Haskell recursive function to read input from user until a condition, and then return the input as list

Question

I'm making a function readUntil that reads lines from user. Reading stops when f returns True for a line. Then the lines are returned as a list (the line that returned True is not included in that list). Here's what I have at the moment (some really raw/pseudo code now, excuse me):

readUntil :: (String -> Bool) -> IO [String]
readUntil f = fmap reverse (go []) where
    go xs = do
    x <- getLine
    if f = True then return xs
                else go (x : xs)

I would also like to make readUntil recursive (or implement a recursive helper function).

@WillemVanOnsem I did a bit of a pseudo code to start with (since I'm kinda stuck), as in "check if f is True here". Sorry I should've explained it. — Aelin
– Aelin, Commented Apr 21, 2018 at 12:49
Well I have the idea that you are quite close to a valid solution. Here you want that f applied with parameter x is True, so I guess with a few hints you can come up with a solution. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Apr 21, 2018 at 12:53

rprospero · Accepted Answer · 2018-04-22 17:01:35Z

1

I've included a small example below. You're pretty much at the solution. The important bit is just remembering how you'd generate a non-monad recusive list, then just putting that in the monad.

collectUntil :: (Monad m) => m a -> (a -> Bool) -> m [a]
collectUntil act f = do
  x <- act
  if f x
    then return []
    else (x:) <$> collectUntil act f

readUntil :: (String -> Bool) -> IO [String]
readUntil = collectUntil getLine

answered Apr 22, 2018 at 17:01

rprospero

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