2

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))

这无法编译并出现以下错误：

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；该调用merge可能使用与整个函数的约束所选择的实例不同的实例Into（具有不同outer但相同的实例）。但我想不出解决它的方法。innerinto

我尝试过的事情：

  • 使用函数依赖来暗示outerfrom inner; 这接近工作（抱怨需要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东西推inner入两个不同outer的 s
  • 使用关联的类型同义词来做同样的事情；除了丑化类型签名（outer=> Outer inner）之外，我还摔倒了，因为outer我在实例中使用的类型变量（其中一个幻像）比inner，这意味着我无法合法地实例化关联的类型实例声明
  • merge在使用in intowith时添加显式类型签名ScopedTypeVariables以将其绑定到类型签名 for into; 但由于类型merge不涉及outer它并没有帮助

有什么方法可以明确地使用相同类型的类实例merge作为整个into？或者我可以限制类型系统需要这样做的任何其他方式？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：

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
4

1 回答 1

6

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

解决方案是使类层次结构更加精确。在这里，您可以创建第二个类merge：

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-hocMergeable类，但这取决于应用程序的详细信息和merge.

于 2013-05-28T11:13:56.787 回答