haskell - 为 MultiParamTypeClasses 注释不明确的类型变量

Question

我正在使用一个类型类，我可以在其中“测量”类型 v 的某些属性。在 a 类型的对象上.例如，请考虑以下定义:

{-# LANGUAGE MultiParamTypeClasses #-}

data SizedElem a =
  SizedElem
    { getSize :: Size
    , getElem :: Elem a
    }

newtype Size =
  Size Integer

newtype Elem a =
  Elem a

class Measured a v where
  measureTypeClass :: a -> v

instance Measured (Elem a) (SizedElem a) where
  measureTypeClass (Elem a) = SizedElem (Size 1) (Elem a)

尝试结合测量元素的功能，然后像这样提取尺寸

broken :: Elem a -> Size
broken = getSize . measureTypeClass

导致 GHC 的投诉:

Ambiguous type variable ‘a0’ arising from a use of ‘measureTypeClass’
prevents the constraint ‘(Measured
                            (Elem a) (SizedElem a0))’ from being solved.
...
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instance exist:
  instance Measured (Elem a) (SizedElem a)

天真地，我只是尝试在中间注释我期望的类型:

broken' :: Elem a -> Size
broken' xs =
  let meas = measureTypeClass xs :: SizedElem a
   in getSize meas

但是，这会导致稍微不同的错误:

No instance for (Measured (Elem a) (SizedElem a2))
  arising from a use of ‘measureTypeClass’
In the expression: measureTypeClass xs :: SizedElem a
In an equation for ‘meas’:
    meas = measureTypeClass xs :: SizedElem a

如果我理解正确，这意味着 a从注释解释 :: SizedElem a与类型签名中的类型不同。我如何告诉编译器这些应该是相同的类型变量？只需申请 {-# LANGUAGE ScopedTypeVariables #-}没有解决问题。
顺便说一句，对于特定类型，一切正常，例如

specificWorks :: Elem Char -> Size
specificWorks xs =
  let meas = measureTypeClass xs :: SizedElem Char
   in getSize meas

或者如果类型签名对于输入和输出具有相同的类型变量

elemWorks :: Elem a -> Elem a
elemWorks = getElem . measureTypeClass

此外，完全放弃类型类也有效，但对于我的实际应用程序，由于其他原因我需要类型类:

measureDirect :: Elem a -> SizedElem a
measureDirect (Elem a) = SizedElem (Size 1) (Elem a)

directWorks :: Elem a -> Size
directWorks = getSize . measureDirect

Answer 1

使用扩展 ScopedTypeVariables , 添加 forall函数签名的量词:

getSize' :: forall a. Elem a -> Size
getSize' xs =
  let meas = measureTypeClass xs :: SizedElem a
  in getSize meas

另一种适用于此的技术是 "constraint trick" .经验法则是永远不要有一个变量出现两次的实例头(在 => 的右侧)，而是使用等式约束。

instance (a ~ b) => Measured (Elem a) (SizedElem b) where
  measureTypeClass (Elem a) = SizedElem (Size 1) (Elem a)