Syntax extensions: mdo and do...rec

[Amr A Sabry]

> Another approach to value recursion was recently given by Sabry and Moggi
> in their latest FICS paper: They have an mfix-like operator for
> continuations there, but it doesn't satisfy left-shrinking either.

The paper in question is available at:

    http://www.cs.indiana.edu/~sabry/papers/valuerecursion_extended.ps

> (Their treatment of value recursion is rather different than ours as
> well.) See Amr Sabry's papers for details. 

The typing of mfix given in our paper is: (m a -> m a) -> m a; by
changing the (ret e) to e in rule M.2 you get exactly the same mfix of
Erkok/Launchbury. 

The paper also shows why left-shrinking cannot hold in general and gives
an example for the continuation monad.

--Amr