Records in Haskell: Type-Indexed Records (another proposal)

[AntC]

Matthew Farkas-Dyck <strake888 <at> gmail.com> writes:

> 
> > I think you don't want all those type vars in your record decls -- but only
> > vars for the mutatable types, like this:
> >
> >   type R c = { X ::. Int, Y::. String, Z ::. c, ... }
> >
> > Then you don't need a Quasifunctor instance for every field, only the
> > mutatable ones.
> 
> Yes, I know. That is just a very general example.
> 
> > Oh, and how do you deal with multiple record constructors as in H98:
> >    data T a = T1 { x :: a, y :: Bool }
> >             | T2 { x :: a }
> >
> Not sure what you mean. With an argument of such a multiconstructed
> type, I would do as ever in Haskell: pattern-match.

So please show what record declarations look like. And how they get turned 
into Has instances. Your example decl on the wiki for R a b c is not valid 
Haskell. What's more the wiki hss it as a `type`. Did you mean a `data`? I'm 
confused.

> 
> > You don't give full details for your Has instances, but presumably you'd do
> > the same equality constraint style as SORF and DORF.
> 
> I assume you mean
> instance (v~a) => Has k v (R a) where ...
> 
> I'm not sure why we need this, but I assume that we do, since it was
> written by SPJ, so yes.
> 

Matthew, you really, really need to understand why SPJ put it that way, if you 
want your proposal to be taken seriously. He wasn't just making it up.

> > You don't show how you'd do record update.
> 
> Yep. It's on the wiki.
> "qfmap X f r is r mutated by f at X"
> 

So do you mean this is what developers put in the code?

> > what is the type for:
> >     r{ X = True }
> > That is: update record r, set its X field to True.
> 
> This is written as
> qfmap X (const True) (r :: r) :: Quasifunctor X a Bool r s => s;
> 

You mean this is what to put in the code?

DORF is getting beaten up for the amount of boilerplate the programmer is 
expected to add (for fieldLabels, etc.) I can't compare apples with apples for 
your proposal, because I can't see what the code looks like that would appear 
in the program.

So far (apart from Quasifunctor) all I can see is that you're varying the 
sugar, without adding anything to the semantics -- except you've not given the 
surface syntax, so I'm only guessing.


AntC