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Re: [Haskell-cafe] Design Patterns by Gamma or equivalent
by wren ng thornton 18 Mar '09

18 Mar '09

Mark Spezzano wrote: > Because Haskell is not OO, it is functional, I was wondering if there is > some kind of analogous “design pattern”/”template” type concept that > describe commonly used functions that can be “factored out” in a general > sense to provide the same kind of usefulness that Design Patterns do for > OOP. Basically I’m asking if there are any kinds of “common denominator” > function compositions that are used again and again to solve problems. If > so, what are they called? Most of the (particular) problems OO design patterns solve are non-issues in Haskell because the language is more expressive. The issues giving rise to patterns like Visitor and Factory are non-issues, so their "solutions" are trivial. A number of other patterns can actually be written down once and for all (in higher-order functions like foldr, map,...) instead of needing repetition. But that's not to say we don't have our own expressiveness deficiencies ;) The analogous term for the genre is "functional pearl", though the individual pearls don't tend to be as codified as in OOP. One example is using coproducts for open unions[1] which solves the dual problem to Visitor. Another is using open recursion[2]. A third example along the same track is using two-level types[3]. But again a lot of the patterns, once discovered, get turned into libraries that can be used off the shelf: e.g. the two-continuation solution for efficient and fair backtracking search[4], and list-fusion techniques[5]. There also a number of "idioms" which are similar in scope to the idioms that arise in other languages: using tail recursion, accumulators, continuation-passing transformations, closures over recursion[6], Schwartzian transforms, etc. And then there are some things like monoids which fall somewhere between idiom and pearl. Some specific OO patterns have direct analogues in "defunctionalization"[7]. For the most part defunctionalization is something best left to the compiler, but on occasion we want to be explicit about it and this too is an idiom/pearl border case IMO. And, of course, there's the entire cottage industry of using category theory for fun and profit. [1] http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf [2] http://www.cs.utexas.edu/~wcook/Drafts/2006/MemoMixins.pdf [3a] http://web.cecs.pdx.edu/~sheard/papers/generic.ps [3b] http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps [4a] http://okmij.org/ftp/papers/LogicT.pdf [4b] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logict [5a] http://www.cse.unsw.edu.au/~dons/papers/CSL06.html [5b] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring [5c] http://www.cse.unsw.edu.au/~dons/papers/CLS07.html [5d] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stream-fusion [5e] http://homepages.inf.ed.ac.uk/wadler/papers/vanish/vanish.pdf [6] For lack of a better name. I mean doing this: > -- Here the a,b,c,d variables are captured in the closure for go > recursive a b c d = go > where > go 0 = ...a...b... > go n = ...c...d...go (n-1) instead of this: > -- ...instead of needing to be passed through the recursion each time > recursive a b c d 0 = ...a...b... > recursive a b c d n = ...c...d...recursive a b c d (n-1) [7a] http://blog.plover.com/prog/defunctionalization.html -- Live well, ~wren

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Re: [Haskell-cafe] Re: Haskell Logo Voting has started!
by Daniel Schüssler 18 Mar '09

18 Mar '09

Hi, > Even worse, the buttons for moving items up and down are buggy - at > least on my browser (Firefox 3.1 beta 2 on Linux). They sometimes > reorder my other votes! Even assuming that the list box code is not > buggy (which I now doubt), not being able to use the buttons makes this > form almost unusable! I guess it works like this: "Move down" on an element X will make the /smallest/ change (with respect to the abstract ordering) that makes X worse than it's current value. This may require changing more than one number, for example if we have: (105: ) (106: A) (107: X,B) (108: C,D) (109: E ) (110: ) moving down X will result in either (105: A) (106: B) (107: X ) (108: C,D) (109: D ) (110: ) or equivalently (105: ) (106: A) (107: B ) (108: X ) (109: C,D) (110: D) of course, maybe you already know that but you're saying that it is doing this wrongly ;) Btw, I really think the logos should be on the same page as the voting form. Hotkeys for the buttons would be good too. It's usable though if you (incorrectly) consider the numbers as a sort of reverse "star rating" to make a O(n) pass, possibly finetuning later. Greetings, Daniel

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Re: [Haskell-cafe] Who generates Haskell code and uses type information at runtime?
by Martin Hofmann 18 Mar '09

18 Mar '09

> GHC is the current state of the art, which will give you a full > Haskell AST enriched with type information. You can then just modify the > AST directly. I experimented with the AST from HscTypes.CoreModule in the ghc api. However, it seems that this representation is too far away from my theory. I would prefer something similar as Template Haskell. > The ghc-api is somewhat sparsely documented That might be the reason, why I have not seen how it can help me. > It really depends on what you are trying to do... Putting in a nutshell, I generalize an extensional defined function definition into a recursive one. This is done in a number of steps by modifying expressions and exploiting type information of sub-expressions. For example: rev [] = [] rev [a] = [a] rev [a,b] = [b,a] rev [a,b,c] = [c,b,a] ~~> rev x = y ~~> rev [] = [] rev (x:xs) = (y:ys) ~~> rev [] = [] rev (x:xs) = (last (x:xs)) : (reverse (x:xs)) The initial set of rules is given by the user (in a file, via IO, ...). The problem later is to infer the type of an intermediate variable, e.g. 'y'. Thanks, Martin

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Who generates Haskell code and uses type information at runtime?
by Martin Hofmann 18 Mar '09

18 Mar '09

I try to modify Haskell code, parsed from an external source, at runtime. Therefore I need type information about this code. Sometimes to this is referred to as treating "data as code" and "code as data". In homoiconic languages such as Lisp this would be less cumbersome, but in Haskell this seems to be not trivial. Therefore I would like to know how other people solved this. Do you, for example, * use ghc-as-api and manipulate the code snippet in GHC's core language? * use Language.Haskell.Syntax or Language.Haskell.TH and get your type information via IO (from Hint or ghc) as a String? * use your own language representation for code and type and use a tailored type inference/unification algorithm? * use your own language representation combined with Data.Typeable and Data.Dynamics, but what about polymorphic types? * ...? Thanks a lot, Martin

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Re: [Haskell-cafe] OpenGL Linking Issue
by Sean Lee 18 Mar '09

18 Mar '09

Hi Mark, >From what I remember -- I haven't used OpenGL for about a year -- you need something like ghc --make -package GLUT -lglut Test1.hs assuming that you have both HOpenGL haskell package and OpenGL c library. Cheers, Sean 2009/3/18 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > > > I hope I’m posting to the right forum. > > > > I’ve got OpenGL up and running on my Windows Vista machine (finally!) and it > runs perfectly well under Eclipse, bringing up a window with a rendered > image as expected. > > > > The only issue starts when I try to compile the program without Eclipse > (using GHC rather than GHCi like Eclipse does). > > > > I type: > > > > C:\Users\Mark\workspace2\OpenGLPractice\src>ghc --make Test1.hs > > > > And get the following output > > > > Linking Test1.exe ... > > C:\Program > Files\Haskell\GLUT-2.1.1.2\ghc-6.10.1/libHSGLUT-2.1.1.2.a(Extensions. > > o):fake:(.text+0xcc): undefined reference to `glutGetProcAddress@4' > > collect2: ld returned 1 exit status > > > > What’s happening here? Obviously it’s a linking issue, but I was wondering > whether I need to include some library or file or option to ghc so that it > links correctly. > > > > Cheers, > > > > Mark Spezzano > > > > No virus found in this outgoing message. > Checked by AVG. > Version: 7.5.557 / Virus Database: 270.11.18/2008 - Release Date: 17/03/2009 > 4:25 PM > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Sean Lee PhD Student Programming Language and Systems Research Group School of Computer Science and Engineering University of New South Wales

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OpenGL Linking Issue
by Mark Spezzano 18 Mar '09

18 Mar '09

Hi, I hope I’m posting to the right forum. I’ve got OpenGL up and running on my Windows Vista machine (finally!) and it runs perfectly well under Eclipse, bringing up a window with a rendered image as expected. The only issue starts when I try to compile the program without Eclipse (using GHC rather than GHCi like Eclipse does). I type: C:\Users\Mark\workspace2\OpenGLPractice\src>ghc --make Test1.hs And get the following output Linking Test1.exe ... C:\Program Files\Haskell\GLUT-2.1.1.2\ghc-6.10.1/libHSGLUT-2.1.1.2.a(Extensions. o):fake:(.text+0xcc): undefined reference to `glutGetProcAddress@4' collect2: ld returned 1 exit status What’s happening here? Obviously it’s a linking issue, but I was wondering whether I need to include some library or file or option to ghc so that it links correctly. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.18/2008 - Release Date: 17/03/2009 4:25 PM

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Haskell on your system? Information wanted!
by Don Stewart 18 Mar '09

18 Mar '09

Hey all, I noticed we didn't have an easy page to find out how to get hold of the Haskell toolchain for various systems. So there's now a link from haskell.org to (existing) pages on how to obtain Haskell on windows, mac osx and linux and bsd. If you're a distro maintainer for these systems, please consider adding relevant pointers to the pages, so that users of these systems can find all the info they need. http://haskell.org/haskellwiki/Windows http://haskell.org/haskellwiki/OSX http://haskell.org/haskellwiki/GNU/Linux http://haskell.org/haskellwiki/BSD -- Don

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Re: [Haskell-cafe] Design Patterns by Gamma or equivalent
by Gregg Reynolds 18 Mar '09

18 Mar '09

2009/3/11 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > I’m very familiar with the concept of Design Patterns for OOP in Java and > C++. They’re basically a way of fitting components of a program so that > objects/classes fit together nicely like Lego blocks and it’s useful because > it also provides a common “language” to talk about concepts, like Abstract > Factory, or an Observer to other programmers. In this way one programmer can > instantly get a feel what another programmer is talking about even though > the concepts are fundamentally abstract. > > Because Haskell is not OO, it is functional, I was wondering if there is > some kind of analogous “design pattern”/”template” type concept that > describe commonly used functions that can be “factored out” in a general > sense to provide the same kind of usefulness that Design Patterns do for > OOP. Basically I’m asking if there are any kinds of “common denominator” > function compositions that are used again and again to solve problems. If > so, what are they called? > You might find it useful to read the original works upon which the whole "design pattern" economy is based, namely Christopher Alexander's books (http://www.patternlanguage.com/leveltwo/booksframe.htm?/leveltwo/../booksto…) I liked "Notes on the Synthesis of Form", which predates his pattern language stuff. My $0.02: design patterns became popular in the imperative language community precisely because such languages lack the formal discipline of functional languages. Alexander came up with the notion of a pattern language in order to bring some kind of discipline and abstraction to observed regularities in architecture, urban design, etc. By definition such a language cannot have anything close to formal semantics, any more than a natural language lexicon can have formal semantics. Alexander's writing is very interesting and thought provoking, but I'm not sure it has much to offer the world of functional programming. Programmers used Alexander's ideas to try to bring some order to both specifications and programs. A pattern language is a natural for trying to describe a real-world problem one is trying to solve. Imperative programmers also used it to describe programming patterns. Implementations of things like Observer/VIsitor etc. are ad-hoc, informal constructions; the equivalent in a functional language is a mathematical structure (feel free to fix my terminology). I don't think one needs design patterns for Haskell; it has mathematics and more-or-less formal semantics. Who needs "Visitor" when you have "For All"? Although design patterns may still be useful for describing a real-world problems I suspect one would be better off dumping the notion of design pattern, the better to avoid cognitive dissonance when trying to think in purely functional terms. Cheers, -gregg

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How can I check which thunks are piling up on the stack?
by Gü?nther Schmidt 18 Mar '09

18 Mar '09

Hi, How can I check which thunks are piling up on the stack? Günther

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OpenGL and Cabal installation
by Mark Spezzano 18 Mar '09

18 Mar '09

Hi all, I’m trying desperately to get OpenGL up and running. I type >cabal.exe configure and get the following ... checking GL/glut.h usability... no checking GL/glut.h presence... no checking for GL/glut.h... no checking for GLUT library... no checking for GL/glut.h... (cached) no checking GLUT/glut.h usability... no checking GLUT/glut.h presence... no checking for GLUT/glut.h... no configure: error: no GLUT header found, so this package cannot be built See `config.log' for more details. Where is it looking for these glut.h files? I’ve tried putting them everywhere in my PATH but msys won’t find them. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.18/2008 - Release Date: 17/03/2009 4:25 PM

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