将函数依赖类转换为类型族实例 [英] Converting Functional Dependency class to Type Family instances

问题描述

是否可以从fundep类创建类型族实例?例如，假设我有一个课程

Is it possible to create type family instances from a fundep class? For example, let's say that I have the class

class A a b | a -> b

具有某些实例(导入了外部库)，并且想要为类型族创建所有对应的实例

with some instances (imported an external library) and want to create all corresponding instances for the type family

type family A' a :: *

表示A' a ~ b iff A a b，而无需从外部源手动复制和修改实例.

such that A' a ~ b iff A a b, without having to manually copy and modify the instances from the external source.

如果可能的话，我该怎么办?

How would I do that (if it is possible)?

迄今为止我最有前途的尝试，

My most promising attempt so far,

class A' a where
    type A'_b a :: *

instance forall a b. A a b => A' a where
    type A'_b a = b

给出错误消息

    The RHS of an associated type declaration mentions ‘b’
      All such variables must be bound on the LHS

所以，我的猜测是答案是否定的? :/

So, my guess is that the answer is no? :/

推荐答案

不要考虑太多.

class C a b | a -> b
instance C T U

大致翻译为

class C' a where
    type B a
instance C' T where
    type B T = U

即使它们的行为类似，它们在语义上也有所不同. C是一个两参数类，但是其第二个参数由其第一个唯一地确定. C'是具有关联类型的单参数类.关联类型被解释为FC强制系统的顶级公理，而眼底基本上只影响统一.

They're semantically somewhat different, even though they behave in a similar way. C is a two-parameter class, but its second parameter is uniquely determined by its first. C' is a one-parameter class with an associated type. Associated types are interpreted as top-level axioms for FC's coercion system whereas fundeps basically just affect unification.

可以在两种样式之间进行转换，但是必须使用newtype来绑定type公式中的变量.

It is possible to convert between the two styles, but you have to use a newtype to bind the variables in the type equation.

newtype WrappedB a = WrappedB { unwrapB :: B a }
instance C' a => C a (WrappedB a)  -- you can't use a type synonym family in an instance

newtype Wrap a b = Wrap { unWrap :: a }
instance C a b => C' (Wrap a b) where
    type B (Wrap a b) = b  -- b needs to be bound on the LHS of this equation

通常我不建议编写您在问题中尝试过的通用实例，因为这样实例有重叠的趋势.

In general I don't advise writing the sort of generic instance you've attempted in your question, because such instances have a tendency to be overlapping.

功能依赖项少，嗯，很奇怪而不是类型家庭.在其他所有条件都相同的情况下，我倾向于选择资助人.

Functional dependencies are less, um, weird than type families, though. All other things being equal I tend to prefer a fundep.

这篇关于将函数依赖类转换为类型族实例的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！