Haskell ViewPatterns example

Main.hs

#! /usr/bin/env nix-shell
#! nix-shell -i 'runghc -XViewPatterns' -p 'ghc.withPackages (pkgs: [ pkgs.containers ])'

module Main where

import Data.Map (Map)
import qualified Data.Map as Map

f :: Int -> Map Int String -> String
f k (Map.lookup k -> Just s) = s
f _ _                        = ""

main = do
  let m = Map.fromList [(0, "a"), (1, "b")]
  print $ f 0 m
  print $ f 1 m
  print $ f 2 m