"Could not deduce" 多参数类型错误 class

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

我得到了那个错误：

[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，例如：

eval :: gen -> light -> Float

但我不想添加一个不会被使用的新值参数。是否有其他类型解析方法？

Answer 1

问题是 eval 不使用 gen，因此在选择 Shadow 实例时对其类型进行特化不足以决定使用哪个 gen使用。一种可能的解决方案是使用 functional dependency 强制每个 light:

的选择只有一个 gen

{-# LANGUAGE FunctionalDependencies #-}

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

但是，您可能不想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 class 中删除 gen 的相反替代方案——如果 gen 和 light 不相关，则您不需要需要多参数类型 class 来关联它们：

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

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