[ghc-steering-committee] Unsaturated type families (#242)
arnaud.spiwack at tweag.io
Wed Dec 2 15:30:37 UTC 2020
I'm a bit worried by the limited response from the committee on this
proposal. It is a non-trivial proposal, and I think it deserves more eyes.
So please have a look at them so that we can commit to this with confidence.
On Tue, Dec 1, 2020 at 7:28 PM Richard Eisenberg <rae at richarde.dev> wrote:
> This thread has seen only positive responses, and with no responses for
> the past 6 days. I'm thus inclined to accept the proposal.
> However, there are open questions around the following points:
> * the concrete syntax (pending discussion on #370)
> * defaulting rules (as raised by Alejandro in this thread)
> Conveniently, these are both listed as Unresolved Questions in the
> proposal itself. We need a way of resolving these questions. The syntax
> question may become clearer once we know what to do about #370. The
> defaulting question is harder. I vote to return to this question once #378
> has settled somewhat -- but even then, it will be hard. Still, I think we
> should move forward with accepting the main proposal, and we can continue
> to debate the defaulting strategy in a further thread, perhaps in parallel
> with reviewing the (already existing) implementation.
> I will accept this proposal as written at the end of the week, barring
> commentary here (or on GitHub) to stop me.
> > On Nov 25, 2020, at 10:28 AM, Simon Peyton Jones <simonpj at microsoft.com>
> > I'm on board with unsaturated type families.
> > Like Linear Haskell, it's quite a "big" proposal, but accepting it is
> compatible with idea of Haskell as a laboratory for exploration. I think
> we should flag it as experimental, with the implication that details are
> liable to change as we gain experience.
> > Like others, I'd like us to converge on #370 before fixing syntax.
> > I don’t have a strong opinion about the defaulting stuff.
> > Declaration of interest: I'm a co-author on the paper.
> > Simon
> > | -----Original Message-----
> > | From: ghc-steering-committee <ghc-steering-committee-
> > | bounces at haskell.org> On Behalf Of Richard Eisenberg
> > | Sent: 20 November 2020 19:36
> > | To: Simon Peyton Jones via ghc-steering-committee <ghc-steering-
> > | committee at haskell.org>
> > | Subject: [ghc-steering-committee] Unsaturated type families (#242)
> > |
> > | Hi committee,
> > |
> > | Csongor Kiss has proposed -XUnsaturatedTypeFamilies.
> > |
> > | Proposal:
> > |
> > | ub.com%2Fkcsongor%2Fghc-proposals%2Fblob%2Funsaturated-type-
> > | families%2Fproposals%2F0000-unsaturated-type-
> > | families.rst&data=04%7C01%7Csimonpj%40microsoft.com
> > | 40385b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C6374
> > | 14978666235646%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2l
> > | uMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=WdLjCp2ReG9ZXOjmE
> > | Ow6VCJlAO7Yf1aWkVAXHxrsmMM%3D&reserved=0
> > | ICFP'19 paper:
> > | https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww
> > | microsoft.com%2Fen-
> > | us%2Fresearch%2Fuploads%2Fprod%2F2019%2F03%2Funsaturated-type-
> > | families-icfp-
> > | 2019.pdf&data=04%7C01%7Csimonpj%40microsoft.com
> > | 5b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C63741497
> > | 8666235646%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzI
> > | iLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=dgowMlsNpH0X9fPAr498F
> > | Y9u8xML9n0G1nwvPN4R9HA%3D&reserved=0
> > | Discussion:
> > |
> > | ub.com%2Fghc-proposals%2Fghc-
> > | proposals%2Fpull%2F242&data=04%7C01%7Csimonpj%40microsoft.com
> > | 9e62e3137e40385b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%
> > | 7C1%7C637414978666235646%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiL
> > | CJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=C9N0y7l
> > | KhZsepv0mkXbJOMDVLFi4bN5kaVW7DbXW1ro%3D&reserved=0
> > |
> > | The central idea is to allow type functions (both type families and
> > | type synonyms) to appear unsaturated. (Currently, all type functions
> > | are required syntactically to be applied to all parameters they are
> > | declared with.) This poses a problem for type inference, as detailed
> > | in both the proposal and the paper. The key question: if we have (f a
> > | ~ g b), can we conclude (f ~ g) and (a ~ b)? Not if either of f or g
> > | is a type function. This proposal thus describes a mechanism to
> > | introduce a new flavor of arrow, such that we can identify type
> > | functions by their kind. Specifically, we have regular types like
> > | Maybe :: Type -> @M Type (where the M stands for "matchable"), but
> > | type functions like F :: Type -> @U Type (where the U stands for
> > | "unmatchable"). Unmatchable applications can not be decomposed during
> > | type inference.
> > |
> > | Much of the proposal is concerned with backward-compatibility: most
> > | users will not want to write @M or @U after each of their arrows, so
> > | the proposal describes ways of defaulting this behavior to match
> > | (most) programmers' expectations.
> > |
> > | The proposal also includes matchability polymorphism, the ability to
> > | abstract over a matchability parameter.
> > |
> > | Pros:
> > | + This proposal greatly increases the expressiveness of Haskell's type
> > | system.
> > | + With this proposal, we can finally do proper functional programming
> > | in types, rather than just in terms.
> > | + This proposal is a key ingredient toward having dependent types, as
> > | + unsaturated functions are commonplace in terms, and thus should also
> > | be supported in types. (Allowing unsaturated functions in types was a
> > | key difference between Adam Gundry's proposal for dependent types
> > |
> > | .gundry.co.uk
> > | .com%7Cbd9e62e3137e40385b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011
> > | db47%7C1%7C1%7C637414978666235646%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4w
> > | LjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdat
> > | a=wk7Xc%2Bb5FLSrndKYZ2ytJh6gO2oYiCXLDhQhdEOfSOg%3D&reserved=0,
> > | which requires a notion of a subset of the type and term languages
> > | shared in common, and mine, which makes no distinction between terms
> > | and types.) There is a prototype implementation.
> > | + The ideas are backed up by peer-reviewed research.
> > | + Despite focusing on type families, this work applies equally to
> > | ordinary functions which might be used in types once we have stronger
> > | support for dependent types.
> > |
> > | Cons:
> > | - This adds a new dimension of complexity to our kind system, by
> > | separating out matchable and unmatchable arrows.
> > | - The rules for defaulting appear convenient in practice, but are
> > | somewhat arbitrary.
> > | - The rules for defaulting care about context -- does an arrow appear
> > | in the type of a term or the type of a type? These rules thus go
> > | against the spirit of #378, which advocates for not accepting features
> > | that distinguish between types and terms.
> > |
> > | Recommendation: With reservations, I recommend acceptance. I think
> > | that the power to use higher-order programming should not be
> > | restricted to terms, and allowing unsaturated functions at compile
> > | time is necessary in order to have convenient dependent types.
> > | However, I am concerned about the extra complexity of matchability. A
> > | key open question for me is how much matchability is apparent to users
> > | -- even ones using some higher-order type-level programming. If
> > | matchability is pervasive, then I would lean against. But my
> > | expectation is that matchability fades into the background -- much
> > | like levity polymorphism (unless you want it).
> > |
> > | Open question: What to do about syntax? The proposed syntax is
> > | sensible. However, #370 suggests an alternative syntax that might be
> > | more forward-thinking.
> > |
> > | Richard
> > | _______________________________________________
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> > | 85b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C6374149
> > | 78666235646%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMz
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