Haskell While Loop « It Seemed Like a Good Idea at the Time

15Sep/092

Haskell While Loop

In the past week, while working on three completely unrelated Haskell projects, I have found myself in need of a while loop. So I came up with this one:

-- | For some reason Control.Monad doesn't provide a 'while'
-- function, even though it has a 'forever' function.
while :: Monad m => m Bool -> m a -> m ()
while predicate action = do
b <- predicate
if b then action >> while predicate action
else return ()

Hoogle shows no functions with that type signature, and I couldn't find anything in the documentation that seemed to fit the bill. This seems like a rather fundamental function for getting stuff done, is there something I'm missing?

EDIT: The most code-golfed version I have yet come up with is while p a = p >>= flip when (a >> while p a) can anyone improve on that?

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brian
September 15th, 2009 - 22:20

Maybe

when b $ action >> while predicate action

is a little better.
Will Donnelly
September 15th, 2009 - 22:33

True. I hadn’t thought of that.

flip when

seems to be even more concise in the long run:

while p a = p >>= flip when (a >> while p a)

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