bracketOnError, while, forever

[Ashley Yakeley]

In article <878y60v8hk.fsf at peti.cryp.to>, Peter Simons <simons at cryp.to> 
wrote:

>   while :: (Monad m) => m Bool -> m a -> m ()
>   while cond f = cond >>= flip when (f >> while cond f)

I use this:

    while :: (Monad m) => m (Maybe a) -> m [a];
    while mma = do
    {
        ma <- mma;
        case ma of
        {
            Just a -> do
            {
                as <- while mma;
                return (a:as);
            };
            _ -> return [];
        };
    };

It is not tail-recursive however. I have a different function if I don't 
need results:

    whileDo :: (Monad m) => m Bool -> m ();
    whileDo mb = mb >>= \b -> if b then whileDo mb else return ();

I also have a highly generalised "for" function, but it uses my own 
Functor classes:

    for :: (ExtractableFunctor f,FunctorApplyReturn m) =>
     (a -> m b) -> (f a -> m (f b));
    for foo fa = fextract (fmap foo fa);

In my libraries, [] is an instance of ExtractableFunctor, and Monad is a 
subclass of FunctorApplyReturn (which has return and liftM2).

-- 
Ashley Yakeley, Seattle WA