我一直在尝试构建一个替代品Lens.para，以在 para 函数工作时为它提供镜头上下文。但是，我似乎在某处的递归中犯了错误。
根据我对它的理解，它是递归代数数据类型中值Lens.para的一个变形函数。也就是说，它使用并采用一个函数来展开选项列表，以用于遍历一段数据的“自相似语法空间”，同时使其遍历数据上下文对函数可用这是工作。它的类型是，其中是数据上下文值的列表，并且是每个值的类型，它知道如何“查看”连续级别。platedLens.Plated a => (a -> [r] -> r) -> a -> r[r]a
我用于概念验证的极其简单的玩具示例数据类型如下：
data EExp a
= ELit a
| EAdd (EExp a) (EExp a)
deriving (Show, Eq)
所以，这是我的代码，包括现有的工作版本showOptions和我的新版本，showOptions'它使用我的自定义Lens.para，称为paraApp. 不同之处在于，Pretext它在工作时将 a 与数据一起传递，以便稍后我可以调整我的代码，以便在需要时利用它Pretext来调整原始数据结构。
{-# LANGUAGE RankNTypes, TemplateHaskell, ExplicitForAll, DeriveDataTypeable, StandaloneDeriving #-}
module StepThree where
import qualified Control.Lens as Lens
import qualified Data.Data as DD
import qualified Data.Data.Lens as DDL
import qualified Data.Maybe as DM
import qualified Data.List as DL
import Text.Read (readMaybe)
import StepThreeGrammar (EExp(..), pretty, run)
import Control.Comonad.Store.Class (pos, peek, ComonadStore)
import Control.Lens.Internal.Context (Pretext(..), sell)
import qualified Language.Haskell.Interpreter as I
import Language.Haskell.Interpreter (Interpreter, GhcError(..), InterpreterError(..))
instance DD.Data a => Lens.Plated (EExp a)
deriving instance DD.Data a => DD.Data (EExp a)
eg3' :: EExp Int
eg3' = EAdd (EAdd (EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)) (ELit 5)) (ELit 0)
showOptions :: (Lens.Plated a, Show a) => (a -> String) -> a -> [String]
showOptions showFn = Lens.para $ \a xs ->
let
sa = showFn a
(_,is) = DL.mapAccumL mapAccumFn (0, sa) xs
in
sa : concat is
where
mapAccumFn (n, acc) x =
let
i = pfxIndex (head x) acc
in
( (n+i+length (head x)
, drop (i+length (head x)) acc)
, map (replicate (n+i) ' ' ++) x)
showOptions' :: (Lens.Plated a, Show a) => (a -> String) -> a -> [String]
showOptions' showFn = paraApp $ \(a, ctx) xs ->
let
sa = showFn a
(_, is) = DL.mapAccumL mapAccumFn (0, sa) xs
in
sa : concat is
where
mapAccumFn (n, acc) x =
let
i = pfxIndex (head x) acc
in
( (n+i+length (head x)
, drop (i+length (head x)) acc)
, map (replicate (n+i) ' ' ++) x)
paraApp :: Lens.Plated a => ((a, Pretext (->) a a a) -> [r] -> r) -> a -> r
paraApp f x = go id (x, makePretextFocussingOnSelfFor x)
where
go p a =
let p' = Lens.plate . p
holes = Lens.holesOf p' x
in f a (go p' <$> (map (\c -> (pos c, c)) holes))
makePretextFocussingOnSelfFor x = Pretext ($ x)
pfxIndex :: Eq a => [a] -> [a] -> Int
pfxIndex x y = maybe 0 id (DL.findIndex (x `DL.isPrefixOf`) (DL.tails y))
如果我进入GHCi并执行以下代码，它会提供预期的输出：
*Main EditorTest StepThree Control.Lens> mapM_ putStrLn $ StepThree.showOptions show eg3'
EAdd (EAdd (EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)) (ELit 5)) (ELit 0)
EAdd (EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)) (ELit 5)
EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)
EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 1
ELit 5
ELit 0
当我不想对上下文做任何事情（比如更新原始值的特定部分）时，这很好
因此，当我尝试替换功能时，会发生以下情况（应该与上述相同）：
*Main EditorTest StepThree Control.Lens> mapM_ putStrLn $ StepThree.showOptions' show eg3'
EAdd (EAdd (EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)) (ELit 5)) (ELit 0)
EAdd (EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)) (ELit 5)
EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)
EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 1
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 5
EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 1
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 0
EAdd (EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)) (ELit 1)
EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 1
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 5
EAdd (EAdd (ELit 11) (ELit 9)) (ELit 3)
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
ELit 1
EAdd (ELit 11) (ELit 9)
ELit 11
ELit 9
ELit 3
ELit 11
ELit 9
显然我的递归在某个地方出错了，但我无法解决。与往常一样，我们将不胜感激任何帮助。
如果您不熟悉 的原始定义Lens.para，可以在https://hackage.haskell.org/package/lens-4.15.2/docs/src/Control.Lens.Plated.html#para找到