我可以写一个实例
-- In Data.Sequence.Internal
instance Lift a => Lift (Seq a) where
...
让用户将完全实现的序列提升到拼接中。但是假设我想要一些不同的东西来构建用于创建序列的函数?sequenceCode :: Quote m => Seq (Code m a) -> Code m (Seq a)
sequenceCode = ???
我的想法是我可以写出类似的东西triple :: a -> a -> a -> Seq a
triple a b c = $$(sequenceCode (fromList [[|| a ||], [|| b ||], [|| c ||]]))
并让该函数直接使用底层序列构造函数构建其序列,而不必在运行时构建和转换列表。写
sequenceCode
这样的东西并不难直接用于序列,使用它们的内部结构(看下面的跳转)。但是,顾名思义,sequenceCode
看起来很像 sequence
.有没有办法概括它?片刻反射(reflection)表明Traversable
是不够的。可以用 Generic1
做点什么吗?类(class) staged generics ?我做了一些尝试,但我对那个包的理解不够好,无法知道从哪里开始。即使只使用普通的旧 GHC 泛型也有可能吗?我开始怀疑是这样,但我还没有尝试过,它肯定会毛茸茸的。这是
Data.Sequence
的代码版本:{-# language TemplateHaskellQuotes #-}
import Data.Sequence.Internal
import qualified Language.Haskell.TH.Syntax as TH
class Functor t => SequenceCode t where
traverseCode :: TH.Quote m => (a -> TH.Code m b) -> t a -> TH.Code m (t b)
traverseCode f = sequenceCode . fmap f
sequenceCode :: TH.Quote m => t (TH.Code m a) -> TH.Code m (t a)
sequenceCode = traverseCode id
instance SequenceCode Seq where
sequenceCode (Seq t) = [|| Seq $$(traverseCode sequenceCode t) ||]
instance SequenceCode Elem where
sequenceCode (Elem t) = [|| Elem $$t ||]
instance SequenceCode FingerTree where
sequenceCode (Deep s pr m sf) =
[|| Deep s $$(sequenceCode pr) $$(traverseCode sequenceCode m) $$(sequenceCode sf) ||]
sequenceCode (Single a) = [|| Single $$a ||]
sequenceCode EmptyT = [|| EmptyT ||]
instance SequenceCode Digit where
sequenceCode (One a) = [|| One $$a ||]
sequenceCode (Two a b) = [|| Two $$a $$b ||]
sequenceCode (Three a b c) = [|| Three $$a $$b $$c ||]
sequenceCode (Four a b c d) = [|| Four $$a $$b $$c $$d ||]
instance SequenceCode Node where
sequenceCode (Node2 s x y) = [|| Node2 s $$x $$y ||]
sequenceCode (Node3 s x y z) = [|| Node3 s $$x $$y $$z ||]
然后在另一个模块中,我们可以定义triple
如上:triple :: a -> a -> a -> Seq a
triple a b c = $$(sequenceCode (fromList [[|| a ||], [|| b ||], [|| c ||]]))
当我用 -ddump-splices
编译它时(或 -ddump-ds
),我可以验证序列是直接构建的,而不是使用 fromList
.
最佳答案
我有 uploaded a package这样做。
原来GHC.Generics
足够了。但是,我实际上会使用 linear-generics
相反,因为它有一个更通用的 Generic1
版本.这个想法是,通过检查一个值的通用表示,我们可以构建我们需要的所有信息来为它生成一个模板 Haskell 代码。都是很低级的!首先,一些清嗓子:
{-# language TemplateHaskellQuotes #-}
{-# language FlexibleContexts #-}
{-# language FlexibleInstances #-}
{-# language ScopedTypeVariables #-}
{-# language DataKinds #-}
{-# language TypeOperators #-}
{-# language EmptyCase #-}
{-# language DefaultSignatures #-}
module Language.Haskell.TH.TraverseCode
( TraverseCode (..)
, sequenceCode
, genericTraverseCode
, genericSequenceCode
) where
import Generics.Linear
import Language.Haskell.TH.Syntax
(Code, Lift (..), Exp (..), Quote, Name)
import qualified Language.Haskell.TH.Syntax as TH
import Language.Haskell.TH.Lib (conE)
import Data.Kind (Type)
-- for instances
import qualified Data.Functor.Product as FProd
import qualified Data.Functor.Sum as FSum
import Data.Functor.Identity
import qualified Data.Sequence.Internal as Seq
import Data.Coerce
现在我们将进入事情的本质:class TraverseCode t where
traverseCode :: Quote m => (a -> Code m b) -> t a -> Code m (t b)
default traverseCode
:: (Quote m, GTraverseCode (Rep1 t), Generic1 t)
=> (a -> Code m b) -> t a -> Code m (t b)
traverseCode = genericTraverseCode
sequenceCode
:: (TraverseCode t, Quote m)
=> t (Code m a) -> Code m (t a)
sequenceCode = traverseCode id
genericSequenceCode
:: (Quote m, GTraverseCode (Rep1 t), Generic1 t)
=> t (Code m a) -> Code m (t a)
genericSequenceCode = TH.unsafeCodeCoerce . gtraverseCode id . from1
genericTraverseCode
:: (Quote m, GTraverseCode (Rep1 t), Generic1 t)
=> (a -> Code m b) -> t a -> Code m (t b)
genericTraverseCode f = TH.unsafeCodeCoerce . gtraverseCode f . from1
class GTraverseCode f where
gtraverseCode :: Quote m => (a -> Code m b) -> f a -> m Exp
为什么我们在这里使用无类型的模板 Haskell?简单:构建我们需要的表达式非常容易,但是弄清楚如何使类型对子表达式有用会很棘手。那么,当然,我们需要泛型实例。我们将一步一步地从外到内,一路收集信息。首先我们看一下类型的东西:
instance (Datatype c, GTraverseCodeCon f)
=> GTraverseCode (D1 c f) where
gtraverseCode f m@(M1 x) = gtraverseCodeCon pkg modl f x
where
pkg = packageName m
modl = moduleName m
这为我们提供了 GHC 用于包和模块的名称。接下来我们看看构造函数的东西:
class GTraverseCodeCon f where
gtraverseCodeCon :: Quote m => String -> String -> (a -> Code m b) -> f a -> m Exp
instance GTraverseCodeCon V1 where
gtraverseCodeCon _pkg _modl _f x = case x of
instance (GTraverseCodeCon f, GTraverseCodeCon g)
=> GTraverseCodeCon (f :+: g) where
gtraverseCodeCon pkg modl f (L1 x) = gtraverseCodeCon pkg modl f x
gtraverseCodeCon pkg modl f (R1 y) = gtraverseCodeCon pkg modl f y
instance (Constructor c, GTraverseCodeFields f)
=> GTraverseCodeCon (C1 c f) where
gtraverseCodeCon pkg modl f m@(M1 x) = gtraverseCodeFields (conE conN) f x
where
conBase = conName m
conN :: Name
conN = TH.mkNameG_d pkg modl conBase
有趣的情况是当我们到达一个实际的构造函数时(C1
)。这里我们从 Constructor
中获取构造函数的(非限定)名称。实例,并将其与包和模块名称结合起来以获得模板 Haskell Name
构造函数,我们可以从中构建一个引用它的表达式。这个表达式被传递到最低级别,我们处理字段。其余的基本上是这些领域的左折叠。class GTraverseCodeFields f where
gtraverseCodeFields :: Quote m => m Exp -> (a -> Code m b) -> f a -> m Exp
instance GTraverseCodeFields f => GTraverseCodeFields (S1 c f) where
gtraverseCodeFields c f (M1 x) = gtraverseCodeFields c f x
instance (GTraverseCodeFields f, GTraverseCodeFields g)
=> GTraverseCodeFields (f :*: g) where
gtraverseCodeFields c f (x :*: y) =
gtraverseCodeFields (gtraverseCodeFields c f x) f y
instance Lift p => GTraverseCodeFields (K1 i p) where
gtraverseCodeFields c _f (K1 x) = [| $c x |]
instance GTraverseCodeFields Par1 where
gtraverseCodeFields cc f (Par1 ca) =
[| $cc $(TH.unTypeCode (f ca)) |]
instance GTraverseCodeFields U1 where
gtraverseCodeFields cc _f U1 = cc
-- Note: this instance is *different* from the one that we'd
-- write if we were using GHC.Generics, because composition works
-- differently in Generics.Linear.
instance (GTraverseCodeFields f, TraverseCode g) => GTraverseCodeFields (f :.: g) where
gtraverseCodeFields cc f (Comp1 x) =
gtraverseCodeFields cc (traverseCode f) x
现在我们可以编写各种实例:instance TraverseCode Maybe
instance TraverseCode Identity
instance TraverseCode []
instance TH.Lift a => TraverseCode (Either a)
instance TH.Lift a => TraverseCode ((,) a)
instance (TraverseCode f, TraverseCode g)
=> TraverseCode (FProd.Product f g)
instance (TraverseCode f, TraverseCode g)
=> TraverseCode (FSum.Sum f g)
instance TraverseCode V1
-- The Elem instance isn't needed for the Seq instance
instance TraverseCode Seq.Elem
instance TraverseCode Seq.Digit
instance TraverseCode Seq.Node
instance TraverseCode Seq.FingerTree
对于Seq
例如,我需要手写一些东西,因为 Seq
不是 Generic1
的实例(我们不希望这样)。此外,我们并不真正想要派生实例。使用一点强制魔法,并知道一些关于如何zipWith
和 replicate
在序列上工作,我们可以最小化拼接的大小以及 GHC 编译到 Core 后必须处理的类型数量。instance TraverseCode Seq.Seq where
-- Stick a single coercion on the outside, instead of having a bunch
-- of `Elem` constructors on the inside.
traverseCode f s = [|| coerceFT $$(traverseCode f ft') ||]
where
-- Use zipWith to make the tree representing the sequence
-- nice and shallow.
ft' = coerceSeq (Seq.zipWith (flip const) (Seq.replicate (Seq.length s) ()) s)
coerceFT :: Seq.FingerTree a -> Seq.Seq a
coerceFT = coerce
coerceSeq :: Seq.Seq a -> Seq.FingerTree a
coerceSeq = coerce
关于haskell - 如何使用 Template Haskell 构建多态结构?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/70778223/