a =
  case f x of
    1 -> 1
    2 -> 3
    3 -> 4
    _ -> 100

a =
  case f x of
    1 -> 1
    2 -> 3
    3 -> 4
    _ -> 100

a =
  case f x of
    1 -> 1
    2 -> 3
    3 -> 4
    _ -> 100

a =
  case f x of
    Nothing -> 1
    Just x  -> 3

a =
  case f x of
    Nothing -> 1
    Just x  -> 3

a =
  case f x of
    Nothing -> 1
    Just x  -> 3

a =
  case f x of
    Nothing -> do
      return 3
    Just x -> return ()

a =
  case f x of
    Nothing -> do
      return 3
    Just x -> do
      return 5