函子超型具有冲突的实例

问题描述

我定义了一个细粒度函子类(FgFunctor)，以便将约束应用于类型可能会映射到我的Ordered Triple数据类型(OrdTriple)上的函数，这要求所包含的类型是可排序的.

I have defined a Fine-Grained Functor class (FgFunctor) in order to apply a constraint to the type of functions that may map over my Ordered Triple datatype (OrdTriple), which requires the contained type be orderable.

import Data.List (sort)

-- 'fine-grained' functor
class FgFunctor f a b where
    fgmap :: (a -> b) -> f a -> f b

data OrdTriple a = OrdTriple a a a deriving Show

instance (Ord a, Ord b) => FgFunctor OrdTriple a b where
    fgmap f (OrdTriple n d x) = OrdTriple n' d' x'
        where [n', d', x'] = sort [f n, f d, f x]

main :: IO ()
main = do
    let test = fgmap (* 10^4) $ OrdTriple 1 6 11 
    print test

在我将所有其他Functor都定义为FgFunctor之前，代码也可以正常工作，就像这样:

The code works fine until I define all other Functors to also be FgFunctors, like so:

-- all regular functors are also fine-grained ones
instance Functor f => FgFunctor f a b where
    fgmap = fmap

有了该实例声明，当我尝试在OrdTriple类型上使用fgmap时，编译器就会抱怨实例声明重叠

With that instance declaration, as soon as I try to use fgmap on my OrdTriple type, the compiler complains about overlapping instance declarations

Overlapping instances for FgFunctor OrdTriple b0 b0
  arising from a use of ‘fgmap’
Matching instances:
  instance Functor f => FgFunctor f a b
    -- Defined at OrdTriple.hs:15:10
  instance (Ord a, Ord b) => FgFunctor OrdTriple a b
    -- Defined at OrdTriple.hs:18:10
In the expression: fgmap (* 10 ^ 4)
In the expression: fgmap (* 10 ^ 4) $ OrdTriple 1 6 11
In an equation for ‘test’:
    test = fgmap (* 10 ^ 4) $ OrdTriple 1 6 11

但是，它所引用的第一个匹配实例"永远都不适用于OrdTriple，因为OrdTriple不是Functor，因此我正在努力确定是什么导致了假定的重叠.

However, the first 'matching instance' it refers to should never apply to OrdTriple, as OrdTriple is not a Functor, and so I am struggling to determine what is causing the supposed overlap.

推荐答案

当编译器(GHC)查找特定类型的类型类的实例时，它不会考虑每个实例的上下文.这就是为什么即使OrdTriple不是Functor的实例，它也为OrdTriple找到两个FgFunctor实例的原因.

When the compiler (GHC) looks for instances of a type class for a specific type, it doesn't take contexts of each instance into account. That's why it found two instances of FgFunctor for OrdTriple even though OrdTriple is not an instance of Functor.

GHC/AdvancedOverlap 向您展示了如何解决这种情况.

GHC/AdvancedOverlap shows you how you can solve this situation.

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