镜片和类型家庭 [英] Lenses and TypeFamilies

问题描述

我遇到了将Control.Lens与
一起使用的问题 -XTypeFamilies GHC编译指示时使用数据类型.

I've encountered a problem of using Control.Lens together with
datatypes while using the -XTypeFamilies GHC pragma.

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies    #-}

import Control.Lens (makeLenses)

class SomeClass t where
  data SomeData t :: * -> *

data MyData = MyData Int

instance SomeClass MyData where
  data SomeData MyData a = SomeData {_a :: a, _b :: a}

makeLenses ''SomeData

错误消息是:reifyDatatype: Use a value constructor to reify a data family instance.

有什么办法可以克服它，也许使用Control.Lens中的某些功能?

Is there any way to overcome it, maybe using some functional from Control.Lens?

推荐答案

最明智的做法是自行定义这些镜头……这并不困难:

The most sensible thing would be to just define those lenses yourself... it's not like it's very difficult:

a, b :: Lens' (SomeData MyData a) a
a = lens _a (\s a' -> s{_a=a'})
b = lens _b (\s b' -> s{_b=b'})

甚至

a, b :: Functor f => (a -> f a) -> SomeData MyData a -> f (SomeData MyData a)
a f (SomeData a₀ b₀) = (`SomeData`b₀) <$> f a₀
b f (SomeData a₀ b₀) =   SomeData a₀  <$> f b₀

...它根本不使用镜头库中的任何内容，但与所有镜头组合器完全兼容.

...which doesn't use anything from the lens library at all, but is fully compatible to all lens combinators.

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