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

ys could have a more informative name. line?

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 ++.

...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.

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

ys could have a more informative name. line?

(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.)

...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''

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

ys could have a more informative name. line?

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 the use of ++.

...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.

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

ys could have a more informative name. line?

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 ++.

...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.