问题描述
此刻,我正在学习关于存在性量化,幻像类型和GADT的知识.如何创建带有幻像变量的数据类型的异构列表?例如:
I'm learning about existential quantification, phantom types, and GADTs at the moment. How do I go about creating a heterogeneous list of a data type with a phantom variable? For example:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ExistentialQuantification #-}
data Toy a where
TBool :: Bool -> Toy Bool
TInt :: Int -> Toy Int
instance Show (Toy a) where
show (TBool b) = "TBool " ++ show b
show (TInt i) = "TInt " ++ show i
bools :: [Toy Bool]
bools = [TBool False, TBool True]
ints :: [Toy Int]
ints = map TInt [0..9]
具有如下功能可以:
isBool :: Toy a -> Bool
isBool (TBool _) = True
isBool (TInt _) = False
addOne :: Toy Int -> Toy Int
addOne (TInt a) = TInt $ a + 1
但是,我希望能够这样声明一个异类列表:
However, I would like to be able to declare a heterogeneous list like so:
zeros :: [Toy a]
zeros = [TBool False, TInt 0]
我尝试使用空类型类通过以下方式限制a
上的类型:
I tried using an empty type class to restrict the type on a
by:
class Unify a
instance Unify Bool
instance Unify Int
zeros :: Unify a => [Toy a]
zeros = [TBool False, TInt 0]
但是以上内容无法编译.我能够使用存在量化来获得以下信息:
But the above would fail to compile. I was able to use existential quantification to do get the following:
data T = forall a. (Forget a, Show a) => T a
instance Show T where
show (T a) = show a
class (Show a) => Forget a
instance Forget (Toy a)
instance Forget T
zeros :: [T]
zeros = [T (TBool False), T (TInt 0)]
但是这样,我不能将基于Toy a
中特定类型的a
的函数应用于T
. addOne
以上.
But this way, I cannot apply a function that was based on the specific type of a
in Toy a
to T
e.g. addOne
above.
总而言之,在不忘记/丢失幻象变量的情况下,我可以通过哪些方式创建异构列表?
In conclusion, what are some ways I can create a heterogeneous list without forgetting/losing the phantom variable?
推荐答案
以Toy
类型开头:
data Toy a where
TBool :: Bool -> Toy Bool
TInt :: Int -> Toy Int
现在,您可以将其包装在存在的中,而不会因类系统而过度概括:
Now you can wrap it up in an existential without over-generalizing with the class system:
data WrappedToy where
Wrap :: Toy a -> WrappedToy
由于包装器仅容纳Toy
个,因此我们可以解开它们并收回Toy
个:
Since the wrapper only holds Toy
s, we can unwrap them and get Toy
s back:
incIfInt :: WrappedToy -> WrappedToy
incIfInt (Wrap (TInt n)) = Wrap (TInt (n+1))
incIfInt w = w
现在您可以区分列表中的内容:
And now you can distinguish things within the list:
incIntToys :: [WrappedToy] -> [WrappedToy]
incIntToys = map incIfInt
编辑
正如Cirdec所指出的那样,可以将一些不同的部分分开来弄弄:
Edit
As Cirdec points out, the different pieces can be teased apart a bit:
onInt :: (Toy Int -> WrappedToy) -> WrappedToy -> WrappedToy
onInt f (Wrap t@(TInt _)) = f t
onInt _ w = w
mapInt :: (Int -> Int) -> Toy Int -> Toy Int
mapInt f (TInt x) = TInt (f x)
incIntToys :: [WrappedToy] -> [WrappedToy]
incIntToys = map $ onInt (Wrap . mapInt (+1))
我还应该指出,到目前为止,没有什么可以真正证明Toy
GADT的合理性. bheklilr使用简单的代数数据类型的更简单方法应该可以正常工作.
I should also note that nothing here so far really justifies the Toy
GADT. bheklilr's simpler approach of using a plain algebraic datatype should work just fine.
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