Question

这是我的最小例子：

{-# 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))

无法使用以下错误进行编译：

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>

我理解为什么发生此错误;对merge的调用可能正在使用Into的不同实例（与outer相同但inner}不同于整个约束所选择的实例into功能。但我无法找到解决问题的方法。

我尝试过的事情：

使用函数依赖关系来暗示outer中的inner;这接近工作（抱怨需要UndecidableInstances），但似乎不太对劲;理想情况下，我真的希望能够将同一inner推送到两个不同的outer s
使用关联类型同义词来做同样的事情;除了丑化类型签名（outer =＆gt; Outer inner）之外，我也失败了，因为我在实例中使用的outer有更多的类型变量（其中一个是幻像）比inner，意味着我无法在实例声明中合法地实例化相关类型
使用merge into ScopedTypeVariables添加显式类型签名，并将into与merge的类型签名绑定;但由于outer的类型未引用merge，因此无效

对于整个into，我是否可以明确地使用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的相同类型类实例？或者任何其他方式我可以约束类型系统需要这样做？理想情况下，我想保留这个类，所以我的实例声明仍然很简单：

{{1}}

Answer 1

拥有一个没有提及所有类变量的类方法很少是个好主意（除非这些类变量由函数依赖项唯一确定）。

解决方案是使类层次结构更精确。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ad-hoc Mergeable类，但这取决于应用程序的详细信息和{{1}的属性}。

如何解决这种类型歧义？

1 个答案: