August 2010 - Haskell-Cafe

Re: [Haskell-cafe] ATs vs FDs
by Antoine Latter 14 Aug '10

14 Aug '10

What's wrong with fun-deps? The associated type synonym syntax is prettier, but I didn't tknow that fun-deps were evil. Do you have any links? Take care, Antoine On Aug 14, 2010 5:19 AM, "Andrew Coppin" <andrewcoppin(a)btinternet.com> wrote: As I understand it, ATs were invented because FDs are "evil" and must never be used ever for any purpose. However, it doesn't seem to be possible to use ATs to do the same things that FDs can do. You can use ATs to write type functions, which take one type and return another type. This allows you to express type relationships in a very elegant way. However, what it does /not/ seem to allow you to do is express one-to-one relationships. For example, I'd like to be able to say that the next vector up from a Vector3 is a Vector4, and the next vector down is a Vector2. And I can say that. What I can't say is that the *only* next vector up is a Vector4. And thus, all my code is littered with ambiguous type warnings because although /currently/ there's only one class instance, somebody could come along some day and write another one. What am I missing? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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ATs vs FDs
by Andrew Coppin 14 Aug '10

14 Aug '10

As I understand it, ATs were invented because FDs are "evil" and must never be used ever for any purpose. However, it doesn't seem to be possible to use ATs to do the same things that FDs can do. You can use ATs to write type functions, which take one type and return another type. This allows you to express type relationships in a very elegant way. However, what it does /not/ seem to allow you to do is express one-to-one relationships. For example, I'd like to be able to say that the next vector up from a Vector3 is a Vector4, and the next vector down is a Vector2. And I can say that. What I can't say is that the *only* next vector up is a Vector4. And thus, all my code is littered with ambiguous type warnings because although /currently/ there's only one class instance, somebody could come along some day and write another one. What am I missing?

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Re: [Haskell-cafe] Is bumping the version number evil, if it's not mandated by the PVP?
by Sebastian Fischer 14 Aug '10

14 Aug '10

[CC'ing café again] On Aug 14, 2010, at 12:25 PM, Max Rabkin wrote: > On Sat, Aug 14, 2010 at 11:13 AM, Sebastian Fischer > <sebf(a)informatik.uni-kiel.de> wrote: >> Hello, >> >> I wonder whether (and how) I should increase the version number of >> a library >> when the API does not change but the implementation gets more >> efficient. >> >> Should I bump a.b.C or even a.B to signal that it's worth using the >> new >> version or should I bump only a.b.c.D such that packages that >> depend on a.b >> get installed with the new version automatically? > > My understanding is that the PVP only describes the *minimum* version > bump, not the maximum. There is a third option though: give the > updated version two version numbers, one with an a.b.c.D bump so that > reverse dependencies get the performance improvement, and one with an > a.b.C bump so that users see a new version as worthwhile. That's an interesting idea! In my case I'll probably bump version a.b.c.D with unchanged API but better performance and additionally release a new major version with a new API as well (as I planned to extend the API independently). Thanks! Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Fwd: [Haskell-cafe] ATs vs FDs
by Gábor Lehel 14 Aug '10

14 Aug '10

---------- Forwarded message ---------- From: Gábor Lehel <illissius(a)gmail.com> Date: 2010/8/14 Subject: Re: [Haskell-cafe] ATs vs FDs To: Andrew Coppin <andrewcoppin(a)btinternet.com> You're missing equality constraints for classes. In other words: class (a ~ Smaller (Bigger a), a ~ Bigger (Smaller a)) => Vector a where type Bigger a; type Smaller a FDs and TFs are equivalent in theory, however their implementation in GHC has shortcomings on (I believe) both sides. In this case, GHC unfortunately doesn't yet support equality constraints in class contexts, which you would need to do some of the things FDs can do. I believe there are workarounds for this in some cases, including one which uses plain type classes to encode not-quite-equality, and one where you just add the constraints to every method of the class instead, but I don't know the details. On Sat, Aug 14, 2010 at 12:19 PM, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: > As I understand it, ATs were invented because FDs are "evil" and must never > be used ever for any purpose. However, it doesn't seem to be possible to use > ATs to do the same things that FDs can do. > > You can use ATs to write type functions, which take one type and return > another type. This allows you to express type relationships in a very > elegant way. However, what it does /not/ seem to allow you to do is express > one-to-one relationships. > > For example, I'd like to be able to say that the next vector up from a > Vector3 is a Vector4, and the next vector down is a Vector2. And I can say > that. What I can't say is that the *only* next vector up is a Vector4. And > thus, all my code is littered with ambiguous type warnings because although > /currently/ there's only one class instance, somebody could come along some > day and write another one. > > What am I missing? > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Work is punishment for failing to procrastinate effectively. -- Work is punishment for failing to procrastinate effectively.

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ANNOUNCE: Sifflet visual programming language, release 1.0!
by gdweber＠iue.edu 14 Aug '10

14 Aug '10

Sifflet 1.0 is now available on Hackage. Sifflet is a visual, functional programming language and support system for students learning about recursion. Sifflet programmers define functions by drawing diagrams, and the Sifflet interpreter uses diagrams to show how the function calls are evaluated. New in This Release ------------------- - Sifflet now has exporters to Scheme, Python, and Haskell and a small supporting "library" for each of those languages. (These exporters are intended for beginning students who are learning these languages, not for "industrial-strength" code production.) - Although the exporters apparently generate correct code, there are at present too many parentheses -- a few extras in Python, many extras in Haskell. (No extra parentheses in Scheme, of course.) - Sifflet has a new number type, Sifflet.Data.Number.Number, which allows numbers to be exact or inexact, almost as in Scheme. Originally intended for the Scheme exporter, it is now also used internally by Sifflet's interpreter. - The code is reorganized. There is now a separate library package, sifflet-lib, in addition to the sifflet package which provides only the executable. At present, the library's API is highly unstable. - Sifflet has a new parser for user inputs, made with Parsec, allowing more intuitive input of numbers (liberation from Haskell number syntax: for example, 0.0 can now be entered as 0.0, 0., or .0) and providing more intelligible error messages for some kinds of invalid input. - Sifflet requires Gtk2hs packages (gtk, glib, cairo) version 0.11; support for the 0.10 Gtk2hs packages is dropped. About Sifflet ------------- Sifflet is a visual, functional programming language intended as an aid for learning about recursion. * A picture explains Sifflet better than words: please see the screenshot showing how to evaluate 3!: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Features: - Visual editor. - Visual tracer/debugger which shows how recursive and other function calls are evaluated. To support active learning and avoid screen clutter, Sifflet displays only as much of the computation as the user requests. - A carefully crafted tutorial with 31 pictures and about as many pages. - Number, string, and list data types. - A small collection of primitive functions. - Runnable examples of compound functions. - (New!) Sifflet functions can be "exported" to Scheme, Python 3, and Haskell. * Download: http://hackage.haskell.org/package/sifflet-lib http://hackage.haskell.org/package/sifflet * Home page: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Tutorial: http://mypage.iu.edu/~gdweber/software/sifflet/doc/tutorial.html -- ___ ___ __ _ / _ \ / _ \| | | | Gregory D. Weber, Associate Professor / /_\// / | | | /\ | | Indiana University East / /_\\/ /__| | |/ \| | http://mypage.iu.edu/~gdweber/ \____/\_____/\___/\__/ Tel. (765) 973-8420; FAX (765) 973-8550

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MOF and functional language paradigm
by Vasili I. Galchin 13 Aug '10

13 Aug '10

Hello, In my company (and I believe many, many companies) they use MOF heavily: http://en.wikipedia.org/wiki/Meta-Object_Facility MOF seems to be tightly coupled with object oriented program and has nothing to do with functional language approach?? Regards, Vasili

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Compiled OpenGL program has DOS window?
by Eitan Goldshtrom 13 Aug '10

13 Aug '10

Hi. I'm working in Windows on an OpenGL application. I finally got everything working, doing all of my testing through the interpreter. When I finally compiled and ran the program from an executable for the first time I noticed that before creating my OpenGL window an empty DOS prompt popped up. I think I understand why the computer would want to do that, but I don't know how to make it stop. I've done some OpenGL in C++ and I remember having a similar problem. If I remember correctly the problem was fixed with a compiler flag. I'm using GHC and I looked through their compiler flags, but I didn't find anything that looked like it would deal with this. Anyone have any ideas? -Eitan

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Beautiful Differentiation using VectorSpace
by Christopher Witte 13 Aug '10

13 Aug '10

Hi all, I'm trying to go through the paper Beautiful Differentiation by Conal Elliott and got a bit stuck. So I decided to download the VectorSpace package, which as far as I know implements the paper, and just try and play around with it. Unfortunately when I tried the simple examples at the beginning of the paper I got an error: Prelude Data.VectorSpace> :m Data.Derivative Prelude Data.Derivative> let f1 z = sqrt(3*sin z) Prelude Data.Derivative> f1 (D 2 1) Top level: Couldn't match expected type `Data.VectorSpace.Scalar a' against inferred type `Data.VectorSpace.Scalar (Data.VectorSpace.Scalar a)' NB: `Data.VectorSpace.Scalar' is a type function, and may not be injective <interactive>:1:0: No instances for (Data.MemoTrie.HasTrie (Data.Basis.Basis a), Floating (Data.VectorSpace.Scalar a), Data.VectorSpace.VectorSpace (Data.VectorSpace.Scalar a)) arising from a use of `f1' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Data.MemoTrie.HasTrie (Data.Basis.Basis a), Floating (Data.VectorSpace.Scalar a), Data.VectorSpace.VectorSpace (Data.VectorSpace.Scalar a)) In the expression: f1 (D 2 1) In the definition of `it': it = f1 (D 2 1) <interactive>:1:8: No instance for (Num (a Data.LinearMap.:-* (a :> Data.VectorSpace.Scalar a))) arising from the literal `1' at <interactive>:1:8 Possible fix: add an instance declaration for (Num (a Data.LinearMap.:-* (a :> Data.VectorSpace.Scalar a))) In the second argument of `D', namely `1' In the first argument of `f1', namely `(D 2 1)' In the expression: f1 (D 2 1) Which was completely incomprehensible to me. I tried looking at the documentation but I couldn't find anything about actually using the library. Does anyone have any experience using this library and maybe have some examples of it's use? Thanks Chris Witte

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Build failure for PortMidi on mac: incorrect Block.h
by Brian Victor 13 Aug '10

13 Aug '10

I'm trying to build PortMidi on the mac, and get the error shown below. It appears that an OSX header is trying to include a Block.h file and getting the GHC Block.h instead of the one it expects. I'm new to Haskell and even newer to cabal, so I'm not sure how to go about solving this. Incidentally, cabal, cabal-install, and the system PortMidi libraries are all up-to-date. %cabal install PortMidi Resolving dependencies... Configuring PortMidi-0.1.3... Preprocessing library PortMidi-0.1.3... Building PortMidi-0.1.3... [1 of 1] Compiling Sound.PortMidi ( Sound/PortMidi.hs, dist/build/Sound/PortMidi.o ) In file included from /System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/FSEvents.h:28, from /System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/CarbonCore.h:218, from /System/Library/Frameworks/CoreServices.framework/Frameworks/AE.framework/Headers/AE.h:20, from /System/Library/Frameworks/CoreServices.framework/Headers/CoreServices.h:21, from portmidi/pm_mac/pmmacosxcm.c:38:0: /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:51:0: error: expected specifier-qualifier-list before ‘StgPtr’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:108:0: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘bdescr’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:163:0: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:174:0: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:187:0: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:198:0: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:219:0: error: expected ‘)’ before ‘n’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:223:0: error: expected ‘)’ before ‘n’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:235:0: error: expected declaration specifiers or ‘...’ before ‘nat’ /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/include/Block.h:241:0: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘StgWord’ cabal: Error: some packages failed to install: PortMidi-0.1.3 failed during the building phase. The exception was: exit: ExitFailure 1 -- Brian

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ANN: hswip-0.3 embedding swi-prolog in haskell
by Evgeny Tarasov 13 Aug '10

13 Aug '10

I would like to announce a small library hswip. It is an analogue of pyswip [1] python library in haskell. Hswip allows to embed swi-prolog [2] interpreter into your haskell applications in a convenient way. Let't look at an example: main = do prologInit ["-q","-nosignals"] prologCall "asserta(a(b,c))." prologCall "asserta(parent(pam, bob))." prologCall "asserta(parent(tom, bob))." prologCall "asserta(parent(tom, liz))." prologCall "asserta(parent(bob, ann))." prologCall "asserta(parent(bob, pat))." prologCall "asserta(parent(pat, jim))." res <- prologCall "parent(X,Y)." print res Output: [[("X","pat"),("Y","jim")],[("X","bob"),("Y","pat")],[("X","bob"),("Y","ann")],[("X","tom"),("Y","liz")],[("X","tom"),("Y","bob")],[("X","pam"),("Y","bob")]] The library still has alpha quality and there are many things missing including exception handling and BSD license. Any feedback is welcome! -Evgeny Tarasov [1] http://code.google.com/p/pyswip/ [2] http://www.swi-prolog.org/ [3] http://hackage.haskell.org/package/hswip-0.3

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