haskell - 类型类声明中 "type"声明的含义

Question

我刚刚陷入了那段代码:

-- | Gathers common slice operations.
class Slice a where
    type Loc a

    sliceEvents :: a -> [ResolvedEvent]
    -- ^ Gets slice's 'ResolvedEvent's.
    sliceDirection :: a -> ReadDirection
    -- ^ Gets slice's reading direction.
    sliceEOS :: a -> Bool
    -- ^ If the slice reaches the end of the stream.
    sliceFrom :: a -> Loc a
    -- ^ Gets the starting location of this slice.
    sliceNext :: a -> Loc a
    -- ^ Gets the next location of this slice.
    toSlice :: a -> SomeSlice
    -- ^ Returns a common view of a slice.

我不明白type Loc a在做什么...

Answer 1

Loc a 是关联类型，它是声明与类实例关联的类型族实例的一种方式。 Loc a表示的类型由a的类型决定，并在实例中指定:例如

instance Slice Foo where
    type Loc Foo = Bar
    ...

无论 Loc a 出现在类声明中，它都会被替换为实例中相应的类型 - 因此 Foo 的实例函数将如下所示

sliceEvents :: Foo -> [ResolvedEvent]
...
sliceFrom :: Foo -> Bar
...

通过给出类约束，关联类型还可以在类声明之外的其他函数中使用:例如

myFunction :: (Slice a) => a -> Loc a