On Mon, 10 Dec 2018 at 9:36 PM, Tom Schrijvers wrote: > Maybe our Haskell'17 paper about Elaboration on Functional Dependencies > sheds some more light on your problem: > > https://people.cs.kuleuven.be/~tom.schrijvers/Research/papers/haskell2017a.pdf > > Thank you Tom, looks interesting and very applicable. It'll be a few days before I can take a proper look.
I see you compare GHC vs Hugs: good, they differ significantly in the details of implementation. I found Hugs well-principled, whereas GHC is far too sloppy in what instances it accepts (but then you find they're unusable). Maybe I scanned your paper too quickly, but it looks like you insist on non-overlapping instances (or that if instances overlap, they are 'confluent', in the terminology of the 2005~2008 work; or 'coincident' in Richard E's work). And the 2006 CHRs paper takes the same decision. That's a big disappointment: I think the combo of Overlap+FunDeps makes sense (you need to use (~) constraints -- hence this thread), and there's plenty of code using the combo -- including the HList 2004 paper (with `TypeCast`). You can make that combo work with GHC, as did HList. But it's hazardous because of GHC's sloppiness/bogusness. You can't make that combo work with Hugs as released; but because it's a better principled platform, a limited-scope tweak to the compiler makes it work beautifully. Documented here: https://ghc.haskell.org/trac/ghc/ticket/15632#comment:2 AntC > On Sat, Dec 8, 2018 at 6:42 AM Anthony Clayden < > anthony_clay...@clear.net.nz> wrote: > >> On Fri, 7 Dec 2018 at 10:57 PM, Adam Gundry wrote: >> >>> >>> Regarding inference, the place that comes to mind is Vytiniotis et al. >>> OutsideIn(X): >>> >> >> Thanks Adam for the references. Hmm That OutsideIn paper is formidable in >> so many senses ... >> >> I'm not sure I grok your response on my specific q; to adapt your example >> slightly, without a signature >> >> foo2 x@(_: _) = x >> >> GHC infers `foo2 :: [a] -> [a]`. That's not general enough. So I give a >> signature with bare `b` as the return type, using the (~) to improve it: >> >> foo2 :: [a] ~ b => [a] -> b >> >> ... and GHC infers `foo2 :: [a] -> [a]`. Oh. Is that what I should expect? >> >> I'm chiefly concerned about improving bare tyvars under a FunDep. So to >> take the time-worn example, I want instances morally equivalent to >> >> data TTrue = TTrue; data TFalse = TFalse >> >> class TEq a b r | a b -> r where tEq :: a -> b -> r >> >> instance TEq a a TTrue where tEq _ _ = TTrue >> instance TEq a b TFalse where tEq _ _ = TFalse >> >> The FunDeps via CHRs 2006 paper tells me that first instance is supposed >> to be as if >> >> instance (r ~ TTrue) => TEq a a r where ... >> >> but it isn't >> >> tEq 'a' 'a' :: TFalse -- returns TFalse, using the first pair of >> instances >> tEq 'a' 'a' :: TFalse -- rejected, using the (~) instance: >> 'Couldn't match type TFalse with TTrue ...' >> >> The 'happily' returning the 'wrong' answer in the first case has caused >> consternation amongst beginners, and a few Trac tickets. >> >> So what magic is happening with (~) that it can eagerly improve `foo2` >> but semi-lazily hold back for `tEq`? >> >> Of course I lied about the first-given two instances for TEq: instances >> are not consistent with Functional Dependency. So I need to resort to >> overlaps and a (~) and exploit GHC's bogus consistency check: >> >> instance {-# OVERLAPPABLE #-} (r ~ TFalse) => TEq a b r where ... >> >> > ... like automatically inferring where `typeCast` needs to be placed >> >> Yes the non-automatic `TypeCast` version >> >> instance (TypeCast TTrue r) => TEq a a r where >> tEq _ _ = TTrue -- rejected 'Couldn't match expected type >> 'r' with actual type 'TTrue' ...' >> >> tEq _ _ = typeCast TTrue -- accepted >> >> Even though `typeCast` is really `id` (after it's jumped through the >> devious indirection hoops I talked about). >> >> So what I'm trying to understand is not just "where" to place `typeCast` >> but also "when" in inference it unmasks the unification. >> >> >> AntC >> >> >>> About your specific question, if a wanted constraint `a ~ b` can be >>> solved by unifying `a` with `b`, then GHC is free to do so (in a sense, >>> either parameter of (~) can be improved from the other). The real >>> difference with `TypeCast` arises when local equality assumptions (given >>> constraints) are involved, because then there may be wanted constraints >>> that cannot be solved by unification but can be solved by exploiting the >>> local assumptions. >>> >>> For example, consider this function: >>> >>> foo :: forall a b . a ~ b => [a] -> [b] >>> foo x = x >>> >>> When checking the RHS of `foo`, a wanted constraint `[a] ~ [b]` arises >>> (because `x` has type `[a]` and is used in a place where `[b]` is >>> expected). Since `a` and `b` are universally quantified variables, this >>> cannot be solved by unification. However, the local assumption (given >>> constraint) `a ~ b` can be used to solve this wanted. >>> >>> This is rather like automatically inferring where `typeCast` needs to be >>> placed (indeed, at the Core level there is a construct for casting by an >>> equality proof, which plays much the same role). >>> >>> Hope this helps, >>> >>> Adam >>> >>> >>> On 06/12/2018 12:01, Anthony Clayden wrote: >>> > The (~) constraint syntax is enabled by either `GADTs` or >>> `TypeFamilies` >>> > language extension. >>> > >>> > GADTs/Assoc Data Types/Assoc Type Synonyms/Type Families arrived in a >>> > series of papers 2005 to 2008, and IIRC the development wasn't finished >>> > in full in GHC until after that. (Superclass constraints took up to >>> > about 2010.) >>> > >>> > Suppose I wanted just the (~) parts of those implementations. Which >>> > would be the best place to look? (The Users Guide doesn't give a >>> > reference.) ICFP 2008 'Type Checking with Open Type Functions' shows >>> > uses of (~) in user-written code, but doesn't explain it or motivate it >>> > as a separate feature. >>> > >>> > My specific question is: long before (~), it was possible to write a >>> > TypeCast class, with bidirectional FunDeps to improve each type >>> > parameter from the other. But for the compiler to see the type >>> > improvement at term level, typically you also need a typeCast method >>> and >>> > to explicitly wrap a term in it. If you just wrote a term of type `a` >>> in >>> > a place where `b` was wanted, the compiler would either fail to see >>> your >>> > `TypeCast a b`, or unify the two too eagerly, then infer an overall >>> type >>> > that wasn't general enough. >>> > >>> > (For that reason, the instance for TypeCast goes through some >>> > devious/indirect other classes/instances or exploits separate >>> > compilation, to hide what's going on from the compiler's enthusiasm.) >>> > >>> > But (~) does some sort of magic: it's a kinda typeclass but without a >>> > method. How does it mesh with type improvement in the goldilocks zone: >>> > neither too eager nor too lazy? >>> > >>> > >>> > AntC >>> >>> >>> _______________________________________________ >> Glasgow-haskell-users mailing list >> Glasgow-haskell-users@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > > >> > > -- > prof. dr. ir. Tom Schrijvers > > Research Professor > KU Leuven > Department of Computer Science > > Celestijnenlaan 200A > <https://maps.google.com/?q=Celestijnenlaan+200A+3001+Leuven+Belgium&entry=gmail&source=g> > 3001 Leuven > <https://maps.google.com/?q=Celestijnenlaan+200A+3001+Leuven+Belgium&entry=gmail&source=g> > Belgium > <https://maps.google.com/?q=Celestijnenlaan+200A+3001+Leuven+Belgium&entry=gmail&source=g> > Phone: +32 16 327 830 > http://people.cs.kuleuven.be/~tom.schrijvers/ >
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