多参数类型类上的"Could not deduce"错误

Question

我有这个代码：

{-# LANGUAGE MultiParamTypeClasses #-}
import System.Random (RandomGen(..))
class RandomGen gen => Shadow gen light where
    shadowRay :: gen -> light -> Float
    eval      :: light -> Float

我得到那个错误：

[1 of 1] Compiling Main             ( problem.hs, problem.o )
problem.hs:6:5: error:
    * Could not deduce (Shadow gen0 light)
      from the context: Shadow gen light
        bound by the type signature for:
                   eval :: Shadow gen light => light -> float -> float
        at problem.hs:6:5-40
      The type variable `gen0' is ambiguous
    * In the ambiguity check for `eval'
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        eval :: forall gen light.
                Shadow gen light =>
                forall float. light -> float -> float
      In the class declaration for `Shadow'

这是GHC 7.10+的问题。在它工作之前。如果我将"gen"参数添加到"eval"，则有一个修复方法，例如：

eval :: gen -> light -> Float

但我不想添加一个不会被使用的新值参数。有没有其他方法进行类型解析？

Answer 1

问题是eval不使用gen，因此在选择要使用的Shadow实例时，专门化其类型不足以决定使用哪个gen。一种可能的解决方案是使用功能依赖关系来强制每个light选择只有一个gen：

{-# LANGUAGE FunctionalDependencies #-}
class RandomGen gen => Shadow gen light | light -> gen where
    shadowRay :: gen -> light -> Float
    eval      :: light -> Float

但是，您可能不希望或不需要以这种方式耦合light和gen。在这种情况下，您可能需要考虑从类型类中删除gen的相反替代方法 - 如果gen和light不相关，则不需要多参数类型类来关联它们：

class Shadow light where
    shadowRay :: RandomGen gen => gen -> light -> Float
    eval      :: light -> Float