- Glasgow-haskell-users

Types differ, but they look the same
by Joachim Breitner 08 Apr '22

08 Apr '22

Hi, sorry for not providing a mwe, but while I dig deeper, maybe someone has a “this is typically a problem with X” idea… This is in the context of a Core plugin (Conal’s Compiling to Categories), and I observe an oddity: I have two 'Types' that _should_ be equal, that ppr’int equal, but where eqType says False. The type looks like this: (SymRAD1 windowFunctor_a3e0Q kernelFunctor_a3e0R channelsFunctor_a3e0S numType_a3e0T, SymRAD2 paddedWindowFunctor_a3e0P windowFunctor_a3e0Q kernelFunctor_a3e0R channelsFunctor_a3e0S numType_a3e0T) what might make eqType go False that is _not_ visible in the pretty- printed view? Cheers, Joachim -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

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Are constructors matched against using "switch" or chained if-else
by Clinton Mead 23 Feb '22

23 Feb '22

Hi All I'm developing an unbounded integer type, which I won't go into the details here but in some circumstances has better performance than the standard "Integer". Anyway, whilst there are complex cases, the most common case is a standard machine int multiplication. Hence I want the type to be optimised for that case. I'm going to have a few constructors, anyway, so I first considered something like this: `data MyInt = BasicZero | BasicPos Word | BasicNeg Word | ComplexPosA ... | ComplexNegA ... | ComplexPosB ... | ComplexNegB ...` I'd naturally make the "Word"s in "BasicPos" and "BasicNeg" strict/unpack, hopefully eliminating the indirection, or perhaps just making them primitive directly. This has 7 constructors, which quite nicely I believe fits into the three spare bits in a 64 bit pointer which GHC optimises I believe. However, this approach I thought of because I assumed that GHC would do a switch style statement on the constructors, so once I have more than one I might as well have as many as I want (well, up to 7, until I lose the optimisation). But if GHC compiles this to a "if ... else if..." chain, a better representation is the following: `data MyInt = BasicInt Int | ComplexPosA ... | ComplexNegA ... | ComplexPosB ... | ComplexNegB ...` That way I can match against BasicInt first, and knock that out of the way as my "hot" case. However, using "Int" instead of "Word" does make the logic a bit more complex, but it may be worth it if I'm reducing the number of patterns I have to check against for all arguments. I was just wondering if anyone could share some insight on what GHC does in these circumstances? For example, does the order I list my cases in a case statement matter if they're non-overlapping? Will GHC match them in the order I list, or will it just make them into a switch statement so it doesn't matter if I reorder them? I guess I could benchmark all this (and probably will) but some insights and general guidance would be good. Thanks, Clinton

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[Haskell] [ANNOUNCE] GHC 9.0.2 released
by Zubin Duggal 23 Jan '22

23 Jan '22

The GHC developers are very happy to at long last announce the availability of GHC 9.0.2. Binary distributions, source distributions, and documentation are available at the [usual place](https://downloads.haskell.org/ghc/9.0.2/). Download Page: https://www.haskell.org/ghc/download_ghc_9_0_2.html Blog Post: https://www.haskell.org/ghc/blog/20211225-ghc-9.0.2-released.html GHC 9.0.2 adds first class AArch64/Darwin support using the LLVM backend, as well as fixing a number of critical correctness bugs with the 9.0.1 release, along with numerous improvements to compiler performance and memory usage: * First-class support for Apple M1 hardware using GHC's LLVM ARM backend. * Fix a number of incorrect rewrite rules for `fromInteger` (#19345, #20066). * Fix a number of runtime system and GC bugs which typically manifested in crashes (#19417, #19297, #19645, #19667, #18033, #20132, #19715, #20399, #20414, #20649, #19147, #20051, #20093) * Fixes for various compiler performance and memory usage issues (#20134, #13586, #20509, #19668, #19996, #19471) * Improvements and bug fixes for the code generator and optimiser (#18638, #19581, #19700, #19996, #19181, #19822) * Fixes for typechecker bugs (#19410 , #19191, #19522, #19400, #19849) * Bug fixes for the frontend and parser (#19397, #19838, #19518, #19777, #20242, #19786) * Emit warnings if Unicode Bidirectional Formatting characters are found in the source, possibly misleading code reviewers (#20263). * Many packaging related fixes, including versioned `ghc-pkg` executables (#20087), and actually distributing GHC versions linked against the `native` big integer backend (#18967, #19953) on both Windows and Alpine Linux. Some previous releases were still linked against the `GMP` library due to a misconfiguration of the builders. * A significant refactoring of `process` fixing numerous bugs mostly on Apple platforms (#19994, [process refactoring](https://github.com/haskell/process/pull/208)) * Bug fixes for the linker on Darwin platforms (#20004, #19968, #19950). A complete list of bug fixes and improvements can be found in the [release notes](https://downloads.haskell.org/ghc/9.0.2/docs/html/users_guide/9.0.2-…. Finally, thank you to Microsoft Research, GitHub, IOHK, the Zw3rk stake pool, Tweag I/O, Serokell, Equinix, SimSpace, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Moreover, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release. As always, do open a [ticket][] if you see anything amiss. Happy Holidays and Happy Hacking! - Zubin

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Typing of Pattern Synonyms: Required vs Provided constraints
by Anthony Clayden 07 Jan '22

07 Jan '22

There was an interesting exchange between the authors of Haskell compilers back in 1999 (i.e. when there were multiple compilers) http://web.archive.org/web/20151001142647/http://code.haskell.org/~dons/has… I was trying to simulate SPJ's point of view, using PatternSynonyms. > > {-# LANGUAGE DatatypeContexts, PatternSynonyms #-} > > data Ord a => TSPJ a = MkTSPJ a a > > pattern PatTSPJ :: (Ord a) => () => a -> a -> TSPJ a -- empty Provided > pattern PatTSPJ x y = MkTSPJ x y > > unPatTSPJ :: TSPJ a -> (a, a) -- no constraints > unPatTSPJ (PatTSPJ x y) = (x, y) > `unPatSPJ`'s binding rejected `No instance for (Ord a) arising from a pattern`. If I don't give a signature, inferred `unPatTSPJ :: Ord b => TSPJ b -> (b, b)`. Taking the DatatypeContext off the `data` decl doesn't make a difference. So Pattern syns seem to be giving exactly the 'stupid theta' behaviour. The User Guide https://downloads.haskell.org/~ghc/8.10.7/docs/html/users_guide/glasgow_ext… says - ⟨CProv⟩ are the constraints *made available (provided)* by a successful pattern match. But it doesn't mean that. It's more like "⟨CProv⟩ are the constraints made available *in addition to ⟨CReq⟩* (provided union required) by a successful pattern match." Or ... is there some way to simulate the up-to-1999 GHC behaviour? (I happen to agree with SPJ; Wadler wasn't thinking it through; consider for example constructor classes over type constructors with constraints: there's nothing in the instance head for the constraint to attach to. Now that we have GADTs -- which are appropriate for a different job -- that DT Contexts perhaps were being (ab-)used for -- I agree more strongly with SPJ.) With GADTs I could get a `unGSPJ` that doesn't expose/provide the constraint, but it does that by packing the constraint/dictionary (polluting) inside the data constructor. Not what I want. As SPJ says down-thread "when you take a constructor *apart*, the invariant must hold by construction: you couldn't have built the thing you are taking apart unless invariant held. So enforcing the invariant again is redundant; and in addition it pollutes the type of selectors." AntC

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Fwd: Typing of Pattern Synonyms: Required vs Provided constraints
by Anthony Clayden 07 Jan '22

07 Jan '22

Thanks Gergö, I do find Richard's tendency to use ViewPattern examples distracts me from understanding the point. That arrow-to-nowhere or arrow-to-the-wrong-value syntax is nausea-inducing. Your "instructive" example is really nothing to do with PatSyns, just ordinary Haskell numeric patterns: the Language Report (section 3.17.2 point 7., same wording in 2010 and '98) says a numeric literal in a pattern gets desugared to an `(==)` test against `fromInteger <something>`. So that explains where the constraints come from. (Annoyingly, the <something> you can't express in legit Haskell source.) Avoiding distractions is why my o.p. used an `Ord` constraint, also to follow SPJ's 1999 example. And the canonical example in those old discussions (like Hughes' 1999 paper) is an `Ord` constraint for elements of a `Set` structured as a BST. With your "polymorphic literal" how do I follow the Language Report to desugar? I'd rather be able to write Richard's example using an as-Pattern like one of these: > pattern x@Is3 | x == fromInteger (#3#) = x -- where #3# is the inexpressible <something> > pattern x@3@Is3 = x (It's not that I'm "new to patsyn typing"; it's more that patsyn typing and design keeps falling short of what I hope from such a promising feature. So I always want to check I'm not mis-understanding. Specifically, the design for Pattern Synonyms was a lost opportunity to expunge ViewPatterns entirely.) AntC On Fri, 7 Jan 2022 at 09:43, Gergő Érdi <gergo(a)erdi.hu> wrote: > Fwiw, a less contrived, and much more relatable, version of Richard's > example would be > > > pattern Is3 :: (Num a, Eq a) => a -- only a Required constraint > pattern Is3 = 3 -- a polymorphic literal! > > I think it can be quite instructive for people new to patsyn typing to > work out why this is exactly the same as the one in Richard's email! > > > On Thu, Jan 6, 2022, 21:11 Richard Eisenberg <lists(a)richarde.dev> wrote: > >> >> >> ... >> >>

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Re: [Haskell] [ANNOUNCE] GHC 9.0.2 released
by Zubin Duggal 25 Dec '21

25 Dec '21

Oops. Pushed! On 21/12/25 15:37, Shayne Fletcher wrote: >Awesome. Thank-you! There appears not to be a ghc-9.0.2-release tag. Might >this be addressed please? >

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[ANNOUNCE] GHC 9.2.1 now available
by Ben Gamari 18 Nov '21

18 Nov '21

Hi all, The GHC developers are very happy to at long last announce the availability of GHC 9.2.1. Binary distributions, source distributions, and documentation are available at https://downloads.haskell.org/ghc/9.2.1 GHC 9.2 brings a number of exciting features including: * A native code generation backend for AArch64, significantly speeding compilation time on ARM platforms like the Apple M1. * 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. * Merging 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] * Significant improvements in the bytecode interpreter, allowing more programs to be efficently run in GHCi and Template Haskell splices. * Support for profiling of pinned objects with the cost-centre profiler (#7275) * Faster compilation and a smaller memory footprint * Introduction of Haddock documentation support in TemplateHaskell (#5467) Finally, thank you to Microsoft Research, GitHub, IOHK, the Zw3rk stake pool, Tweag I/O, Serokell, Equinix, SimSpace, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Moreover, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release. As always, do open a [ticket] if you see anything amiss. Happy testing, - Ben [apple-m1]: https://www.haskell.org/ghc/blog/20210309-apple-m1-story.html [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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osx conundrum
by David Duke 15 Oct '21

15 Oct '21

I have a conundrum on which advice would be appreciate. Does anyone know how to successfully install ghc on OSX I've tried various binary instalation routes: macports, brew, direct binary downloads from haskel.org All have the same result. when I try to compile a basic hello world program I get Undefined symbols for architecture x86_64: "_iconv", referenced from: _hs_iconv in libHSbase-4.14.3.0.a(iconv.o) (maybe you meant: _base_GHCziIOziEncodingziIconv_iconvEncoding1_closure, _base_GHCziIOziEncodingziIconv_iconvEncoding1_info , _base_GHCziIOziEncodingziIconv_iconvEncoding4_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding15_info , _base_GHCziIOziEncodingziIconv_iconvEncoding4_info , _base_GHCziIOziEncodingziIconv_iconvEncoding7_info , _base_GHCziIOziEncodingziIconv_iconvEncoding6_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding6_info , _base_GHCziIOziEncodingziIconv_iconvEncoding8_info , _hs_iconv_open , _base_GHCziIOziEncodingziIconv_iconvEncoding9_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding12_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding11_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding9_info , _base_GHCziIOziEncodingziIconv_iconvEncoding12_info , _base_GHCziIOziEncodingziIconv_iconvEncoding13_info , _base_GHCziIOziEncodingziIconv_iconvEncoding11_info , _base_GHCziIOziEncodingziIconv_iconvEncoding7_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding_info , _base_GHCziIOziEncodingziIconv_iconvEncoding13_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding2_info , _base_GHCziIOziEncodingziIconv_iconvEncoding14_bytes , _base_GHCziIOziEncodingziIconv_iconvEncoding15_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding3_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding8_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding2_closure , _hs_iconv , _base_GHCziIOziEncodingziIconv_iconvEncoding10_bytes , _hs_iconv_close , _base_GHCziIOziEncodingziIconv_iconvEncoding5_closure ) "_iconv_open", referenced from: _hs_iconv_open in libHSbase-4.14.3.0.a(iconv.o) (maybe you meant: _hs_iconv_open) "_iconv_close", referenced from: _hs_iconv_close in libHSbase-4.14.3.0.a(iconv.o) (maybe you meant: _hs_iconv_close) "_locale_charset", referenced from: _localeEncoding in libHSbase-4.14.3.0.a(PrelIOUtils.o) ld: symbol(s) not found for architecture x86_64 I've triedgiong through ghcup 8.8.4 8.6.5. 8.10.2 8.10.7 9.0.1 all have the same problem. I'd be happy to build from source. Small problem: what Haskell compiler do I use? Any advice on installs that works along with any changes to paths to avoid the iconv problems would be appreciated as currently my Haskell-related activities have come to a grinding halt. Switchig to a different OS would be nice but its not a feasible option a at present.Writing a compiler is starting to look attractive.. thanks David -- David Duke Emeritus Professor of Computer Science School of Computing University of Leeds UK E:duke.j.david@gmail.com W:https://engineering.leeds.ac.uk/staff/334/Professor_David_Duke

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Re: Glasgow-haskell-users Digest, Vol 212, Issue 5
by Dominic Steinitz 12 Oct '21

12 Oct '21

Hi David I am a long time user of ghc on OSX. I have seen that problem but never on native OSX only when using nix (and then I added it explicitly). Two things spring to mind: Add it explicitly on the compile command `-liconv` Use nix and then you can control the build environment in a totally controllable and reproducible manner. This is actually easier than it sounds: `curl https://nixos.org/nix/install | sh` and `nix-env -I ghc`. If you get the same error with that then we can try adding `iconv` explicitly. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.org Twitter: @idontgetoutmuch > > I have a conundrum on which advice would be appreciate. Does > anyone know how to successfully install ghc on OSX > I've tried various binary instalation routes: > macports, brew, direct binary downloads from haskel.org > All have the same result. when I try to compile a basic hello world program > I get > > Undefined symbols for architecture x86_64: > "_iconv", referenced from: > > > I've triedgiong through ghcup > > 8.8.4 > 8.6.5. > 8.10.2 > 8.10.7 > 9.0.1 > > all have the same problem. > I'd be happy to build from source. Small problem: what Haskell compiler do > I use? > > Any advice on installs that works along with any changes to paths to avoid > the iconv problems would be appreciated as currently my Haskell-related > activities have come to a grinding halt. Switchig to a different OS would > be nice but its not a > feasible option a at present.Writing a compiler is starting to look > attractive.. > > thanks > David > > -- > David Duke > Emeritus Professor of Computer Science > School of Computing University of Leeds UK > E:duke.j.david@gmail.com > W:https://engineering.leeds.ac.uk/staff/334/Professor_David_Duke

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Pattern synonym constraints :: Ord a => () => ...
by Anthony Clayden 06 Oct '21

06 Oct '21

Thank you to Richard for the Tweag tutorials on Pattern Synonyms. That third one on Matchers was heavy going. I didn't find an answer (or did I miss it?) to something that's bugging me: > pattern SmartConstr :: Ord a => () => ... Seems to mean: * Required constraint is Ord a -- fine, for building * Provided constraint is Ord a -- why? for matching/consuming I'm using `SmartConstr` with some logic inside it to validate/build a well-behaved data structure. But this is an ordinary H98 datatype, not a GADT. I don't want to expose the datatype's underlying constructor, because client code could break the abstraction/build an ill-formed data structure. If I pattern-match on `SmartConstr`, the consuming function wants `Ord a`. But I can't always provide `Ord a`, because this isn't a GADT. And the client's function could be doing something that doesn't need `Ord a` -- like counting elements, or showing them or streaming to a List, etc. This feels a lot like one of the things that's wrong with 'stupid theta' datatype contexts. My work-round seems to be to define a second `ReadOnlyConstr` without constraints, that's unidirectional/ can't be used for building. For definiteness, the use case is a underlying non-GADT constructor for a BST > Node :: Tree a -> a -> Tree a -> Tree a > > pattern SmartNode :: Ord a => () => Tree a -> a -> Tree a -> Tree a with the usual semantics that the left Tree holds elements less than this node. Note it's the same `a` with the same `Ord a` 'all the way down' the Tree. If some function is consuming the Tree, it can provide constraints for its own purposes: > member :: Ord a => a -> Tree a -> Bool > dumbElem :: Eq a => a -> Tree a -> Bool > max :: Ord a => Tree a -> a (That again is the same thinking behind deprecating datatype contexts.) > countT (SmartNode l x r) = countT l + 1 + countT r -- why infer Ord a => ? > > class FancyShow t where > fancyShow :: Show a => Int -> t a -> String > instance FancyShow Tree where > fancyShow indent (SmartNode l x r) = ... -- rejected: Could not deduce Ord a > (Ref the parallel thread on Foldable: client code can't declare an instance for a Constructor class using SmartConstr.) I can see commonly your Provided would be at least the constraints inside the GADT constructor. But why presume I have a GADT? (And yes I get that a devlishly smart pattern might be building different GADT constrs/various constraints, so this is difficult to infer.) AntC

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