组合镜头和棱镜用于产品的总和？ [英] Combo lenses and prisms for sums of products?

问题描述

如果我有记录类型，我可以用镜头做任何我想做的事情。如果我有一个总和类型，我可以做任何我想要的棱镜。但是，如果我有一笔包含记录的金额， makeFields 不会将镜头放入字段中（当然），但只能对它们进行遍历。 declarePrisms 似乎更有前景。根据文件，

  declarePrisms [d | 
数据Exp = Lit Int | Var字符串| Lambda {bound :: String，body :: Exp} 
 |] 
  

将创建

 数据Exp = Lit Int | Var字符串| Lambda {bound :: String，body :: Exp} 
 _Lit :: Prism'Exp Int 
 _Var :: Prism'Exp String 
 _Lambda :: Prism'Exp（String，Exp）
  

这让我几乎在那里，但我真正想要的更像是这样：

  data Exp = Lit Int | Var字符串| Lambda字符串Exp 
数据LambdaRec = {_bound :: String，_body :: Exp} 
 ... 
 _Lambda :: Prism'Exp LambdaRec 
  - LambdaRec，
  - 理想情况下也可以遍历在Exp中查看它们。 
 class MightBeLambda t其中
类型BoundOptic t 
类型BodyOptic t 
 bound :: BoundOptic t 
 body :: BodyOptic t 
实例MightBeLambda Exp其中
 type BoundOptic Exp = Traversal'Exp String 
 ... 
 instance MightBeLambda LambdaRec其中
类型BoundOptic LambdaRec = Lens'LambdaRec字符串
  

是否有任何方法可以自动执行此类操作，还是需要手动执行？

< hr>

有人可能希望这样做：

  data ExpTag = LitT | VarT | LambdaT 
 
数据Exp':: ExpTag  - > *其中
 Lit':: Int  - > Exp'LitT 
 Var':: String  - > Exp'VarT 
 Lambda':: {_bound :: String，_body :: Exp}  - > Exp'LambdaT 
  

然后棱镜可以被定义为evilly，使用 unsafeCoerce ，以避免任何复制记录的风险。 行为。 （makePrisms在应用于具有单个构造函数的类型时生成Isos）

  { - ＃LANGUAGE TemplateHaskell＃ - } 
 
模块演示其中
 
导入Control.Lens 
 
数据Exp = Lit Int | Var字符串| Lambda字符串Exp 
数据LambdaRec = LambdaRec {_bound :: String，_body :: Exp} 
 
 makePrisms''Exp 
 makePrisms''LambdaRec 
 makeLenses''LambdaRec 
 
 _ExpLambdaRec :: Prism'Exp LambdaRec 
 _ExpLambdaRec = _Lambda。 from _LambdaRec 
 
  - 使用_ExpLambdaRec的示例
 expBound :: Traversal'Exp字符串
 expBound = _ExpLambdaRec。绑定
  

请注意，由于GHC可以做到的优化，因此不需要使用此中间记录类型生成的代码。

  getBound :: Exp  - >可能String 
 getBound =预览expBound 
 
  - 为getBound生成的核心
  -  
  -  getBound1 = 
  -  \ eta_B1  - > 
  -  _ {
  -  __DEFAULT  - >的案例eta_B1 （Nothing）`cast` ...; 
  -  Lambda y1_a6XB y2_a6XC  - > （Just y1_a6XB）`cast` ... 
  - } 
  

If I have a record type, I can do pretty much anything I want to it with lenses. If I have a sum type, I can do pretty much whatever I want to it with prisms. But if I have a sum that includes a record, makeFields doesn't give me lenses into the fields (of course), but only traversals for them. declarePrisms seems a bit more promising. According to the documentation,

declarePrisms [d|
  data Exp = Lit Int | Var String | Lambda{ bound::String, body::Exp }
  |]

will create

data Exp = Lit Int | Var String | Lambda { bound::String, body::Exp }
_Lit :: Prism' Exp Int
_Var :: Prism' Exp String
_Lambda :: Prism' Exp (String, Exp)

This gets me almost there, but what I really want is more like this:

data Exp = Lit Int | Var String | Lambda String Exp
data LambdaRec = { _bound::String, _body::Exp }
...
_Lambda :: Prism' Exp LambdaRec
-- bound and body lenses into LambdaRec,
-- and ideally also traversals to look at them in Exp.
class MightBeLambda t where
  type BoundOptic t
  type BodyOptic t
  bound :: BoundOptic t
  body :: BodyOptic t
instance MightBeLambda Exp where
  type BoundOptic Exp = Traversal' Exp String
  ...
instance MightBeLambda LambdaRec where
  type BoundOptic LambdaRec = Lens' LambdaRec String

Is there any way to do something like this automatically, or would I have to do it by hand?

---

An even crazier way one might wish to do it:

data ExpTag = LitT | VarT | LambdaT

data Exp' :: ExpTag -> * where
  Lit' :: Int -> Exp' LitT
  Var' :: String -> Exp' VarT
  Lambda' :: { _bound::String, _body::Exp } -> Exp' LambdaT

Then the prisms can be defined evilly, using unsafeCoerce to avoid any risk of copying records.

解决方案

You can take a detour through another generated Iso to get this behavior. (makePrisms generates Isos when applied to a type with a single constructor)

{-# LANGUAGE TemplateHaskell #-}

module Demo where

import Control.Lens

data Exp = Lit Int | Var String | Lambda String Exp
data LambdaRec = LambdaRec { _bound::String, _body::Exp }

makePrisms ''Exp
makePrisms ''LambdaRec
makeLenses ''LambdaRec

_ExpLambdaRec :: Prism' Exp LambdaRec
_ExpLambdaRec = _Lambda . from _LambdaRec

-- Example using _ExpLambdaRec
expBound :: Traversal' Exp String
expBound = _ExpLambdaRec . bound

Note that thanks to optimizations that GHC can do that this intermediate record type isn't necessarily used in the generated code.

getBound :: Exp -> Maybe String
getBound = preview expBound

-- Generated core for getBound
--
-- getBound1 =
--   \ eta_B1 ->
--     case eta_B1 of _ {
--       __DEFAULT -> (Nothing) `cast` ...;
--       Lambda y1_a6XB y2_a6XC -> (Just y1_a6XB) `cast` ...
--     }

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