[英]Haskell-style type families
In Haskell, I might write a typeclass with a type
declaration to create a type family, like so: 在Haskell中,我可能会编写一个带type
声明的类型类来创建一个类型族,如下所示:
class ListLike k where
type Elem :: * -> *
fromList :: [Elem k] -> k
And then write instances like this: 然后写这样的实例:
instance ListLike [a] where
type Elem [a] = a
fromList = id
instance ListLike Bytestring where
type Elem Bytestring = Char
fromList = pack
I'm aware that you can create typeclasses and type-level functions in Idris, but the methods operate on data of the given type, not the type itself. 我知道您可以在Idris中创建类型类和类型级函数,但这些方法对给定类型的数据进行操作,而不是类型本身。
How can I make a typeclass-constrained type family in Idris like the one above? 如何在Idris中创建类型类约束类型系列,如上所述?
I have no clue if you will ever find a use for this, but I think the obvious translation should be 我不知道你是否会找到它的用途,但我认为显而易见的翻译应该是
class ListLike k where
llElem : Type
fromList : List llElem -> k
instance ListLike (List a) where
llElem = a
fromList = id
instance ListLike (Maybe a) where
llElem = a
fromList [] = Nothing
fromList (a::_) = Just a
λΠ> the (Maybe Int) (fromList [3])
Just 3 : Maybe Int
λΠ> the (List Int) (fromList [3])
[3] : List Int
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