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

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 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 _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee