[Haskell-cafe] Instance Chains selection by class membership [was: [ghc-proposals/cafe] Partially applied type families]

Anthony Clayden anthony_clayden at clear.net.nz
Mon Jun 5 01:11:00 UTC 2017

> On Sun Jun 4 13:11:47 UTC 2017, J. Garrett Morris wrote:

>> On Sun, Jun 4, 2017 at 1:00 PM, Anthony Clayden wrote:
>> ...
> You have reiterated a point that is made at length ...

Good, so we're agreed that the 2010 paper is inconsistent
to reject `(f :+: f)` but accept `((f :+: f) :+: f)`.

> I shouldn't, however, have suggested in 2015 that
> the deduplicated version is necessarily preferable.

I don't follow. The 2015 version is consistent.

>  If you omit the deduplication constraints, you simply end
> up with Leijen-style scoped rows [Leijen, TFP05] ...

Ouch! Its "A novel aspect of this work is that
 records can contain duplicate labels, " is about the most
blood-chilling claim I've seen in the many papers on records
It is true, though, compared to what you seem to suggest
"effectively introducing a form of scoping over the labels.

If `((f :+: g) :+: (h :+: f))` is to be supported,
what's the basis for preferring `f` to the left or right?

> ... Of course, if, as we did in 2010, you use classes
> of branching to implement elimination on variants, you
have no way to
> tell the difference between the two models of variants,

Is the only point I was wanting to make:
using a single class fails to separate concerns,
in this example.

>> I'm working on showing how the Swierstra encoding
>> could be catered for in Haskell 2010 + MPTCs+TypeEq.

> You keep talking about guarding instances or equations. 
> Guards are equivalent to closed type families,

I think not. They're somewhere between CTFs and Instance
In that guarded instances are not closed.
Which also appears to be your claim wrt Instance Chains.

> ... so of course they're sufficient to implement
> Swierstra's encoding (following any of ...), 

(I was specifically aiming to implement
 using Haskell as at 2010, but let it pass ...)

Yes any of these approaches are "sufficient".
That's not in debate.

> and will require auxiliary definitions and indirections

Yes, see code I posted subsequent to your message.

> that would not be required using instance chains.

Huh? Your 2015 paper uses auxiliaries class `In`
Figure 3 in the Instance Chains version.

True that the 2010 paper uses no auxiliaries,
but it thereby ends up inconsistent.

What is "wrong" with using auxiliaries?
(Again, in terms of readability.)


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