Haskell：测试列表长度是否小于4

Question

I have the following haskell function that returns Nothing if the String is shorter than 4 characters:

  stringToMove :: String -> Maybe Move
  stringToMove [] = Nothing
  stringToMove [a] = Nothing
  stringToMove [a, b] = Nothing
  stringToMove [a, b, c] = Nothing
  stringToMove (l1:n1:l2:n2:rest) = Just (stringToCoords l1 n1, stringToCoords l2 n2)

I could test on length string , but as far as I read, this is not the haskell way of writing this, since if String is long, all of it will be evaluated.

Is there an elegant way of writing this without having to repeat Nothing in each line?

Answer 1

  stringToMove :: String -> Maybe Move
  stringToMove (l1:n1:l2:n2:rest) = Just (stringToCoords l1 n1, stringToCoords l2 n2)
  stringToMove _ = Nothing

Answer 2

I just feel like chiming in, if you have a lot of checks like this and you want to return a Maybe , you can use do notation:

stringToMove :: String -> Maybe Move
stringToMove s = do
    l1:n1:l2:n2:_ <- return s
    return (stringToCoords l1 n1, stringToCoords l2 n2)

You can do failable bindings as many times as you like, and if any of them fails, the function will return Nothing .