How to change this into a while loop in Haskell?

Question

Haskell is challenging! What I figured out so far is that I can do the following to simulate a for-loop in Haskell to get a list of numbers from the user:

myList <- sequence [putStr "Enter an integer: " >> (
    \s -> read s :: Int) <$> getLine | t <- [1..5]]

Great! So myList contains five integers that I have entered. Great! But here's the catch. Instead of a for-loop that iterates five times (or any finite number of times) how can I convert the above to the equivalent while-loop?

So what I'm thinking of is this, but it won't work, unfortunately. Is there some "magic" way to make it work?

takeWhile (\x -> x > 0) $ sequence [putStr "Enter an integer: " >> (
    \s -> read s :: Int) <$> getLine | t <- [1..]]

The problem is that (\x -> x > 0) works with Ints. (Or any Num type.) But what's coming from that list is really a bunch of IO Ints. x > 0 returns a Bool. I need a function that returns an IO Bool? I'm a little lost. Can someone point the way to Haskell enlightenment for me?! Studying this on my own isn't exactly easy!!! Thank you so much!!!

Also I don't know if you noticed, but your code snippets don't compile. — MikaelF
– MikaelF, Commented Mar 23, 2020 at 5:14
It's hard to see how forever could ever yield something that acts like a while loop. — amalloy
– amalloy, Commented Mar 23, 2020 at 5:14
The first code snippet DOES compile! Unless there's a typo in there, but trust me, I tested it several times and it compiles. The second code snippet, with "takeWhile" however, does not compile, but I just wanted people to get an idea of what I was trying to do. — Douglas Lewit
– Douglas Lewit, Commented Mar 24, 2020 at 17:18

amalloy · Accepted Answer · 2020-03-23 08:26:59Z

6

You cannot write this program with a sequence of an infinite list of IO actions. Any operations you perform "outside" of the sequence will be unable to inspect its contents, and any operations inside the sequence will be unable to stop it from continuing.

Instead, you must write an IO action which reads an Int, inspects it, and decides whether to continue or to stop at that time.

positiveInts :: IO [Int]
positiveInts = do
  putStr "Enter an integer: "
  i <- readLn
  if i <= 0
    then pure []
    else (i:) <$> positiveInts

edited Mar 23, 2020 at 8:26

answered Mar 23, 2020 at 5:19

amalloy

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Comments

Redu · Accepted Answer · 2020-03-23 16:18:00Z

0

@amalloy's answer is great. It's kind of conditional sequencing.

Perhaps we may attempt generalizing it further by inventing takeWhileM (basically conditional sequencing) which sequences only an initial part of an indefinitely long list of actions while a predicate satisfies.

takeWhileM :: Monad m => (a -> Bool) -> [m a] -> m [a]
takeWhileM f (a:as) = a >>= \n -> if f n then (n:) <$> takeWhileM f as
                                         else pure []

So for this particular case i run it like

λ> takeWhileM (> 0) . repeat $ putStr "Enter an integer:" >> readLn
Enter an integer:1
Enter an integer:2
Enter an integer:3
Enter an integer:4
Enter an integer:5
Enter an integer:0
[1,2,3,4,5]

edited Mar 23, 2020 at 16:18

answered Mar 23, 2020 at 15:02

Redu

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7 Comments

amalloy Over a year ago

This isn't what I'd expect a function named takeWhileM to do. The one in monad-loops has type Monad m => (a -> m Bool) -> [a] -> m [a], which better fits with functions like filterM or mapM.

Redu Over a year ago

@amalloy Yes.. you are possibly right.. we may perhaps call this function sequenceWhile for a better naming convention.

Douglas Lewit Over a year ago

Hey there @amalloy, wow! Thanks so much!!!! This is exactly what I was looking for. Is Haskell your primary language? I'm still at the dabbler stage. I think Haskell is really interesting, and I want to continue working with it, but not sure if I'll ever reach any high level or professional level of skill in the language. Maybe? Could be Nothing or Just success!!! Hahahaha. Just an FP joke. Okay, thanks again. This was super helpful, and your takeWhileM function was not hard to read. By the way, can you recommend any resources for Haskell wannabes who are not geniuses?

Redu Over a year ago

@amalloy I have just discovered that it was needless to rediscover sequenceWhile since it already lives in Control.Monad.Extra which fits the job like a glove. So yes.. it should be called squenceWhile. :)

Redu Over a year ago

@Douglas Lewitt I believe you address me... As @amalloy mentions this function should not be called takeWhileM since by convention that name fits to another type signature as mentioned in his above comment. This function should be called sequenceWhile instead. Don't let Haskell intimidate you. Once you (can) forget about what you know about programming Haskell starts making so much sense. All you need is time so never give up. Learn You a Haskell for Great Good is a good starting point.

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