我如何编写函数将泛型类型转换为与DSum一起使用的标记形状类型? [英] How can I write function to convert generic type to Tag-shaped type for use with DSum?
问题描述
如何实现这个 toDSum
函数?我已经设法得到基本情况进行编译,但我不知道如何通过递归调用来承载所有类型信息。在尝试递归之前,是否必须从类型中去掉 Code >?
(这是我该如何编写这个GEq实例?)
{ - #LANGUAGE GADTs# - }
{ - #LANGUAGE DataKinds# - }
{ - # LANGUAGE TypeOperators# - }
{ - #LANGUAGE KindSignatures# - }
{ - #LANGUAGE RankNTypes# - }
{ - #LANGUAGE ScopedTypeVariables# - }
module Foo其中
导入Data.Dependent.Sum
导入Data.GADT.Compare
导入Data.Proxy
导入Generics.SOP
导入限定的GHC。泛型为GHC
类型GTag t = GTag_(代码t)
新类型GTag_ t(as :: [*])= GTag(NS((:〜:) as)t)
实例GEq(GTag_t)其中
geq(GTag(Z Refl))(GTag(Z Refl))= Just Refl
geq(GTag(S x))(GTag (S y))= GTag x`geq` GTag y
geq _ _ = Nothing
toDSum :: forall t。通用t => t - > DSum(GTag t)(NP I)
toDSum = foo。 unSOP。 from
where
foo ::()
=> NS(NP I)(代码t)
- > DSum(GTag t)(NP I)
foo = bar(Proxy :: Proxy t)
bar :: forall t1。 ()
=>代理t1 - > NS(NP I)(代码t1)
- > DSum(GTag t1)(NP I)
bar _(Z x)= GTag(Z Refl):=> x
bar _(S x)=未定义
<这段代码的一个版本在我的 other 答案,但类型略有不同,这实际上简化了代码。
正如你在实例GEq(GTag_t)
中看到的那样,当你想在 NS>
>上编写归纳函数时,或 NP
,你需要保持索引参数化 - 你会看到这个通用模式与'依赖'编程有很大关系(都是真正的依赖编程,并在Haskell中伪造)。
这恰恰是 bar
的问题:
forall t1。 ()=>代理t1 - > NS(NP I)(代码t1) - > DSum(GTag t1)(NP I)
^^^^^^^^^
这样的函数没有办法递归 - 仅仅因为如果 S rep :: NS(NP I)(Code t1)
,那么它不一定是这种情况(实际上,这里从来就不是这种情况):对于某些 t2 $ c $
rep :: NS(NP I)(代码t2)
即使这个事实是,你也很难说服它的编译器。
你必须做这个函数(重命名至 toTagValG
)参数在指数中:
type GTagVal_ t = DSum (GTag_t)(NP I)
type GTagVal t = DSum(GTag t)(NP I)
toTagValG :: NS f xss - > DSum(GTag_xss)f
toTagValG(Z rep)= GTag(Z Refl):=> rep
toTagValG(S rep)= case toTagValG代表GTag tg:=> args - > GTag(S tg):=> args
然后 xss
被实例化为<$从使用
和至
或时,c $ c> Code t
$ c> from :: a - > Rep a Rep a = SOP I(Code a)
:
toTagVal :: Generic a => a - > GTagVal
toTagVal = toTagValG。 unSOP。从
注意这种类型是推断的(如果关闭MonomorphismRestriction)
另一个方向更简单:
fromTagVal :: Generic a => GTagVal a - >从TavVal =到
。 SOP。 (\(GTag tg:=> args) - > hmap(\Refl-> args)tg)
尽管您可以在函数中使用导入函数来编写函数:
fromTagValG :: DSum(GTag_ xss)f - > (GTag(Z Refl):=> rep)= Z rep
fromTagValG(GTag(S tg):=> args)= S $ fromTagValG $ GTag tg:=> ; args
请注意,您可以为此函数指定一个非常一般的类型,并且 toTagValG
- 事实上,它根本没有提及 NP I
。你也应该能够说服自己,这些函数是彼此反转的,因此见证了 NS f xss
和 DSum(GTag_ xss)之间的同构性。 f
。
How can I implement this toDSum
function? I've managed to get the base case to compile, but I don't know how to carry all the type information across a recursive call. Do I have to strip off the Code
from the type before trying to recurse?
(this is a followup to How can I write this GEq instance?)
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Foo where
import Data.Dependent.Sum
import Data.GADT.Compare
import Data.Proxy
import Generics.SOP
import qualified GHC.Generics as GHC
type GTag t = GTag_ (Code t)
newtype GTag_ t (as :: [*]) = GTag (NS ((:~:) as) t)
instance GEq (GTag_ t) where
geq (GTag (Z Refl)) (GTag (Z Refl)) = Just Refl
geq (GTag (S x)) (GTag (S y)) = GTag x `geq` GTag y
geq _ _ = Nothing
toDSum :: forall t . Generic t => t -> DSum (GTag t) (NP I)
toDSum = foo . unSOP . from
where
foo :: ()
=> NS (NP I) (Code t)
-> DSum (GTag t) (NP I)
foo = bar (Proxy :: Proxy t)
bar :: forall t1 . ()
=> Proxy t1 -> NS (NP I) (Code t1)
-> DSum (GTag t1) (NP I)
bar _ (Z x) = GTag (Z Refl) :=> x
bar _ (S x) = undefined
A version of this code was in my other answer, but the types are slightly different, which actually simplifies the code.
As you have seen with instance GEq (GTag_ t)
, when you want to write inductive functions on NS
or NP
, you need to keep the index parametric - you will see this general pattern quite a bit with 'dependant' programming (both real dependant programming and faking it in Haskell).
This is precisely the issue with bar
:
forall t1 . () => Proxy t1 -> NS (NP I) (Code t1) -> DSum (GTag t1) (NP I)
^^^^^^^^^
There is no way for such a function to be recursive - simply because if S rep :: NS (NP I) (Code t1)
, then it is not necessarily the case (indeed, it is never the case here) that rep :: NS (NP I) (Code t2)
for some t2
- and even if this fact were true, you would struggle to convince the compiler of it.
You must make this function (renaming to toTagValG
) parametric in the index:
type GTagVal_ t = DSum (GTag_ t) (NP I)
type GTagVal t = DSum (GTag t) (NP I)
toTagValG :: NS f xss -> DSum (GTag_ xss) f
toTagValG (Z rep) = GTag (Z Refl) :=> rep
toTagValG (S rep) = case toTagValG rep of GTag tg :=> args -> GTag (S tg) :=> args
Then xss
is instantiated with Code t
when you use to
or from
, since from :: a -> Rep a
and Rep a = SOP I (Code a)
:
toTagVal :: Generic a => a -> GTagVal a
toTagVal = toTagValG . unSOP . from
Note this type is inferred (if you turn off the MonomorphismRestriction)
The other direction is even simpler:
fromTagVal :: Generic a => GTagVal a -> a
fromTagVal = to . SOP . (\(GTag tg :=> args) -> hmap (\Refl -> args) tg)
Although you can write the function in the lambda with induction as well:
fromTagValG :: DSum (GTag_ xss) f -> NS f xss
fromTagValG (GTag (Z Refl) :=> rep) = Z rep
fromTagValG (GTag (S tg) :=> args) = S $ fromTagValG $ GTag tg :=> args
Note that you can assign a very general type to this function, and toTagValG
- indeed, it does not mention NP I
at all. You should also be able to convince yourself that these functions are each others inverses, and so witness an isomorphism between NS f xss
and DSum (GTag_ xss) f
.
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