haskell - 仅从模块中导出 getter 或 setter

Question

有没有办法让我只从带有镜头的模块中导出特定的 getter xor setter？

例如，让我们假设一个数据结构具有始终为的不变量，>= 0仅通过递增它进行修改，并且仅使用初始值创建0：

module Something
    ( Counter
    -- export only `count` getter
    , make
    , increment
    ) where

data Counter = Counter { _count :: Int } deriving (Eq)
makeLenses ''Positive

make :: Counter
make = Counter 0

increment :: Counter -> Counter
increment c = c ^. count %~ (+1)

我怎么能只导出count吸气剂？

Answer 1

A lens isn't, in fact, "a getter and a setter", it just happens to be isomorphic to such a pair. So you can't just export one of them, rather you have to define something new and export that. Fortunately, this is extremely simple:

data Counter = Counter { _count' :: Int } deriving (Eq)
makeLenses ''Counter

count :: Getter Counter Int
count = count'

Answer 2

如果您只想生成Getter和Fold光学（视情况而定），您可以使用新generateUpdateableOptics设置

{-# LANGUAGE TemplateHaskell #-}
import Control.Lens

data Counter = Counter { _count :: Int } deriving (Eq)

let rules = set generateUpdateableOptics False lensRules in
  makeLensesWith rules ''Counter

-- Generates:
-- count :: Getter Counter Int