TypeFamilies vs. FunctionalDependencies & type-level recursion

dm-list-haskell-prime at scs.stanford.edu dm-list-haskell-prime at scs.stanford.edu
Tue Jun 21 18:41:49 CEST 2011

At Tue, 21 Jun 2011 10:01:24 +0200,
José Pedro Magalhães wrote:
>     | One thing you could do to help in this specific case would be to use a
>     | different M1 tag--e.g., M1 S ... for selectors and M1 NS ... for
>     | fields without selectors (or K1 NS).  I presume you've already
>     | considered this and/or it's too late to make such a change.  (Or to
>     | move the distinction up to the constructor with two different
>     | constructor tags, CR and CN for record and no-record.)
>     I don't think it's too late to make a change.  The stuff has only just
>     gone in, so it's still very malleable.  There may be other considerations,
>     but legacy code isn't one of them!
> I suppose that could be changed, yes, but what exactly are we trying to solve
> here? One can already specify different behavior for constructors with/without
> named fields. Are we trying to avoid OverlappingInstances? Then yes, this
> might help, but I'm not sure this change alone would make all generic
> programming possible without OverlappingInstances.

Sorry, I wasn't necessarily advocating the change.  This came out of a
long discussion of UndecidableInstances.  Oleg's TYPEOF approach is
one way to avoid them (or at least to avoid them in conjunction with
OverlappingInstances).  Before Oleg posted his examples, Simon asked
if it might be possible to use the new Generic class in a similar way.
I was pointing out that no you can't, though the specific example I
gave could be accommodated by a small change to the Generic interface.


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