On Mon, Jan 05, 2009 at 02:16:11PM +0100, Jan Snajder wrote:
Hi,
is there a reason why there is no monadic version of "until" in the Haskell libraries? It would be defined as follows:
untilM :: (Monad m) => (a -> Bool) -> (a -> m a) -> a -> m a untilM p f x | p x = return x | otherwise = f x >>= untilM p f
The same applies to scanM, also not part of the libraries:
scanM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m [a] scanM f q [] = return [q] scanM f q (x:xs) = do q2 <- f q x qs <- scanM f q2 xs return (q:qs)
I often find myself in need for these. To me these seem idiomatic enough to be included in the library. But since they is not, I guess there must be another, more idiomatic way to do this.
There's no particular reason these aren't in the standard libraries that I know of. I've written untilM myself once or twice. Perhaps there are multiple slightly different ways to implement them and no one can agree; or perhaps no one has ever proposed adding them. But there's no more idiomatic way to do this that I know of. -Brent