haskell - 我该如何解决这种类型类歧义？

Question

这是我的最小示例:

{-# LANGUAGE MultiParamTypeClasses, RankNTypes #-}

import Control.Lens

class Into outer inner where
    factory :: inner -> outer
    merge :: inner -> inner -> inner

-- Given an inner item, a lens and an outer item, use factory to construct a new
-- outer around the inner if the Maybe outer is Nothing, or else use merge to combine
-- the argument inner with the one viewed through the lens inside the outer
into :: Into outer inner =>
    inner -> Lens' outer inner -> Maybe outer -> Maybe outer
inner `into` lens = Just . maybe (factory inner) (over lens (merge inner))

编译失败，出现以下错误:

GHCi, version 7.6.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( foo.hs, interpreted )

foo.hs:10:62:
    Could not deduce (Into outer0 inner) arising from a use of `merge'
    from the context (Into outer inner)
      bound by the type signature for
                 into :: Into outer inner =>
                         inner -> Lens' outer inner -> Maybe outer -> Maybe outer
      at foo.hs:9:9-84
    The type variable `outer0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    In the second argument of `over', namely `(merge inner)'
    In the second argument of `maybe', namely
      `(over lens (merge inner))'
    In the second argument of `(.)', namely
      `maybe (factory inner) (over lens (merge inner))'
Failed, modules loaded: none.
Prelude> 

我明白为什么会出现这个错误；对 merge 的调用可能使用与 Into 不同的实例(具有不同的 outer 但相同的 inner)由整个 into 函数的约束选择的那个。但我想不出解决它的方法。

我尝试过的事情:

• 使用函数依赖从内部隐含外部；这接近于工作(提示需要 UndecidableInstances)，但似乎不太正确；理想情况下，我真的希望能够有一种方法将相同的 inner 插入两个不同的 outer
• 使用关联类型的同义词来做同样的事情；除了丑化类型签名(outer => Outer inner)，我还摔倒了，因为我在一个实例中使用的 outer 有比 inner 更多的类型变量(其中一个是幻影)，这意味着我无法在实例声明中合法地实例化关联类型
• 在 into 中使用 merge 和 ScopedTypeVariables 添加显式类型签名，以将其绑定(bind)到 into 的类型签名;但由于 merge 的类型不引用 outer 它没有帮助

有什么方法可以明确地为 merge 使用与整个 into 相同的类型类实例？或者我可以通过任何其他方式将类型系统限制为需要这样做？理想情况下，我想保留该类，以便我的实例声明仍然如此简单:

instance (Hashable v, Eq v) => Into (VarInfo s k v) (HashSet v) where
    -- VarInfo is just a record type with 2 fields, the second being a HashSet v
    factory = VarInfo (return ())
    merge = HashSet.intersection

Answer 1

拥有一个不提及所有类变量的类方法很少是一个好主意(除非这些类变量由函数依赖唯一确定)。

解决方案是使类层次结构更加精确。在这里，您可以为 merge 创建第二个类:

class Mergeable a where
    merge :: a -> a -> a

class Mergeable inner => Into outer inner where
    factory :: inner -> outer

您也可以使用更通用的 Semigroup 类而不是专用的 Mergeable 类，但这取决于您的应用程序的细节和merge 的属性。