[ghc-steering-committee] Proposal: Define Kinds Without Promotion (#106)

[Eric Seidel]

Thanks Iavor and Richard for your comments. This whole discussion has been quite illuminating to me as well. (The more I sleep on it, the more I feel swayed by Richard's perspective.)

That being said, I don't think this proposal is the right place to settle the issue. I think my recommendation at this point would be to:

1. Accept the proposal as is. It uses standard terminology, and within that framework I think it is quite clear.
2. Encourage Richard to write up another proposal to discuss the broader terminology issue. It seems like ghc-proposals would actually be a natural place to discuss this further. It doesn't really affect the implementation, but it would have a big impact on the Manual.

On Tue, Aug 21, 2018, at 01:47, Iavor Diatchki wrote:
> Hello,
> 
> On Tue, Aug 21, 2018 at 7:06 AM Richard Eisenberg <rae at cs.brynmawr.edu>
> wrote:
> 
> >
> > >
> > > Here are some things to think about, if such a proposal is ever written:
> > >   * in this new terminology `4` is not a type, but it also does not fall
> > in any of the other categories, so what is it?
> >
> > It's type-level data.
> >
> > Why? it is not introduced by a `data` declaration.
> 
> 
> 
> > >   * how about `Eq`?
> >
> > It's a "constraint constructor", a generalization of a type class.
> >
> > Yes, we could refer to the things that construct types of kind K, as
> K-constructors, and indeed sometimes people do.   This is consistent, for
> example we could say that `4` is a `Nat` constructor.  However, it is also
> convenient to have the concept of a "type constructor", which ranges over
> all of these things, independent of what their kind is.   And, the question
> of how they were introduced (through a `data` declaration, a class, as a
> primitive or in some other way) is often completely irrelevant.
> 
> 
> >   * how about a type variable `f`?
> >
> > It's a type variable. But this is a great question: should it be something
> > else, perhaps a quantified variable (if `f`'s kind isn't Type)? After all,
> > if we have (f :: Nat), then f wouldn't range over types. Indeed, I think
> > calling an f of non-Type kind something other than a type variable would be
> > an improvement. I don't have a great suggestion of what we should call it,
> > though.
> >
> 
> I think the current terminology works just fine and we don't need to keep
> inventing new names.
> 
> >   * are type functions really type constructors?
> >
> > I'm not sure what this means. Right now, if we have `type family F a`,
> > then F isn't really a type constructor, as we can't ever write a plain F in
> > a Haskell program.
> >
> > I was referring to the fact that some type functions meet your definition
> of a type constructor: when applied to enough arguments they produce
> something of kind `Type`.
> 
> 
> 
> > >   * are we really suggesting that we should start supporting things like
> > `'True` at the value level?
> >
> > No. I'm suggesting that `'True` and `True` are just two different ways of
> > referring to the same thing. The former is accepted only in a type-level
> > context.
> >
> 
> I think that this is the crux of our misunderstanding: if it is really the
> same thing, then why does it have different names, and some of them are
> only available sometimes.
> In what sense does a `type data` declaration introduce a `data`
> constructor, if this `data` constructor can never be used at the value
> level (e.g., in a case statement)?
> 
> 
> > >
> > > Anyway, it seems to me that we have gone a little off-course, and these
> > changes are not really about making #106 easier to understand.
> >
> > But I think they are, for the reasons I've articulated above. The GitHub
> > trail contains several comments to the effect of "this proposal is simply a
> > change in data constructors' namespace" but the proposal describes the new
> > feature as a change to promotion.
> >
> 
> We may be able to implement this proposal as just a change to a
> data-constructor's namespace (I am not sure, last time I tried to do it, it
> seemed more complicated than this, but this was a long time ago, and I a
> much more familiar with how GHC works now).   Explaining it that way is not
> simpler, however, as this discussion has illustrated.
> 
> This is a summary:
>    * this proposal does NOT change `data` declarations---they work just
> like they do at the moment,
>    * this proposal does NOT change anything to do with promotion (or
> whatever we want to call what `DataKinds` does)
>    * this proposal DOES introduce a new language construct, `type data`,
> which looks somewhat like a `data` declaration, but it is not the same
> (e.g. no records, no strictness, unpacking, constructors can't be the same
> as the LHS, etc.)
>    * the purpose of `type data` is to declare some new type-level
> constants, that's all.
> 
> -Iavor
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