[ghc-steering-committee] Unsaturated type families (#242)
rae at richarde.dev
Tue Dec 1 18:27:53 UTC 2020
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> wrote:
> 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.
> | -----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:
> | https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgith
> | ub.com%2Fkcsongor%2Fghc-proposals%2Fblob%2Funsaturated-type-
> | families%2Fproposals%2F0000-unsaturated-type-
> | families.rst&data=04%7C01%7Csimonpj%40microsoft.com%7Cbd9e62e3137e
> | 40385b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C6374
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> | 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%7Cbd9e62e3137e4038
> | 5b5f08d88d8b91e0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C63741497
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> | Discussion:
> | https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgith
> | ub.com%2Fghc-proposals%2Fghc-
> | proposals%2Fpull%2F242&data=04%7C01%7Csimonpj%40microsoft.com%7Cbd
> | 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
> | https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fadam
> | .gundry.co.uk%2Fpub%2Fthesis%2F&data=04%7C01%7Csimonpj%40microsoft
> | .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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