Subject: Re: Haskell-prime Digest, Vol 2, Issue 6
From: Henrik Nilsson <[email-masked]>
To: Simon Peyton-Jones <[email-masked]>
Date: Thu, 02 Feb 2006 15:50:26 +0000

Dear all,

Simon PJ wrote:

 > Now, someone can try to define a conservative version of FDs, as has
 > been suggested; perhaps it could even be kind-of-isomorphic to
 > associated types.  Perhaps something *very* conservative would be
 > enough for most applications.  (It would hurt to standardise FDs if we
 > think we're going to discard them subsequently, but if the spec was
 > somewhat crisp maybe it would not hurt too much.)

What about the "FD Conditions" as defined in the paper by Sulzman, Duck,
Peyton-Jones and Stuckey you referred to? (Mark Jones original
conditions + a so called "bound variable condition").
If those conditions are imposed, one gets sound, complete,
and decidable type inference according to the said paper.

I had a quick look at the monadic libraries and a vector space class in
Yampa, and my impression is that the FDs used there would be compatible
with the FD conditions. But I emphasize that's just an impression.

Or are the FD conditions considered too restrictive?
Clearly, as exemplified in the above paper, they do rule out a number of 
reasonable programs. But as long as the adopted version of FDs
allow the basic usage of FDs for applications like monad transformers,
I don't think I personally would be too unhappy.

Best,

/Henrik

-- 
Henrik Nilsson
School of Computer Science and Information Technology
The University of Nottingham
[email-masked]