Pointless Lenses This cabal package can be installed with: $ cabal install pointless-lenses For a manual install, execute: $ runhaskell Setup.lhs configure $ runhaskell Setup.lhs build $ runhaskell Setup.lhs install You can now start playing with the example code that comes with the library, under Language.Pointless.Examples.Examples. The easiest way is to create a new module that imports some library modules module Test where import Test.QuickCheck.Test import Generics.Pointless.Lenses import Generics.Pointless.Lenses.Examples.Examples and interpret it $ ghci Test.hs We can now test the well-behavedness of some lens, for example: > quickCheck (wb filter_left_lns) +++ OK, passed 100 tests. Or run some example and later check what it is actually doing: > put filter_left_lns ([1,2,3],[Left 0,Right 'a',Left 4]) [Left 1,Right 'a',Left 2,Left 3]