[ghc-steering-committee] #536: Type-level literals as a separate language extension, rec: accept
Adam Gundry
adam at well-typed.com
Mon May 1 13:17:22 UTC 2023
Thanks for your comments Iavor, your input is appreciated.
The ongoing discussion on language extensions made me think about
proposal #536 again. With due respect to Richard, I still think we
should accept it, for the following reasons:
* We owe it to proposal authors to make decisions in a reasonably
prompt fashion. The ongoing discussion is unlikely to be resolved
extremely quickly, and the extension split in the proposal accords with
the de facto approach to language extensions as feature flags.
* Even if we accept the premise that we have too many extensions,
accepting the proposal doesn't really make things worse (if we already
have a hundred extensions too many, what harm one more?).
* As Iavor says, there is no deep reason for type literals and
DataKinds to be grouped as part of the same feature; it is merely a
historical accident. If we later move to a model where features are
enabled by default and controlled using warnings, it would be equally
reasonable to have separate warnings for type literals and DataKinds.
* The proposed change is simple, low effort to implement and maintain,
and is clearly desired by some users.
* The workaround with -Werror=unticked-promoted-constructors doesn't
accurately characterise the language with type literals but not DataKinds.
Feel free to object, of course. :-)
Adam
On 20/03/2023 17:59, Iavor Diatchki wrote:
> Hello,
>
> I am not on the committee but as the original author of the type
> literals extensions I thought I'd give my 2c. I am strongly in favor of
> separating type literals from DataKinds, they are not really together
> for any deep reason.
>
> For me the separate extension would be useful because it makes it
> possible to use type literals with the `TypeData` extension, which for
> me, is a much nicer way to work with type level things than `DataKinds`.
>
> -Iavor
>
>
>
> On Mon, Mar 20, 2023 at 10:08 AM Chris Dornan <chris at chrisdornan.com
> <mailto:chris at chrisdornan.com>> wrote:
>
> I am also in favour of extensions that focus on solving a single
> problem well and giving users precise control over what extensions
> they deploy. If we get this right then we should be able discover
> which extensions people want and those that are free of unintended
> consequences, and can put them on track to be included in a GHC and
> then Haskell standard.
>
> This proposal isn't ambitious but it does otherwise appear to
> qualify so I am in favour.
>
> Chris
>
>
>
>
>> On 20 Mar 2023, at 08:43, Simon Peyton Jones
>> <simon.peytonjones at gmail.com <mailto:simon.peytonjones at gmail.com>>
>> wrote:
>>
>> Personally I'm happy with having orthogonal extensions that enable
>> just one thing. That allows our users to do what they want,
>> rather than telling them what they want.
>>
>> On this particular proposal I don't think it's a big deal either
>> way, but I'd vote in favour. It's not exactly true to say that it
>> duplicates an existing feature. I think Richard's point is that
>> you can get a warning for using an unticked constructor (but
>> nothing for using a ticked one). That's different from getting an
>> error if you use a constructor in a type, ticked or not.
>>
>> This particular proposal is easy to describe and easy to
>> implement. But I don't think we should burn a lot of cycles on
>> it... it's a matter of taste, so we could just vote.
>>
>> Simon
>>
>> On Sun, 19 Mar 2023 at 21:09, Richard Eisenberg
>> <lists at richarde.dev <mailto:lists at richarde.dev>> wrote:
>>
>> This argument gave me pause. Thanks, Adam. I think, for me, it
>> comes down to this: what do we think will be more useful to
>> more of our users? A notion of a core language -- perhaps
>> implemented by different compilers -- with extensions? Or a
>> unified language GHC/Haskell which allows subsetting? I tend
>> to think the latter, though I'm not sure.
>>
>> Regarding this particular proposal: I remain against the
>> proposal, as it duplicates an existing feature. That's a
>> simple reason to be against a proposal.
>>
>> Richard
>>
>> > On Mar 10, 2023, at 9:15 AM, Adam Gundry
>> <adam at well-typed.com <mailto:adam at well-typed.com>> wrote:
>> >
>> > This proposal is very helpful in exposing some of the
>> divisions about the interpretation of extensions that exist
>> within the committee! "Let a thousand flowers bloom" vs
>> "more-extensions-make-me-wince", if you like.
>> >
>> > For what it's worth I tend to be in the former camp. I think
>> we should strive for orthogonality and avoid bundling together
>> distinct features merely because they have related effects.
>> Instead we should construct our language via composition of
>> simple components. And I think it's reasonable to say that
>> TypeLevelLiterals and (the rest of) DataKinds are clearly
>> distinct and can be understood in isolation.
>> >
>> > If the number of extensions is a problem, that points to the
>> need to better structure our documentation and gather coherent
>> subsets of extensions into manageable pieces (much as GHC2021
>> tries to do). Beyond that I'd like to understand the downsides
>> better (e.g. what costs does it actually impose on HLS to have
>> another extension?).
>> >
>> > A conversation with one of the authors of Helium this week
>> made me think that in the GHC proposals process we have tended
>> to neglect the possible presence of other Haskell
>> implementations. It's not realistic to expect that anyone will
>> reimplement the whole of GHC Haskell, but perhaps there is a
>> fragment large enough to be useful for a fair number of
>> Haskell programs but also small enough to be independently
>> implemented (e.g. by Helium). It would strike me as plausible
>> that such an implementation might want to support
>> TypeData+TypeLevelLiterals but not DataKinds.
>> >
>> > Adam
>> >
>> >
>> > On 09/03/2023 16:21, Richard Eisenberg wrote:
>> >> Though I see the arguments in the other direction, I vote
>> against this proposal. As Joachim argues, adding an extension
>> puts a tax on the entire ecosystem by adding one more bit of
>> context a reader has to have to understand code. In addition,
>> HLS will have to be aware of this extension. And the benefits
>> seem pretty marginal here.
>> >> Can we instead simply provide a warning? The warning here
>> would be something like the -Wpuns idea, in that it helps the
>> author keep to a particular subset of the language. (We might
>> want to collect such warnings somehow in our documentation.)
>> >> Actually, it occurs to me we already have the warning:
>> -Wunticked-promoted-constructors. We've put that warning in
>> the -Weverything bin (not even -Wall) because in the past
>> we've decided that we shouldn't recommend/require the ' when
>> using a constructor in a type. But actually, setting
>> -Werror=unticked-promoted-constructors would seem to be a
>> perfect substitute for -XTypeLevelLiterals -XNoDataKinds. OK,
>> not perfect: you could still use a constructor in a type with
>> a ' mark. But I think that's great, actually, because perhaps
>> an author wary of constructors in types still might need to
>> use one, in one place, in a 2,000-line file. This ticks all
>> the boxes (ha ha).
>> >> Having discovered this replacement, I'm now pretty strongly
>> against this proposal. I'll also post on GitHub.
>> >> Richard
>> >>> On Mar 9, 2023, at 4:26 AM, Arnaud Spiwack
>> <arnaud.spiwack at tweag.io <mailto:arnaud.spiwack at tweag.io>
>> <mailto:arnaud.spiwack at tweag.io
>> <mailto:arnaud.spiwack at tweag.io>>> wrote:
>> >>>
>> >>> I can see where the author is coming from. But as Joachim,
>> I find myself unconvinced that it is worth yet one extra
>> extension (but then again, I'm the
>> more-extensions-make-me-wince member of this committee).
>> >>>
>> >>> On Wed, 8 Mar 2023 at 13:56, Joachim Breitner
>> <mail at joachim-breitner.de <mailto:mail at joachim-breitner.de>
>> <mailto:mail at joachim-breitner.de
>> <mailto:mail at joachim-breitner.de>>> wrote:
>> >>>
>> >>> Hi,
>> >>>
>> >>> Am Mittwoch, dem 08.03.2023 um 14:58 +0300 schrieb
>> Vladislav Zavialov:
>> >>> > DataKinds is indeed a "bad" extension because it
>> introduces
>> >>> ambiguity
>> >>> > to name resolution.
>> >>> >
>> >>> > There are two ways to avoid this issue:
>> >>> > 1. Keep namespaces separate and double down on using
>> punned
>> >>> > constructor names.
>> >>> > 2. Merge terms and types, discouraging punned
>> constructor names.
>> >>> >
>> >>> > Each approach could indeed lead to a "dialect" of sorts.
>> >>> >
>> >>> > * The (accepted and recently implemented) TypeData
>> extension
>> >>> moves us
>> >>> > in the direction of option (1), and I imagine the
>> proposed
>> >>> > TypeLevelLiterals would be often used in conjunction
>> with TypeData.
>> >>> > This carves out a limited, conservative language
>> subset for type-
>> >>> > level programming.
>> >>> > * At the same time, #270 "Support pun-free code"
>> moves us in the
>> >>> > direction of option (2), and I expect it to be the
>> better option
>> >>> when
>> >>> > it comes to fully fledged dependently-typed programming
>> >>> envisioned by
>> >>> > #378.
>> >>> >
>> >>> > If I had to choose, I'd go with option (2). I don't
>> see myself using
>> >>> > TypeData, nor TypeLevelLiterals for that matter. But
>> I am of a
>> >>> "let a
>> >>> > thousand flowers bloom" persuasion, hence my
>> recommendation to
>> >>> > accept.
>> >>>
>> >>>
>> >>> Thanks for putting this into a clearer context.
>> >>>
>> >>> Where does DataKind fit in? Is it a natural part of
>> approach (2)
>> >>> (which
>> >>> is the general direction outlined by #378), or is yet
>> another take on
>> >>> the issue?
>> >>> If DataKinds was not what we want according to #378,
>> but the fragment
>> >>> carved out by TypeLevelLiterals was, then that would be
>> a strong
>> >>> reason
>> >>> for me to acccept TypeLevelLiterals.
>> >>> But if DataKinds is the goal (or at least “the” goal),
>> then I
>> >>> think I’d
>> >>> like to see stronger arguments for why it should be split.
>> >>>
>> >>>
>> >>> Cheers,
>> >>> Joachim
--
Adam Gundry, Haskell Consultant
Well-Typed LLP, https://www.well-typed.com/
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27 Old Gloucester Street, London WC1N 3AX, England
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