Generalise type of forever

[Stefan O'Rear]

On Sun, Jan 06, 2008 at 10:33:27PM -0800, Don Stewart wrote:
> It's been pointed out by a couple of people that the type of forever
> is overly constrained. We get a few more useful programs for free
> if we change the type:
> 
>     [generalise type of 'forever' from returning m () to m a
>     Don Stewart <dons at galois.com>**20080107062656] {
>     hunk ./Control/Monad.hs 42
>     -    , forever       -- :: (Monad m) => m a -> m ()
>     +    , forever       -- :: (Monad m) => m a -> m a
>     hunk ./Control/Monad.hs 190
>     -forever     :: (Monad m) => m a -> m ()
>     +forever     :: (Monad m) => m a -> m a
>     }
> 
> In particular, fail breaks out nicely.
> 
>     > forever Nothing
>     Nothing
> 
> Deadline: 12th January.
> 
> -- Don

Your new type is not more general, for it no longer satisfies e.g.
"Maybe Int -> Maybe ()".

I counter-propose using the principal type, m a -> m b.

Stefan
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