July 2009 - Libraries

Adding swap to Data.Tuple
by roconnor＠theorem.ca 11 Jul '09

11 Jul '09

I have a new proposal to add swap and swap' to Data.Tuple http://hackage.haskell.org/trac/ghc/ticket/3298 -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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C includes in packages?
by Maurício 09 Jul '09

09 Jul '09

Is it possible to provide a C header in a cabal package? Maybe as a template for hs2c? I would like to include a few macros in a base package so that all dependent packages could use then to implement C code. Thanks, Maurício

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Dynamically formed list of modules in a Cabal package?
by Dimitry Golubovsky 09 Jul '09

09 Jul '09

Hi, Is this possible to have a list of modules (in the other-modules field of Cabal package description) be formed dynamically? It is a common situation when a Haskell module in a Cabal package is created as a result of preprocessing e. g. by cpp or hsc2hs. This covers the case of one non-Haskell source and one Haskell module. Another scenario, when ffipkg (or domconv*) is used, results in several files (C headers or IDL files) converted to any unpredictable number of Haskell modules (approx. one per C structure/union in the case of ffipkg, or one per IDL interface in the case of domconv). These modules often have meaningless names (esp. in the case of ffipkg) and are not to be used outside of the package that depends on them. So it is not possible to pre-hardcode them all in the hand-written Cabal file. How can Cabal be instructed to add those modules to the list of modules defined in the "other-modules" field of package description? A possible solution might be to create a "private" package before the "main" package is configured, and include the private package into the list of build-depends of the main package. This approach was used for the webidl** package [1] by adding a pre-configuration hook that goes into a sub-directory and builds the private package out of C library headers [2]. This is not a very good solution because private packages will occupy the global package namespace while not being usable by any package other than their main package. A much better solution (one that I am seeking) would be to tell Cabal at some point (via hook, before the configure step starts) that there are more modules in the package that mentioned in the other-modules field. How can this be done? Thanks for any ideas. -------------------------------------------------------- * for applications that run in a web browser via conversion to Javascript and use IDL-defined DOM interfaces ** this package incorporates an IDL lexer generated with flex and used via FFI [1] http://hackage.haskell.org/package/webidl, also see the build log at http://hackage.haskell.org/packages/archive/webidl/0.1.1/logs/success/ghc-6… [2] http://code.haskell.org/yc2js/webidl/Setup.hs -- Dimitry Golubovsky Anywhere on the Web

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Adding a newtype EndoCategory to Control.Category (ticket #3362)
by roconnor＠theorem.ca 08 Jul '09

08 Jul '09

I suggest adding a wrapper to make (a x x) a Monoid for any Category a and type x. This would be added to Control.Category. newtype EndoCategory a x = EndoCategory { runEndoCategory :: a x x } instance (Category a) => Monoid (EndoCategory a x) where mempty = EndoCategory id mappend (EndoCategory f) (EndoCategory g) = EndoCategory (f . g) -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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define gcd 0 0 = 0 (Ticket #3304)
by roconnor＠theorem.ca 08 Jul '09

08 Jul '09

As much as I'd love gcd 0 0 to be 0, the major problem is that this change would contradict the Haskell 98 report. Persumably the Haskell report needs to be changed first. Is there a ticket for this for Haskell prime? I didn't find one. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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Re: define gcd 0 0 = 0 (Ticket #3304)
by Henning Thielemann 08 Jul '09

08 Jul '09

On Wed, 8 Jul 2009, Steve wrote: > On Wed, 2009-07-08 at 10:00 +0200, Henning Thielemann wrote: >> On Tue, 7 Jul 2009, roconnor(a)theorem.ca wrote: >> >>> As much as I'd love gcd 0 0 to be 0, the major problem is that this change >>> would contradict the Haskell 98 report. >> >> There was a long thread about this issue some years ago that came to the >> conclusion that the current behaviour of gcd is algebraically most sound. > > I don't believe there was any such conclusion. But if you can provide a > link to the thread it would be interesting to see it. > > There was a long thread about the issue in 2001 that came to the > conclusion that "there is a lot of support for gcd 0 0 = 0" and "The > strong default for H98 is "no change"". > http://www.mail-archive.com/haskell@haskell.org/msg09883.html You are right, I mixed that up. The mail you mention summarizes the thread with the name "gcd 0 0 = 0" http://www.haskell.org/pipermail/haskell/2001-December/008609.html

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exposing hidden packages with runghc
by Iain Barnett 08 Jul '09

08 Jul '09

I'm trying to install HTTP-3000.0.0 (because I don't have cabal-install, and it's a dependency for cabal-install 0.4.9) This is on a Debian 5.1 machine with GHC 6.8.2 and 6.8.3 on it. The command: sudo runghc Setup configure -p && sudo runghc Setup build && sudo runghc Setup install The error: Could not find module `Data.Array.MArray': it is a member of package array-0.1.0.0, which is hidden ghc-pkg does list the hidden package. I've seen other threads on this "hiding" problem and searched through the GHC user guide, but I haven't actually found a solution, just explanation. I've tried sending the -package flag through to GHC by adding it to the runghc command, but I'm not doing this right, obviously. Would someone be able to point me in the right direction? It would be much appreciated. Iain

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what about moving the record system to an addendum?
by Samuel Bronson 07 Jul '09

07 Jul '09

According to <http://hackage.haskell.org/trac/haskell-prime/wiki/Status>, ticket #99 was rejected, but the tickets own page, <http://hackage.haskell.org/trac/haskell-prime/ticket/99>, says "probably yes". Which is it? I was about to propose this myself, but decided to check the trac just in case it had already been proposed, Haskell being so popular with smart people and all, and was at first rather disappointed and ready to write an angry rant to the list, then rather confused when I noticed the ticket's page said almost the exact opposite of the Status page. In particular, I want records to be considered an extension to Haskell', to be implemented only by compilers that care, and even then only allowed with a LANGUAGE pragma like this: {-# LANGUAGE TraditionalRecordSyntax #-} For pre-Haskell' compilers, we would want something like: {-# LANGUAGE NoTraditionalRecordSyntax #-} which would mean extending the LANGUAGE pragma to support turning extensions off by adding/removing a No from the front of it. Hmm, is it really the case that nobody has proposed LANGUAGE pragmas for Haskell'? I don't see them listed on the Status page. I guess the work involved here is basically: (a) Go through Haskell 98, find all the parts that talk specifically about record syntax, and write up the list of such places. (b) Implement this in GHC: <http://hackage.haskell.org/trac/ghc/ticket/3356> (c) Add support for negating extension names by adding/removing No in front of extension names to Cabal: <http://hackage.haskell.org/trac/hackage/ticket/567> (d) Add {-# LANGUAGE #-} to Haskell' What do you folks think?

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Monoid instances for mtl
by Stefan Holdermans 07 Jul '09

07 Jul '09

Hi all, On a number of occasions I found myself declaring simple instances like instance Monoid a => Monoid (State s a) where mzero = return mzero mappend = liftM2 mappend that make my life considerably easier. Would it be a good idea to add such instances to the Monad Transformer Library? Of course, one could ask where to stop---but for me it make sense to always add sensible instances of the classes from base to a given library. Cheers, Stefan

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Documentation design
by Isaac Dupree 06 Jul '09

06 Jul '09

(for reference, here's the blog-post I wrote that inspired me to ask this list for advice. I'll explain everything in this email anyway though. http://haddock2009.wordpress.com/2009/06/23/how-to-navigate-your-code/ ) My challenge: getting to know an existing code-base quickly and easily, so that I can hack on it. Haddock and HsColour are already pretty helpful at this, but they could be more helpful. (haddock with --ignore-all-exports, i.e. cabal haddock --internal, especially) My dream situation: both haddock-pages and hscolour-pages would be super-hyperlinked and super-readable. For example, haddock would list all a module's definitions, not just its exports. In HsColoured source, every identifier would link to its definition or the haddockumentation of its definition. and so forth. It would be so much easier to generate and browse this in HTML, than to get an IDE working, and it would be so much more readable than a mere text-editor (even with syntax hilighting) and quicker clicking on hyperlinks than grepping for everything. Actually I don't have the resources to worry about designing any HsColour stuff right now (its current design is mainly just a lexeme highlighter). But, with your advices, I can design an improvement on Haddock's ignore-exports mode, which currently has a few shortcomings: 1. you can't tell which identifiers in a module are exported 2. it doesn't document a module's re-exports at all 3. it still obeys {-# OPTIONS_HADDOCK hide #-} et al, even when they're not appropriate for reading documentation of internals 4. 2+3 means that some things may be found nowhere in the documentation at all! Not quite satisfying for something invoked by `cabal haddock --internal`. (Here's a haddock-generated page you can look at so you can figure out the formatting-details I'll be describing: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html and its source code http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/Data-List.ht… ) The ideal haddockumentation-formatting for this purpose isn't quite obvious though. For example, sometimes you want to think about a module in terms of its abstract interface, but sometimes you want to figure out how it's implemented (without reverting completely to text based code browsing). Maybe a compromise of some sort would be good. so... Here's a proposal, for a new mode (`haddock --all-internal`?, to be invoked by `cabal haddock --internal` rather than --internal's current effect of ignore-all-exports) : Files with no explicit export list can be treated as-is anyway. For all files that have an explicit export list, generate the synopsis-of-exports near the top, as usual. But have the index link, generally, to where functions are originally defined (modulo being from a non-internally-documented separate package, where it should link to the appropriate place), and have the fuller documentation below be a compilation of the identifiers *defined* in this module. Actually that would need some revision because the sections and subsections -- *, -- **, etc. defined in the export-list in the .hs, actually are displayed 1. above the table of contents, linking to places in 2. the full list of definitions. Which might be defined in the module in a different order than they're listed in the export list. Why not add the sections into the synopsis? In this case, instead of adding them to the main doc section. But hmm... in ordinary non-internal documentation, would it be nice to *additionally* have the sections marked in the synopsis (in addition to in the Contents and in the main docs section)? Maybe this mode should also abstain from "hiding" any module, because that would cause missing docs. Etc. Questions? Comments? Opinions? Does anyone want this feature, and/or not think it's particularly useful? -Isaac

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