我有以下的Haskell類型可組合倫辛
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
我希望能夠抽象出嵌套檢查值,並給予默認:
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編譯和作品,但getBug不。
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.
是否有可能以這種方式抽象出一個鏡頭?