[Haskell-beginners] untilM and scanM
Brent Yorgey
byorgey at seas.upenn.edu
Sun Jan 11 11:26:46 EST 2009
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
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