Haskell iso lens example

Main.hs

#! /usr/bin/env nix-shell
#! nix-shell -i 'runghc -XOverloadedStrings -XLambdaCase' -p 'ghc.withPackages (pkgs: [ pkgs.lens pkgs.text ])'

module Main where

import Control.Lens
import Data.Text

data Route
  = ForgotPassword
  | Logout
  | GetUser Text
  | FourOhFour
  deriving (Show)

mkRoute :: [Text] -> Route
mkRoute = \case
  ["forgot"]    -> ForgotPassword
  ["logout"]    -> Logout
  ["user", uid] -> GetUser uid
  _             -> FourOhFour

unMkRoute :: Route -> [Text]
unMkRoute = \case
  ForgotPassword -> ["forgot"]
  Logout         -> ["logout"]
  GetUser uid    -> ["user", uid]
  FourOhFour     -> ["404"]

routeIso :: Iso' [Text] Route
routeIso = iso mkRoute unMkRoute

main :: IO ()
main = do
  print $ view routeIso ["logout"]
  print $ view routeIso ["nonsense"]
  print $ view (from routeIso) ForgotPassword

Output:

$ ./Main.hs
Logout
FourOhFour
["forgot"]