lookup environment variables, some of which do not exist. Handle whether exists or not.

Env.hs

module Main where

-- using http://hackage.haskell.org/package/base-4.12.0.0/docs/System-Environment.html#v:lookupEnv
import           System.Environment

main :: IO ()
main = do
  shell <- lookupEnv "SHELL" -- System.Environment.lookupEnv take a String
  doesnotexist <- lookupEnv "DOESNOTEXIST"
  print (checkEnvVars shell)
  print (checkEnvVars doesnotexist)


checkEnvVars :: Maybe String -> String
checkEnvVars Nothing  = "This value does not exist"
checkEnvVars (Just v) = "The enviroment has this, value is " <> v