Syntax extensions: mdo and do...rec

[Brandon Michael Moore]

On Thu, 18 Sep 2003, Ross Paterson wrote:

> The arguments being made here can all be found in the recursive monad
> bindings papers and Levent's thesis.

I don't remember anything about finding smaller binding groups in the
mdo paper. I don't think I've read Levent's thesis.

> On Wed, Sep 17, 2003 at 11:41:24AM -0700, Brandon Michael Moore wrote:
> > In any case, I don't see the need for explicit rec groups. Can't GHC just
> > find the strongly connected components like it already does with let
> > bindings?
>
> That's what GHC and Hugs do now for mdo (actually segments rather than
> components, because actions can't be rearranged).
>
> > Don't the laws for loop and mfix justify the transformation?
>
> The loop axioms do, but Levent didn't assume right tightening, which
> corresponds to moving bindings down from a rec, because monads like
> exceptions don't satisfy it.  The same would go for a loop defined on
> an exception arrow.  And that's the biggest problem with implicit
> segmentation: you need to understand what it does to work out the
> meaning of your program.  Again there's an example in those papers
> and Levent's thesis.

I expected any problems would be like that. I remember hearing about a
fixpoint operator for the continuation monad that satisfied all the laws
but right tightening. Well, this would fall under "If it really turns out
to be frequently necessary".

> BTW, in GHC 6.2 with the -fglasgow-exts -farrows flags, you will be able
> to use either mdo or do...rec for monads and for arrows, as an experiment.
> (Maybe "rec" wasn't such a great keyword to take from the identifier
> space.)

When can we expect 6.2?

Brandon