可组合透镜

Question

我有以下 Haskell 类型

import Control.Lens
import Data.Map.Lens
import qualified Data.Map as M

type RepoStats = M.Map String (M.Map String RepoStat)
data RepoStat = RepoStat {
  _full_name :: String,
  _bug_count :: Maybe Int,
  _commit_count :: Maybe Int
  }
  deriving (Generic, Show, Eq, Read)
makeLenses ''RepoStat

我希望能够抽象出嵌套的值检查并给出默认值：

makeStat name bug commit =
    RepoStat
    { _full_name = name
    , _bug_count = Just bug
    , _commit_count = Just commit
    }

getRepoStat :: String -> String -> (RepoStat -> Identity RepoStat) -> RepoStats -> Identity RepoStats
getRepoStat k1 k2 = at k1 . non (M.empty) . at k2 . non (makeStat k2 0 0)

fakeMap = M.empty :: RepoStats
setBug = fakeMap & (getRepoStat "a" "b") . bug_count ?~ 4
getBug = fakeMap ^. (getRepoStat "a" "b") . bug_count

setBug 可以编译并工作，但 getBug 不能。

test/Spec.hs:65:21: Couldn't match type ‘Identity RepoStats’ …
                   with ‘Const RepoStat RepoStats’
    Expected type: Getting RepoStat RepoStats RepoStat
      Actual type: (RepoStat -> Identity RepoStat)
                   -> RepoStats -> Identity RepoStats
    In the second argument of ‘(^.)’, namely ‘(getRepoStat "a" "b")’
    In the expression: fakeDb ^. (getRepoStat "a" "b")
Compilation failed.

能不能这样抽象出一个镜头？

Answer 1

确实如此。您为 getRepoStat 指定的类型签名是其类型签名作为 setter. If you want to be able to use it as a lens 的完全通用性，那么类型签名应该是

getRepoStat :: Functor f => String -> String -> (RepoStat -> f RepoStat) -> RepoStats -> f RepoStats
-- same definition

这可能也是getRepoStat的推断类型。

Answer 2

只删除类型签名很有用：

getRepoStat k1 k2 = at k1 . non M.empty . at k2 . non (makeStat k2 0 0)

现在我们可以在 ghci 中使用 :t getRepoStat 查看推断类型。

有点乱，主要是因为at超载了：

getRepoStat
  :: (Functor f, At m, IxValue m ~ M.Map String RepoStat) =>
     Index m -> String -> (RepoStat -> f RepoStat) -> m -> f m

我们可以认识到我们 return 一个 Lens' 我们也可以将镜头的上下文限制为 RepoStats:

getRepoStat :: String -> String -> Lens' RepoStats RepoStat
getRepoStat k1 k2 = at k1 . non M.empty . at k2 . non (makeStat k2 0 0)

可组合透镜

Composable Lensing

haskell

haskell-lens

lenses