content: import System.Environment.Blank
  
  main = do
    let envVar = "AN_ENVIRONMENT_VARIABLE"
  
    valueBeforeSettingVariable <- getEnv envVar
    print valueBeforeSettingVariable -- Nothing
  
    valueWithDefaultBeforeSetting <- getEnvDefault envVar "DEFAULT"
    print valueWithDefaultBeforeSetting -- "DEFAULT"
  
    setEnv envVar "" False
  
    valueAfterSettingVariable <- getEnv envVar
    print valueAfterSettingVariable -- Just ""
  
    valueWithDefaultAfterSetting <- getEnvDefault envVar "DEFAULT"
    print valueWithDefaultAfterSetting -- ""
  
    valueFromGetEnvironment <- lookup envVar <$> getEnvironment
    print valueFromGetEnvironment -- Just ""
  
    setEnv envVar "NO_OVERRIDE" False
  
    valueAfterSettingWithExistingValueAndOverrideFalse <- getEnv envVar
    print valueAfterSettingWithExistingValueAndOverrideFalse -- Just ""
  
    setEnv envVar "OVERRIDE" True
  
    valueAfterSettingWithExistingValueAndOverrideTrue <- getEnv envVar
    print valueAfterSettingWithExistingValueAndOverrideTrue -- Just "OVERRIDE"
  
    unsetEnv envVar
  
    valueAfterUnsettingVariable <- getEnv envVar
    print valueAfterUnsettingVariable -- Nothing
  