TypeFamilies vs. FunctionalDependencies & type-level recursion
José Pedro Magalhães
jpm at cs.uu.nl
Wed Jun 22 11:08:47 CEST 2011
2011/6/21 <dm-list-haskell-prime at scs.stanford.edu>
> 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
> > gone in, so it's still very malleable. There may be other
> > but legacy code isn't one of them!
> > I suppose that could be changed, yes, but what exactly are we trying to
> > here? One can already specify different behavior for constructors
> > 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.
Ah, I see. Oleg's TYPEOF encodes only the type arguments to a datatype,
whereas Rep encodes the structure of the datatype. As far as I can tell,
TYPEOF never depends on the right-hand side of the datatype. A simple
example is some datatype with a phantom type, say `data P a = P`. TYPEOF
will encode the `a`, whereas Rep will not.
So now it is very clear to me that you cannot get the TYPEOF if you have the
Rep, in general.
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