bracketOnError, while, forever

[Malcolm Wallace]

Peter Simons <simons at cryp.to> writes:

> On a similar note, I find myself using two rather simple
> combinators frequently but have no place where to put them:
> 
>   while :: (Monad m) => m Bool -> m a -> m ()
>   while cond f = cond >>= flip when (f >> while cond f)
> 
>   forever :: (Monad m) => m a -> m ()
>   forever f = while (return True) f
> 
> 'while', at least, would a nice addition to Control.Monad,

I agree that these sorts of combinators are frequently useful.  However,
there is a reasonable variety in the possible signatures one might
assign to the control-flow notion of "while".  For instance, how about

    while :: Monad m => Bool -> m Bool -> m ()
    while True f  = f >>= \b-> while b f
    while False f = return ()

There are other control-flow analogies like

    until :: Monad m => m Bool -> m ()
    until f = f >>= \b-> if b then return () else until f

    for :: Monad m => Int -> m a -> m ()
    for   0   f = return ()
    for (n+1) f = f >> for n f

which probably also have a few possible monadic variations.

Regards,
    Malcolm