如何使用镜头语法检查地图是否有密钥？

Question

如何使用镜头语法检查地图是否有密钥？

import qualified Map as Map

let x = Map.member "bla" m

如何使用镜头写这个？

Answer 1

使用has。

M.fromList [("bla", ())] & has (ix "bla")

Answer 2

Use the Contains instance.

>>> Map.fromList [("bla", ())] ^. contains "bla"
True

>>> Map.fromList [("bla", ())] & contains "bla" .~ False
Map.fromList []