May 2007 - Haskell-Cafe

enumFrom* strangeness on GHC?
by Felipe Almeida Lessa 04 Jun '07

04 Jun '07

Hello =), I'm puzzled, and maybe someone can help me out. Why does this happens? $ time ghci -e "last $ take 1000000 $ [1..1000000]" 1000000 real 0m0.673s user 0m0.554s sys 0m0.024s $ time ghci -e "last $ take 1000000 $ [1..]" *** Exception: stack overflow real 0m1.305s user 0m1.057s sys 0m0.062s $ ghci -V The Glorious Glasgow Haskell Compilation System, version 6.6 Looking at http://darcs.haskell.org/ghc-6.6/packages/base/GHC/Enum.lhs , I still have no clue of what may be happening. Any ideas? Should I file a bug on GHC? Thanks, -- Felipe.

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Just for a laugh...
by Andrew Coppin 04 Jun '07

04 Jun '07

If you're bored... can you come up with a solution to this? http://warp.povusers.org/ProgrammingChallenge.html (Obviously a pretty silly challenge, but hey.) My first instinct was to use Data.Bits - but I see no instance for Double. (Presumably because performing bitwise operations on a Double is a pretty odd thing to want to do.) So my next guess is to do some bizzare type system hackery that allows you to transform a Double into... whichever integer type is the same size, and proceed from there. Does anybody know how to do that? (Otherwise... wasn't there some library somewhere for serialising values in binary?)

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OpenGL
by Jon Harrop 02 Jun '07

02 Jun '07

I've found HOpenGL and the Debian package libghc6-opengl-dev. The former seems to be very out of date (last release 2003) but I can't find any demos for the latter. Where should I go to get started with OpenGL and Haskell? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

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Resolved: [Haskell-cafe] ffi linking problem
by jeff p 02 Jun '07

02 Jun '07

Hello, In case anyone else finds this useful... My linking problem was finally resolved by using the -fvia-C flag when compiling with ghc. Thanks to Stefan O'Rear who pointed out the possibility and wrote: > > Does using -fvia-C help at all? The C compiler understands header > files and is sometimes better equipped to resolve things. > regards, Jeff

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Has anyone looked into adding subtyping to Haskell?
by Al Falloon 01 Jun '07

01 Jun '07

OCaml has been getting a lot of mileage from its polymorphic variants (which allow structural subtyping on sum types) especially on problems relating to AST transformations and the infamous "expression problem". Has there been any work on extending Haskell's type system with structural subtyping? What is the canonical solution to the expression problem in Haskell? What techniques do Haskellers use to simulate subtyping where it is appropriate? I bring this up because I have been working on a Scheme compiler in Haskell for fun, and something like polymorphic variants would be quite convinent to allow you to specify versions of the AST (input ast, after closure conversion, after CPS transform, etc.), but allow you to write functions that work generically over all the ASTs (getting the free vars, pretty printing, etc.). -- Alan Falloon

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Crazy idea: overloading function application notation
by Jon Harrop 31 May '07

31 May '07

This is a crazy idea I've been working on: overload the syntax "x y" so it can mean function application "f x = f(x)" or multiplication "x y = x*y". The reason is simply that I appreciate the brevity of MLs function application but I also appreciate the brevity of Mathematica's multiplication. Is it possible to implement this in Haskell using type classes? Is there any way this could actually be practicable? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

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Re: Monads and constraint satisfaction problems (CSP)
by Greg Meredith 31 May '07

31 May '07

Brandon, Jeremy, et al, Thanks for the pointers. The paper by OlegK, et al, articulates exactly the structure i was noticing, except that i was coming at it from the other end. i was noticing that a wide range of these CSP-style problems could be decomposed into a trivial monad (e.g., list, multiset, set) and some additional search machinery (over which a good solution should be parametric). They show how to add a reasonable upper limit on the search machinery to any monad. Best wishes, --greg Date: Thu, 31 May 2007 11:36:55 -0700 > From: "Jeremy Shaw" <jeremy.shaw(a)linspireinc.com> > Subject: Re: [Haskell-cafe] Monads and constraint satisfaction > problems (CSP) > To: "Greg Meredith" <lgreg.meredith(a)biosimilarity.com> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: <ejkwj0vs.wl%jeremy.shaw(a)linspireinc.com> > Content-Type: text/plain; charset=US-ASCII > > At Thu, 31 May 2007 10:42:57 -0700, > Greg Meredith wrote: > > > BTW, i think this could have a lot of bang-for-buck because the > literature i > > read exhibited two basic features: > > > > - the "standard" treatments (even by CS-types) are decidedly not > > compositional > > - the people in the field who face industrial strength csp problems > > report that they have to take compositional approaches because the > problems > > are just too large otherwise (both from a human engineering problem > as well > > as a computational complexity problem) > > This paper describes a non-monadic, compositional method for solving CSPs: > > > http://www.cse.ogi.edu/PacSoft/publications/2001/modular_lazy_search_jfp.pdf > > There is also the LogicT monad transformer: > > http://okmij.org/ftp/Computation/monads.html > > j. > -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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New book: Real-World Haskell!
by dons＠cse.unsw.edu.au 31 May '07

31 May '07

Bryan O'Sullivan, Don Stewart and John Goerzen are pleased, and frankly, very excited to announce that were developing a new book for O'Reilly, on practical Haskell programming. The working title is Real-World Haskell. The plan is to cover the major techniques used to write serious, real-world Haskell code, so that programmers can just get to work in the language. By the end of the book readers should be able to write real libraries and applications in Haskell, and be able to: * design data structures * know how to write, and when to use, monads and monad transformers * use Haskells concurrency and parallelism abstractions * be able to write parsers for custom formats in Parsec. * be able to do IO and binary IO of all forms * be able to bind Haskell to foreign functions in C * be able to do database, network and gui programming * know how to do exception and error handling in Haskell * have a good knowledge of the core libraries * be able to use the type system to track and prevent errors * take advantage of tools like QuickCheck, Cabal and Haddock * understand advanced parts of the language, such as GADTs and MPTCs. That is, you should be able to just write Haskell! The existing handful of books about Haskell are all aimed at teaching programming to early undergraduate audiences, so they are ill-suited to people who already know how to code. And while theres a huge body of introductory material available on the web, you have to be both tremendously motivated and skilled to find the good stuff and apply it to your own learning needs. The time has come for the advanced, practical Haskell book. Heres the proposed chapter outline: 1. Why functional programming? Why Haskell? 2. Getting started: compiler, interpreter, values, simple functions, and types 3. Syntax, type system basics, type class basics 4. Write a real library: the rope data structure, cabal, building projects 5. Typeclasses and their use 6. Bringing it all together: file name matching and regular expressions 7. All about I/O 8. I/O case study: a DSL for searching the filesystem 9. Code case study: barcode recognition 10. Testing the Haskell way: QuickCheck 11. Handling binary files and formats 12. Designing and using data structures 13. Monads 14. Monad case study: refactoring the filesystem seacher 15. Monad transformers 16. Using parsec: parsing a bioinformatics format 17. Interfacing with C: the FFI 18. Error handling 19. Haskell for systems programming 20. Talking to databases: Data.Typeable 21. Web client programming: client/server networking 22. GUI programming: gtk2hs 23. Data mining and web applications 24. Basics of concurrent and parallel Haskell 25. Advanced concurrent and parallel programming 26. Concurrency case study: a lockless database with STM 27. Performance and efficiency: profiling 28. Advanced Haskell: MPTCs, TH, strong typing, GADTs 29. Appendices We're seeking technical reviewers from both inside and outside the Haskell community, to help review and improve the content, with the intent that this text will become the standard reference for those seeking to learn serious Haskell. If you'd like to be a reviewer, please drop us a line at book-review-interest(a)realworldhaskell.org, and let us know a little about your background and areas of interest. Finally, a very exciting aspect of this project is that O'Reilly has agreed to publish chapters online, under a Creative Commons License! Well be publishing chapters incrementally, and seeking feedback from our reviewers and readers as we go. You can find more details and updates at the following locations: * The web site, http://www.realworldhaskell.org/blog/welcome/ * The authors, http://www.realworldhaskell.org/blog/about/ * The blog, http://www.realworldhaskell.org/blog/ -- Bryan O'Sullivan, Don Stewart and John Goerzen.

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Frisby grammars that have context
by Mark.Carroll＠Aetion.com 31 May '07

31 May '07

I've been playing with Text.Parsers.Frisby to see how it stacks against other options and, while it's been great so far, I am finding that I can't encode a grammar where what's acceptable depends on what's already been parsed in some nontrivial way. To take a simple example, imagine a grammar where the only lower-case letters that are acceptable are those where their upper-case equivalent occurred earlier in the text. In Parsec I'd code this sort of thing as, nextChar = do allowed <- getState char <- oneOf $ ['A'..'Z'] ++ allowed updateState (union [toLower char]) return char test = runParser (many1 nextChar) [] "" Is this supposed to not be possible in Frisby, or (quite likely) am I missing something that allows me to? I've thought about things like trying to fmap further calls to apply runPeg to rest, but I've not figured out a trick that will actually work. -- Mark

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Monads and constraint satisfaction problems (CSP)
by Greg Meredith 31 May '07

31 May '07

All, All this talk about Mathematica and a reference to monadic treatments of backtracking reminded me that a year ago i was involved in work on a Mathematica-like widget. At the time i noticed that a good deal of the structure underlying LP, SAT and other solvers was terribly reminiscent of comprehension-style monadic structure. i think i asked Erik Meijer if he knew of any work done on this and posted to LtU, but nobody seemed to have understood what i was mumbling about. So, let me try here: does anybody know of references for a monadic treatment of constraint satisfaction? BTW, i think this could have a lot of bang-for-buck because the literature i read exhibited two basic features: - the "standard" treatments (even by CS-types) are decidedly not compositional - the people in the field who face industrial strength csp problems report that they have to take compositional approaches because the problems are just too large otherwise (both from a human engineering problem as well as a computational complexity problem) Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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