我目前正在評估Generics.SOP一個用例，該用例涉及從給定的資料型別定義派生新的相關資料型別。
我想首先定義表示 lambda 術語的資料型別的“de Bruijinized”版本：
-- The reproducer needs only some of the LANGUAGE pragmas and imports,
-- but it might be convenient for your (or my) solutions
{-# LANGUAGE GeneralisedNewtypeDeriving #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TemplateHaskell #-}
module SOP where
import Data.SOP.NS
import Generics.SOP
import Generics.SOP.TH
import Data.Kind
newtype Var = V String
deriving (Eq, Ord, Show)
newtype BindingOcc = B Var
deriving (Eq, Ord, Show)
data Expr
= Var Var
| App Expr Expr
| Lam BindingOcc Expr
deriveGeneric ''Expr -- Code Expr = '[ '[Var], '[Expr, Expr], '[BindingOcc, Expr]]
為了匯出 的 de Bruijinized 版本Expr，我必須洗掉所有BindingOccs（然后添加一個新的'[Int]替代方案，但一步一步地）。我怎么做？也許有一個功能
-- Let's be absolutely explicit about it and inline `Code`
-- Also don't want to confuse the type-checker with a type
-- family that removes the field just yet
deleteBindingOcc_SOP :: SOP I '[ '[Var], '[Expr, Expr], '[BindingOcc, Expr]]
-> SOP I '[ '[Var], '[Expr, Expr], '[Expr]]
deleteBindingOcc_SOP arg = SOP $ trans_NS Proxy {- will be filled in below -} deleteBindingOcc_NP (unSOP arg)
deleteBindingOcc_NP :: NP I xs -> NP I (WithoutBindingOccs xs)
deleteBindingOcc_NP Nil = Nil
deleteBindingOcc_NP (x :* xs)
| B _ <- x = deleteBindingOcc_NP xs
| otherwise = x :* deleteBindingOcc_NP xs
-- I guess I expected to write the following type family
type family WithoutBindingOcc (xs :: [Type]) :: [Type] where
WithoutBindingOcc '[] = '[]
WithoutBindingOcc (BindingOcc ': xs) = WithoutBindingOcc xs
WithoutBindingOcc (x ': xs) = x ': WithoutBindingOcc xs
但是，唉，這不會進行型別檢查；一方面，我xs在 的定義中匹配運行時的引數多型頭deleteBindingOcc_NP，所以我需要一個單例編碼/派生一個型別類只是為了定義我的小輔助函式deleteBindingOcc_NP。
這里是：
-- Now we know the full type of the proxy, carrying the constraint that `deleteBindingOcc_NP` wants
deleteBindingOcc_SOP arg = SOP $ trans_NS (Proxy :: Proxy MyC) deleteBindingOcc_NP (unSOP arg)
class b ~ WithoutBindingOcc a => MyC a b where -- welp
deleteBindingOcc_NP :: NP I a -> NP I b
instance MyC '[] '[] where
deleteBindingOcc_NP Nil = Nil
instance {-# OVERLAPPING #-} MyC a b => MyC (BindingOcc ': a) b where
deleteBindingOcc_NP (_ :* xs) = deleteBindingOcc_NP xs
instance {-# OVERLAPPABLE #-} MyC a b => MyC (x ': a) (x ': b) where
deleteBindingOcc_NP (x :* xs) = x :* deleteBindingOcc_NP xs
但即使這樣也行不通，因為重疊的實體不會進行型別檢查：
? Could not deduce: WithoutBindingOcc (x : a) ~ (x : b)
arising from the superclasses of an instance declaration
from the context: MyC a b
bound by the instance declaration at SOP2.hs:52:31-62
? In the instance declaration for ‘MyC (x : a) (x : b)’
|
52 | instance {-# OVERLAPPABLE #-} MyC a b => MyC (x ': a) (x ': b) where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
仔細考慮一下，這并不奇怪：不能保證后一種型別類永遠不會BindingOcc在其頭部實體化 a x，我們的型別系列應該洗掉它。所以看起來基于型別類的方法不是我想要的。
我的問題是：如何deleteBindingOcc_SOP使用給定的型別簽名以一種適用于不同但相似相關的Codes的方式撰寫？
我擔心Code表示為元語言 Type結構串列的串列可能不適合實作我想要的。不知何故，我們無法知道所有的Types 實際上都是封閉的，不會被進一步替換。
uj5u.com熱心網友回復：
代替使用型別族來關聯原始型別和剝離型別，也許我們可以依賴函式依賴：
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE UndecidableInstances #-} -- required by some of the magic below
{-# LANGUAGE TypeApplications #-}
class MyC a b | a -> b where
deleteBindingOcc_NP :: NP I a -> NP I b
instance MyC '[] '[] where
deleteBindingOcc_NP Nil = Nil
instance {-# OVERLAPPING #-} MyC a b => MyC (BindingOcc ': a) b where
deleteBindingOcc_NP (_ :* xs) = deleteBindingOcc_NP xs
instance {-# OVERLAPPABLE #-} MyC a b => MyC (x ': a) (x ': b) where
deleteBindingOcc_NP (x :* xs) = x :* deleteBindingOcc_NP xs
這似乎有效：
deleteBindingOcc_SOP :: SOP I '[ '[Var], '[Expr, Expr], '[BindingOcc, Expr]]
-> SOP I '[ '[Var], '[Expr, Expr], '[Expr]]
deleteBindingOcc_SOP arg = SOP $ trans_NS (Proxy @MyC) deleteBindingOcc_NP (unSOP arg)
帶有輔助MyC'類而不是重疊實體的替代版本：
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
import Data.Kind
import GHC.TypeLits
import Data.Type.Equality
class MyC a b | a -> b where
deleteBindingOcc_NP :: NP I a -> NP I b
class MyC' (isocc :: Bool) a b | a -> b where
deleteBindingOcc_NP' :: NP I a -> NP I b
instance MyC '[] '[] where
deleteBindingOcc_NP Nil = Nil
instance MyC' (x == BindingOcc) (x ': xs) ys => MyC (x ': xs) ys where
deleteBindingOcc_NP = deleteBindingOcc_NP' @(x == BindingOcc)
instance MyC a b => MyC' True (BindingOcc ': a) b where
deleteBindingOcc_NP' (_ :* xs) = deleteBindingOcc_NP xs
instance MyC a b => MyC' False (x ': a) (x ': b) where
deleteBindingOcc_NP' (x :* xs) = x :* deleteBindingOcc_NP xs
uj5u.com熱心網友回復：
我不知道你在做什么是個好主意......
但除此之外，這里有一些至少適用于您的簡單用例的東西。
type family Equal (a :: k) (b :: k) :: Bool where
Equal a a = True
Equal _ _ = False
type family IfThenElse (b :: Bool) (t :: a) (e :: a) where
IfThenElse True t _ = t
IfThenElse False _ e = e
class CanDecide (b :: Bool) where
ifthenelse :: Proxy b -> ((b ~ True) => r) -> ((b ~ False) => r) -> r
instance CanDecide True where ifthenelse _ x _ = x
instance CanDecide False where ifthenelse _ _ y = y
class CanDecide (Equal a BindingOcc) => IsBindingOcc a
instance CanDecide (Equal a BindingOcc) => IsBindingOcc a
type family DeleteBindingOcc (xs :: [Type]) :: [Type] where
DeleteBindingOcc '[] = '[]
DeleteBindingOcc (x : xs) = IfThenElse (Equal x BindingOcc) (DeleteBindingOcc xs) (x : DeleteBindingOcc xs)
class (All IsBindingOcc xs, DeleteBindingOcc xs ~ ys) => RelDeleteBindingOcc xs ys
instance (All IsBindingOcc xs, DeleteBindingOcc xs ~ ys) => RelDeleteBindingOcc xs ys
deleteBindingOcc_NP :: RelDeleteBindingOcc xs ys => NP f xs -> NP f ys
deleteBindingOcc_NP Nil = Nil
deleteBindingOcc_NP ((x :: f x) :* xs) =
let
ys = deleteBindingOcc_NP xs
in
ifthenelse (Proxy @(Equal x BindingOcc)) ys (x :* ys)
deleteBindingOcc_SOP :: SOP I '[ '[Var], '[Expr, Expr], '[BindingOcc, Expr]]
-> SOP I '[ '[Var], '[Expr, Expr], '[Expr]]
deleteBindingOcc_SOP arg = SOP $ trans_NS (Proxy @RelDeleteBindingOcc) deleteBindingOcc_NP (unSOP arg)
順便說一句，有一個漂亮的庫叫做generic-data-surgery（不幸的是我自己從來沒有用過）它聲稱擅長這種事情。
轉載請註明出處，本文鏈接：https://www.uj5u.com/qiye/389030.html