- Glasgow-haskell-users

[ANNOUNCE] GHC 9.2.1-alpha2 released
by Ben Gamari 23 Apr '21

23 Apr '21

Hi all, The GHC developers are very happy to announce the availability of the second alpha release in the 9.2.1 series. Binary distributions, source distributions, and documentation are available at https://downloads.haskell.org/ghc/9.2.1-alpha2 GHC 9.2 will bring a number of exciting features including: * Many changes in the area of records, including the new `RecordDotSyntax` and `NoFieldSelectors` language extensions, as well as Support for `DuplicateRecordFields` with `PatternSynonyms`. * Introduction of the new `GHC2021` language extension set, giving users convenient access to a larger set of language extensions which have been long considered stable. * Merge of `ghc-exactprint` into the GHC tree, providing infrastructure for source-to-source program rewriting out-of-the-box. * Introduction of a `BoxedRep` `RuntimeRep`, allowing for polymorphism over levity of boxed objects (#17526) * Implementation of the `UnliftedDataTypes` extension, allowing users to define types which do not admit lazy evaluation ([proposal]) * The new [-hi profiling] mechanism which provides significantly improved insight into thunk leaks. * Support for the `ghc-debug` out-of-process heap inspection library [ghc-debug] * Support for profiling of pinned objects with the cost-centre profiler (#7275) * Introduction of Haddock documentation support in TemplateHaskell (#5467) * Proper support for impredicative types in the form of Quick-Look impredicativity. * A native code generator backend for AArch64. This pre-release brings nearly 50 fixes relative to the first alpha, although the long-awaited ARM NCG backend hasn't quite landed yet. As always, do give this a try and open a [ticket] if you see anything amiss. Happy testing, - Ben [proposal]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0265-u… [-hi profiling]: https://well-typed.com/blog/2021/01/first-look-at-hi-profiling-mode/ [ghc-debug]: http://ghc.gitlab.haskell.org/ghc-debug/ [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new

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Guarantees about shape of Generic's Rep
by Alexey Khudyakov 20 Apr '21

20 Apr '21

Hi! I have a question about guarantees that GHC provides about shape of generic representation of data type. To be more concrete if we have two single-constructor data types with N fields will their binary trees built from (:*:) be balanced in same way? Same for (:+:) binary tree. I want to use generics to write conversions between data types with same shape: > λ> data Foo = Foo Int Char deriving (Show,Generic) > λ> to . coerce . from $ Foo 1 'c' :: (Int,Char) > (1,'c') > λ> to . coerce . from $ Foo 1 'c' :: (Sum Int,Char) > (Sum {getSum = 1},'c') > λ> to . coerce . from $ (Sum (1::Int), 'c') :: Foo > Foo 1 'c' Coercion between Reps for different types is only possible when they have same shape. Quick experiments shows that they're same. I think that they should be same but documentation is silent about this. Thanks in advance, Alexey

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How to user-define a type equality constraint?
by Anthony Clayden 06 Apr '21

06 Apr '21

Type equality `(~)` is a fine constraint. It's mildly annoying I need either `-XGADTs` or `-XTypeFamilies` to use it -- because I don't otherwise need those extensions. OTOH it's not H2010 so it needs to be switched on somehow. I see the Committee is discussing what to do. It's to be enabled by `-XTypeOperators`. That would be just as annoying for me -- I don't otherwise use Type Operators. If TypeOperators is to be the default for GHC2021, that more or less convinces me against GHC2021. (I'm not a newbie/I want to fine-tune which extensions I'm using.) @int-index says https://mail.haskell.org/pipermail/ghc-steering-committee/2021-April/002373… > What convention do you have in mind? (~) is not that different from, say, (+), which is a valid type operator: I believe `(~)` is different in that I can't define it in user code(?) Indeed: > (~) is a special, magical constraint, like Coercible and Typeable. ..., the idea is to (eventually) require an explicit import of Data.Type.Equality to indicate the use of (~) in the code, I'd be happy to not import `(~)`/not use TypeOperators, and define an ordinary typeclass/constraint, say `TypeCast a b`. There's a `TypeCast` defined in HList 2004, but it needs FunDeps; more awkwardly it needs a method and for that method to appear in term-level code. Can I user-define a conventional type-class that behaves more like `(~)`? (The "convention I have in mind" is that already in H2010,`~` is a special symbol for irrefutable patterns. Then I'd prefer constraint `(~)` is also special syntax; and at term level I'd prefer it be available as a monadic prefix operator.) AntC

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Re: How to user-define a type equality constraint?
by Anthony Clayden 06 Apr '21

06 Apr '21

> But why does this matter? Because I want the semantics of that equality constraint, without switching on any of these, which I don't otherwise use: GADTs TypeFamilies TypeOperators And if that means I can't use infix `~` in my constraints, I'll put up with that. (I'd user-define a conventional class, say `TypeCast`. Could be that has to be defined in terms of `~`, in which case I'd put that in a shim module as the only place with those extensions.)

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Re: Pattern synonym 'Required' constraints === Datatype Contexts(?)
by Anthony Clayden 12 Mar '21

12 Mar '21

Thank you Richard, Lennart, *Gergő* >* pattern Positive :: (Ord a, Num a) => a* >* pattern Positive <- ((>0) -> True)* *Heh heh, there's another surprise/undocumented 'feature'.* *It's not necessary to give a signature for pattern `Positive`, GHC will infer that from the decl.* *I was surprised to see `True`, and even more surprised there wasn't a `Bool` in the signature. I guess that's so `Positive` can appear as a pattern in a case expr. To dig out the positive value in a lambda expr, it seems I go* *> (\p@Positive -> p) 5 -- returns 5* *Seems I can't use any trick like that to turn `Positive` into explicitly bidirectional. I also tried* >* pattern Positive' <- ((>0) -> ())* *But that's rejected '"** Couldn't match expected type `Bool' with actual type `()'". Is the hidden `Bool` documented somewhere? (Doesn't seem to be in the User Guide nor the wiki nor the paper, on a quick scan.)

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Pattern synonym 'Required' constraints === Datatype Contexts(?)
by Anthony Clayden 12 Mar '21

12 Mar '21

I must be slow on the uptake. I've just grokked this equivalence -- or is it? Consider > data Eq a => Set a = NilSet | ConsSet a (Set a) -- from the Language report > > -- ConsSet :: forall a. Eq a => a -> Set a => Set a -- inferred/per report > > -- equiv with Pattern syn 'Required' constraint > data Set' a = NilSet' | ConsSet' a (Set' a) -- no DT context > > pattern ConsSetP :: (Eq a) => () => a -> (Set' a) -> (Set' a) > pattern ConsSetP x xs = ConsSet' x xs > > ffP ((ConsSet x xs), (ConsSetP y ys)) = (x, y) > > -- ffP :: forall {a} {b}. (Eq a, Eq b) => (Set a, Set' b) -> (a, b) -- inferred The signature decl for `ConsSetP` explicitly gives both the Required `(Eq a) =>` and Provided `() =>` constraints, but the Provided could be omitted, because it's empty. I get the same signature for both `ConsSetP` as `ConsSet` with the DT Context. Or is there some subtle difference? This typing effect is what got DT Contexts called 'stupid theta' and deprecated/removed from the language standard. ("widely considered a mis-feature", as GHC is keen to tell me.) If there's no difference, why re-introduce the feature for Patterns? That is, why go to the bother of the double-context business, which looks weird, and behaves counter to usual signatures: > foo :: (Eq a) => (Show a) => a -> a > -- foo :: forall {a}. (Eq a, Show a) => a -> a -- inferred There is a slight difference possible with Pattern synonyms, compare: > pattern NilSetP :: (Eq a) => () => (Set' a) > pattern NilSetP = NilSet' > > -- NilSetP :: forall {a}. Eq a => Set' a -- inferred > -- NilSet :: forall {a}. => Set a -- inferred/per report Using `NilSetP` somewhere needs giving an explicit signature/otherwise your types are ambiguous; but arguably that's a better discipline than using `NilSet` and allowing a Set with non-comparable element types. AntC

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[ANN] cabal-install-3.4.0.0
by Emily Pillmore 23 Feb '21

23 Feb '21

The Cabal team is excited to announce the release of cabal-install 3.4. This is the third release of the 3.0 release series, bringing a support for the `Cabal-Version: 3.4` `.cabal` file format. This introduces a number of user-facing features including: - Support for GHC 9.0 - Package sublibrary dependency syntax - Allowing omission of the `default-language` declaration - A mixins field allowing the user to specify a sublibrary - [Many other features]( https://cabal.readthedocs.io/en/latest/file-format-changelog.html#cabal-ver… ) In addition, the `cabal-install` frontend itself has gained a number of new features including: - New `active-repositories` `cabal.project` fields allowing users to specify an order and various ways of combining package repositories - Improvements to `cabal init` - A rework of the `cabal v2-sdist` command. `cabal v1-sdist` is removed as of this version - `source-repository-package` directories are no longer local - A new `list-bin` command - `cabal sandbox` is now removed in favor of default nix-style builds - `cabal list` now accepts regular expressions - More rich `index-state` syntax. `v2-freeze` saves `index-state` I'd also like to thank the many contributors who offered patches, tickets, and other help in the preparation of this release. We appreciate all of your help! For a full set of release notes, see https://github.com/haskell/cabal/blob/master/release-notes/cabal-install-3.…. If you have issues, we'd love to hear about there here: https://github.com/haskell/cabal/issues. Happy hacking! Emily

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[ANNOUNCE] GHC 8.10.4 released
by Ben Gamari 10 Feb '21

10 Feb '21

The GHC team is very pleased to announce the availability of GHC 8.10.4. Source and binary distributions are available at the usual place: https://downloads.haskell.org/ghc/8.10.4/ This is a small bug-fix release, fixing two bugs present in 8.10.3: * Fix a linker hang triggered by dynamic code loading on Windows (#19155) * Fix a crash caused by inappropriate garbage of heap-allocated data reachable from foreign exports (#19149) As always, feel free to report any issues you encounter via [gitlab.haskell.org](https://gitlab.haskell.org/ghc/ghc/-/issues/new). Cheers, - Ben

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[ANNOUNCE] GHC 9.0.1 released
by Ben Gamari 08 Feb '21

08 Feb '21

The GHC team is very pleased to announce the availability of GHC 9.0.1. Source and binary distributions are available at the usual place: https://downloads.haskell.org/ghc/9.0.1/ In addition to numerous bug fixes, GHC 9.0.1 will bring a number of new features: * A first cut of the new LinearTypes language extension [2], allowing use of linear function syntax and linear record fields. * A new bignum library, ghc-bignum, improving portability and allowing GHC to be more easily used with integer libraries other than GMP. * Improvements in code generation, resulting in considerable runtime performance improvements in some programs. * Improvements in pattern-match checking, allowing more precise detection of redundant cases and reduced compilation time. * Implementation of the "simplified subsumption" proposal [3] simplifying the type system and paving the way for QuickLook impredicativity in GHC 9.2. * Implementation of the QualifiedDo extension [4], allowing more convenient overloading of `do` syntax. * An experimental new IO manager implementation for Windows platforms, both improving performance and fixing many of the quirks with the old manager built on POSIX-emulation. * Improvements in compilation time. And many more. See the release notes [5] for a full accounting of the changes in this release. As always, feel free to report any issues you encounter via gitlab.haskell.org. Cheers, - Ben [1]: https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#ghc-prim-07 [2]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0111-l… [3]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-s… [4]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0216-q… [5]: https://downloads.haskell.org/ghc/9.0.1/docs/html/users_guide/9.0.1-notes.h…

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[ANNOUNCE] Glasgow Haskell Compiler 9.0.1-rc1 released
by Ben Gamari 26 Jan '21

26 Jan '21

Hello all, The GHC team is very pleased to announce the availability of the first release candidate of GHC 9.0.1 series. Source and binary distributions are available at the usual place: https://downloads.haskell.org/ghc/9.0.1-rc1/ This release candidate comes quite a bit later than expected after difficulty finding a performance neutral fix for a critical soundness bug, #17760. See [1] for details on the solution, particularly if you are a library maintainer currently using the touch# primop or withForeignPtr. Nevertheless, this release has nevertheless seen a considerable amount of testing and consequently we hope that this should be the last pre-release before the final release. In addition to numerous bug fixes, GHC 9.0.1 will bring a number of new features: * A first cut of the new LinearTypes language extension [2], allowing use of linear function syntax and linear record fields. * A new bignum library (ghc-bignum), allowing GHC to be more easily used with integer libraries other than GMP. * Improvements in code generation, resulting in considerable performance improvements in some programs. * Improvements in pattern-match checking, allowing more precise detection of redundant cases and reduced compilation time. * Implementation of the "simplified subsumption" proposal [3] simplifying the type system and paving the way for QuickLook impredicativity in GHC 9.2. * Implementation of the QualifiedDo extension [4], allowing more convenient overloading of `do` syntax. * Improvements in compilation time. And many more. See the release notes [5] for a full accounting of the changes in this release. As always, do test this release and open tickets for whatever issues you encounter. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#ghc-prim-07 [2] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0111-l… [3] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-s… [4] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0216-q… [5] https://downloads.haskell.org/ghc/9.0.1-rc1/docs/html/users_guide/9.0.1-not…

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