April 2010 - Generics

Re: [Hs-Generics] Re: Add everywhereM' to Data.Generics.Schemes
by Neil Mitchell 25 Apr '10

25 Apr '10

Hi Matthias, I started writing an email, but decided a blog post would be a better place to put down my thoughts on everywhere': http://neilmitchell.blogspot.com/2010/04/dangerous-primes-why-uniplate-does… > This said, if I understand your last post correctly, the use of everywhereM' > would be a bug here in your eyes. Do you agree? Because I don't think so. It's possible to use everywhere' correctly, but not trivial. Using descend it's trivial to get right, which is much safer. Read the blog post for my best thoughts why. > I > haven't done a lot of testing yet, but each case where a replacement has to > be performed works as expected. And even if it's not a bug, do you suggest > to drop everywhereM' and use uniplate instead, just to play it safe? I recommend to everyone they drop whatever generics solution they are using and switch to Uniplate :-) [If Uniplate can express what you want - it's one of the least powerful generics libraries - but gets almost all the common uses] I think using descend (from Uniplate) or compos (from Compos paper), or writing a descend/compos operator for SYB, is the right way to go. It's a nicer primitive, and it's perfectly definable in most generics frameworks. I think everywhere' is a bad primitive in any generics framework. Thanks, Neil > > Am 18.04.2010 09:38 schrieb Neil Mitchell: >> >> Hi, >> >> I thought a lot about everywhere' when writing uniplate. I have an >> equivalent of everywhere (transform), but no transform'. In the end I >> decided that it was virtually impossible to use everywhere' correctly, >> and that almost every call of everywhere' was either somewhere an >> everywhere would have worked equally well, or a bug. I then went >> through all the Haskell source code I could find, found about 6 calls >> to everywhere', and all of them would have performed the same job as >> everywhere. >> >> So, my suggestion is that everywhereM' probably isn't very useful, but >> descend/descendM (or compos) is a useful top-down traversal scheme. >> >> Thanks, Neil > >

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Re: Add everywhereM' to Data.Generics.Schemes
by José Pedro Magalhães 18 Apr '10

18 Apr '10

Hi Maciej, Data.Generics.Schemes is now on the syb package, and I'm its maintainer. But I'm not so sure it is wise to start adding more traversal schemes of this form, since there might be many of them: if we include everywhereM', we certainly have to include the bottom-up everywhereM too. Then there are the possible Applicative variants, etc... In general, since these traversal functions are small and everywhere and everywhere' already show the pattern of how to write them, maybe it's best that they are defined by the user, unless there is some general consensus that they would be widely useful. Cheers, Pedro On Mon, Apr 12, 2010 at 20:56, Maciej Podgurski < maciej.podgurski(a)googlemail.com> wrote: > Hello, > > as an analogue to everywhere', I suggest adding everywhereM' to > Data.Generics.Schemes. The implementation would be > > everywhereM' :: Monad m => GenericM m -> GenericM m > everywhereM' f x = do { x' <- f x; > gmapM (everywhereM' f) x' } > > > Best wishes, > > Maciej > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries >

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ANNOUNCE: syb-0.2
by José Pedro Magalhães 09 Apr '10

09 Apr '10

syb: Scrap Your Boilerplate! ================================================================================ Scrap Your Boilerplate (SYB) is a library for generic programming in Haskell. It is supported since the GHC >= 6.0 implementation of Haskell. Using this approach, you can write generic functions such as traversal schemes (e.g., everywhere and everything), as well as generic read, generic show and generic equality (i.e., gread, gshow, and geq). This approach is based on just a few primitives for type-safe cast and processing constructor applications. It was originally developed by Ralf Lämmel and Simon Peyton Jones. Since then, many people have contributed with research relating to SYB or its applications. More information is available on the webpage: http://www.cs.uu.nl/wiki/GenericProgramming/SYB What's new ---------- The biggest improvements from version 0.1 are: * Reintegration of the testsuite; * Introduction of the Generics.SYB.* module hierarchy (as synonyms for their Data.Generics.* counterparts); * Adding gshows :: Data a => a -> ShowS (more efficient than gshow); * Adding gmapAccumA :: forall b d a. (Data d, Applicative a) => (forall e. Data e => b -> e -> (b, a e)) -> b -> d -> (b, a d) ; * Adding ext1B :: (Data a, Typeable1 t) => a -> (forall b. Data b => (t b)) -> a ; * Introduction of two new generic producers ('empty' and 'constrs'). The former produces an empty value of a given datatype, while the latter produces a list of values of a datatype, each using a different constructor. Features -------- * Easy generic programming with combinators * GHC can derive Data and Typeable instances for your datatypes * Comes with many useful generic functions Requirements ------------ * GHC 6.10.1 or later * Cabal 1.6 or later Download -------- Get the package: http://hackage.haskell.org/package/syb Get the source: darcs get http://code.haskell.org/syb/ Bugs & Support -------------- Please report issues or request features at the bug tracker: http://code.google.com/p/scrapyourboilerplate/issues/list For discussion about the library with the authors, maintainers, and other interested persons use the mailing list: http://www.haskell.org/mailman/listinfo/generics

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syb-0.2 release candidate
by José Pedro Magalhães 07 Apr '10

07 Apr '10

Fellow generic programmers, I finally managed to prepare a new release of SYB. Since this is a widely used library I decided to share a "preview" version here before putting it up on Hackage. There is nothing major in this version: basically, the testsuite was reintegrated and I added some basic generic producers. Please let me know if something goes wrong with this new version: http://code.haskell.org/syb/syb-0.2-rc1.tar.gz The draft announcement follows: syb: Scrap Your Boilerplate! > > ================================================================================ > > Scrap Your Boilerplate (SYB) is a library for generic programming in > Haskell. It > is supported in the GHC >= 6.0 implementation of Haskell. Using this > approach, > you can write generic functions such as traversal schemes (e.g., everywhere > and > everything), as well as generic read, generic show and generic equality > (i.e., > gread, gshow, and geq). This approach is based on just a few primitives for > > type-safe cast and processing constructor applications. > > It was originally developed by Ralf Lämmel and Simon Peyton Jones. Since > then, > many people have contributed with research relating to SYB or its > applications. > > More information is available on the webpage: > http://www.cs.uu.nl/wiki/GenericProgramming/SYB > > > What's new > ---------- > > The biggest improvements from version 0.1 are: > * The reintegration of the testsuite; > * The introduction of two new generic producers ('empty' and > 'constrs'). The former produces an empty value of a given datatype, > while > the latter produces a list of values of a datatype, each using a > different > constructor. > > > Features > -------- > > * Easy generic programming with combinators > > * GHC can derive Data and Typeable instances for your datatypes > > * Comes with many useful generic functions > > > Requirements > ------------ > > * GHC 6.10.1 or later > * Cabal 1.2.1 or later > > > Download > -------- > > With cabal-install: > > cabal install syb > > Get the package: > > http://hackage.haskell.org/package/syb > > Get the source: > > darcs get http://code.haskell.org/syb/ > > > Bugs & Support > -------------- > > Please report issues or request features at the bug tracker: > > http://code.google.com/p/scrapyourboilerplate/issues/list > > For discussion about the library with the authors, maintainers, and other > interested persons use the mailing list: > > http://www.haskell.org/mailman/listinfo/generics > Thanks, Pedro

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Rewriting libraries
by Ozgur Akgun 04 Apr '10

04 Apr '10

Hi all, I am implementing something which needs to make use of generic rewriting. I tried implementing a rewriting system of my own, using Uniplate's transform and rewrite methods, and currently I am trying to use the guarded-rewriting package. (whose maintainer is generics(a)haskell.org) I am reading the paper linked from hackage, A Lightweight Approach to Datatype-Generic Rewriting, and I must say I quite liked the approach in most places. Especially not using simple pattern matching in rewrite rules is an important step, to my understanding. However I do have some questions regarding the toolchain: - There is another package, rewriting, which links to the same paper. What is the relationship between these two implementations? Is one of them obsolete? - In the paper, it's mentioned that the Representable instances might be generated automatically. I have quite a complex term structure (and subject to changes), so I wonder if you are planning to work towards achieving this. If not, can you give me some pointers to automate this process myself? - Is it possible to write "nice" show functions for rewrite rules, namely the Rule data type. Thanks a lot for this work, Best, -- Ozgur Akgun PhD Student University of St Andrews

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