如何解决重叠实例

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，在我尝试转变的情况下，这些实例是重叠的Either a b to Either a b并收到以下错误消息（ParsingError是类型别名Either something somethingElse)

    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

我尝试了不同的组合OVERLAPS, OVERLAPPING and OVERLAPPABLE但没有任何作用。我该如何解决这个问题？

您必须更改实例定义之一：

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

无论您在另一个实例上使用哪个重叠，该代码都将起作用。在第二个实例中，您实际上不需要任何编译指示。

原始代码的问题在于实例实际上是语无伦次的，不仅仅是重叠，所以没有组合{-# OVERLAPS / OVERLAPPING / OVERLAPPABLE #-}会拯救你 - 你需要使用{-# INCOHERENT #-}，这是不可取的，我不会推荐它。 GHC 会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，为了从重叠的实例中进行选择，一个实例对于您尝试匹配的类型必须是“最具体的”。详细信息在用户指南 https://downloads.haskell.org/%7Eghc/latest/docs/html/users_guide/glasgow_exts.html#overlapping-instances.