题？

Answer 1

您必须更改其中一个实例定义：

class Transformable a b where 
  transform :: a -> b 

-- this one
instance {-# OVERLAPS #-} (a ~ b) => Transformable a b where
  transform x = x

instance (Transformable l l', Transformable r r' )
       => Transformable (Either l r) (Either l' r') where
  transform = either (Left . transform) (Right . transform) 

test0 :: (Transformable a a', Transformable b b') => Either a b -> Either a' b'
test0 = transform

无论在其他实例上使用哪种重叠，代码都会工作。在第二个实例中，您实际上不需要任何实用程序。

原始代码的问题是，实例实际上是不连贯的，而不仅仅是重叠的，所以{-# OVERLAPS / OVERLAPPING / OVERLAPPABLE #-}的任何组合都不会拯救您--您需要使用{-# INCOHERENT #-}，这是不可取的，我也不推荐使用。GHC将告诉您这种与错误消息不一致的情况：

>:t transform :: (Transformable a a', Transformable b b') => Either a b -> Either a' b'

<interactive>:1:1: Warning:
    Overlapping instances for Transformable
                                (Either a1 b1) (Either a'1 b'1)
      arising from a use of `transform'
    Matching instances:
      instance [overlappable] (Transformable l l', Transformable r r') =>
                              Transformable (Either l r) (Either l' r')
        -- Defined at test6.hs:9:31
      instance [overlap ok] Transformable a a -- Defined at test6.hs:6:27
    (The choice depends on the instantiation of `a1, b1, a'1, b'1'
     To pick the first instance above, use IncoherentInstances
     when compiling the other instance declarations)
    In the expression:
        transform ::
          (Transformable a a', Transformable b b') =>
          Either a b -> Either a' b'

本质上，为了从重叠实例中选择，一个实例必须是您要匹配的类型的“最特定的”。详细内容在用户指南中给出。