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

Tue Aug 28 20:41:47 UTC 2018

OK. I'm convinced now that I should seek further and wider community feedback before pushing through this terminology change. A proposal may be the right way to do that, though it doesn't quite fit within the proposal-process remit.

Given this, I withdraw my objections and support the current proposal.

Richard

> On Aug 21, 2018, at 9:18 AM, Eric Seidel <eric at seidel.io> wrote:
> 
> 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
>> _______________________________________________
>> ghc-steering-committee mailing list
>> ghc-steering-committee at haskell.org
>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
> _______________________________________________
> ghc-steering-committee mailing list
> ghc-steering-committee at haskell.org
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee