Implementing Haskell#lines

Question

Learn You a Haskell shows the lines function:

It takes a string and returns every line of that string in a separate list.

Example:

>lines' "HELLO\nWORLD\nHOWAREYOU"
["HELLO","WORLD","HOWAREYOU"]

Here's my implementation:

lines' :: String -> [String]
lines' [] = []
lines' xs = lines'' xs []
 where lines'' [] ys = ys : []
 lines'' (x:xs) ys 
 | x == '\n' = ys : lines'' xs []
 | otherwise = lines'' xs (ys ++ [x])

Please critique it. Also, when using an accumulator value (such as ys in lines''), I don't know how to use the : function instead of ++.

Answer 1

1. What happens when a string ends in a newline? lines "\n" ⇒ [""], but lines' "\n" ⇒ ["", ""].

2. ys could have a more informative name. line?

3. (削除) When you find yourself wanting to add new items to the end of a list, consider keeping the list in reverse order, adding items to the front and reversing it when it's done. This avoids needing to use ++. (削除ここまで) ("Done" is a fuzzy concept when working lazily with infinite lists.)

4. ...but when reinventing the wheel, you needn't reinvent the axle and bearings as well. There is a Prelude function that will parse one line from a string: break. If you use that, you need only handle recursing on the remainder. This is how lines is really implemented.

   lines s = let (l, s') = break (== '\n') s
    in l : case s' of
    [] -> []
    (_:s'') -> lines s''
   

• Kevin Meredith: thanks, Anonymous. Excellent suggestions for improvement. Would you implement #1 by simply adding that special case lines' "\n" = [""]?
• Anonymous: No, because it wouldn't cover other strings ending in newlines (e.g. lines "a\n" ⇒ ["a"]), and because it can be done without a special case by recursing appropriately.