Haskell Exercise Mooc FI Do notation-CodePudding

This is the exercise from https://haskell.mooc.fi/ training

-- Ex 5: define the IO operation readUntil f, which reads lines from
-- the user and returns them as a list. Reading is stopped when f
-- returns True for a line. (The value for which f returns True is not
-- returned.)
--
-- Example in GHCi:
--   *Set11> readUntil (=="STOP")
--   bananas
--   garlic
--   pakchoi
--   STOP
--   ["bananas","garlic","pakchoi"]

readUntil :: (String -> Bool) -> IO [String]
readUntil f = todo

Would you be able to provide me a hint / solution using do notation ? I am a beginning with the do notation and the "conditional logic" as well as looping is too complex for me at the moment.

Thank you so much

CodePudding user response：

With only do-notation and conditional statements I found the following solution:

readUntil :: (String -> Bool) -> IO [String]          
readUntil f = do x <- getLine; 
                 if f x then (return []) else (do k <- readUntil f
                                                  return (x : k))

The function first reads one line with getLine and then checks whether (f x) is true. It then returns just the empty list. We can't just write ... if f x then [] ... because [] has not the type IO [String] but just [String]. To make [] the type IO [String] we can use the function return or pure but with do-notation I use the return function, because it is included in the Monad typeclass. If f x equals False we then use a second do-block to recursively call the function again and again until we get an Input for which f x == True and therefore returns the empty list. The do-notation is necessary because k must have type [String], but readUntil has the type IO [String]. We can't use the : ("cons") operator with object of type IO String and therefore can't generate the list we want. We then add x to the list k of all our other inputs and return it.