指定来自 Haskell 术语的上下文

Question

这是一个虚拟示例：

class Test a b where
  witness :: a

f :: Test a b => a
f = witness

Haskell 然后说

Could not deduce (Test a b0) arising from a use of ‘witness’
from the context (Test a b)
  bound by the type signature for f :: Test a b => a
  at test.hs:8:6-18
The type variable ‘b0’ is ambiguous
Relevant bindings include f :: a (bound at test.hs:9:1)
In the expression: witness
In an equation for ‘f’: f = witness

错误来自 Haskell 无法推断类型变量b0的事实，解决方案是从类型类Test定义中删除参数b 。 但是，实际上，我不能。

我的问题是：是否存在一种方法可以使用f :: Test ab => a b给出的显式参数b显式识别b0 ？

谢谢。

Answer 1

充实Joachim Breitner 的建议，如果您可以将其类型签名更改为proxy b -> a argument 或Constant ab ，则可以使用witness 。

这两种方法大多是等价的，所以这是一个偏好问题：

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
module SO33958506 where

import Data.Functor.Constant
import Data.Proxy

class Test a b where
  constantWitness :: Constant a b
  proxyWitness :: proxy b -> a

constantWitnessWithProxy :: forall proxy a b. Test a b => proxy b -> a
constantWitnessWithProxy _ = getConstant $ (constantWitness :: Constant a b)

proxyWitnessAsConstant :: forall a b. Test a b => Constant a b
proxyWitnessAsConstant = Constant $ proxyWitness (Proxy :: Proxy b)