January 2008 - Haskell-Cafe

Fwd: ByteString Parsec clone
by Johan Tibell 25 Jan '08

25 Jan '08

I tried to get Dan's thoughts on cloning parts of the Parsec interface and some of the documentation but none of the emails addresses I've tried seem to work. What's allowed when it comes to duplicating something like an API? What about the documentation? I intend it to be released under BSD3 which I believe is used for Parsec as well. Anyone know how I can reach Dan? -- Johan ---------- Forwarded message ---------- From: Johan Tibell <johan.tibell(a)gmail.com> Date: Jan 22, 2008 2:56 PM Subject: ByteString Parsec clone To: daan(a)cs.uu.nl I've written, but yet not published, a ByteString version of parts of the Parsec API as a part of a web application server project. I've currently used almost the exact wording from your documentation (as the semantics of the functions is intended to be the same as in Parsec.) Are you OK with that? P.S. I intend to give my library a different name as it has different module structure and doesn't support a lot of Parsec's features. It will be BSD licensed. Best regards, Johan Tibell

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Binary IO of a list of ints
by Jamie Love 24 Jan '08

24 Jan '08

Hi there I have a list of ints, with values between 0 and 255 and I need to print them out in little endian form to a file. I've been using Data.Binary for single values, e.g. runPut $ do put 'B' put (0 :: Int32) I'm wondering how I can go about writing a list of Ints out. My thought was to do something like: foldr (\x B.hPut output (runPut $ do put (x :: Word8))) data (where output is my file handle), but apart from giving me type errors, it seems a rather arduous way to do it. Could anyone suggest a better way to do this? -- Jamie Love Senior Consultant Aviarc Australia Mobile: +61 400 548 048 ------------------------------------------------------------ This message has been scanned for viruses and dangerous content by MailScanner and is believed to be clean.

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ANNOUNCE: Harpy 0.4 - Runtime code generation for x86 machine code
by Martin Grabmueller 24 Jan '08

24 Jan '08

We are pleased to announce the release of Harpy 0.4, a library for runtime code generation for x86 machine code. Harpy is available from Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/harpy-0.4 Also see Harpy's homepage, which features two tutorials and access to the darcs repo (and to the Haddock documentation, until it is available from Hackage, too): http://uebb.cs.tu-berlin.de/harpy/ Harpy is supposed to be built with GHC 6.8 and GHC 6.6 (with Cabal 1.2 or greater). Please report any build problems you encounter. This release features the following additions and fixes: - New convenience top-level module "Harpy", which re-exports Harpy.CodeGenMonad, Harpy.Call and Harpy.X86Assembler - It is now possible to override Harpy's automatic code buffer management. The new field 'customCodeBuffer' in the type 'CodeGenConfig' can be set to 'Just (buf, size)', where 'buf' is a pointer to a memory region of 'size' bytes. Harpy will then use the supplied code buffer and will not perform any automatic code buffer allocation on overflow. Overflow checking is still performed and will result in an exception in the CodeGen monad. - When using the high-level assembler in X86Assembler, the code buffer is automatically protected from overflow. - Floating point operations added to X86Assembler (only for double operands yet). - Preliminary support for SSE instructions. Currently, only the packed and scalar floating-point arithmetic operations are supported (both in the low-level module Harpy.X86CodeGen and as methods in Harpy.X86Assembler) - Code buffer default size has been increased from 128 to 4096 bytes. - The CodeGenMonad fails when a label is defined twice. - It is now possible to associate names with labels, using the new operation newNamedLabel. The given names will show up in the disassembly, which makes debugging of generated code much easier. - The doc directory contains a second, slightly larger tutorial now. - The examples/evaluator directory contains a small example interpreter for arithmetic expressions, which translates expressions entered at the keayboard to machine code on the fly. This is the demo program we presented at the Haskell Workshop 2007. Happy Haskell Hacking with Harpy, Martin and Dirk

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Re: [Haskell-cafe] Re: Why functional programming matters
by John Lato 24 Jan '08

24 Jan '08

In addition to STM, another item that should interest serious programmers is forkIO. Lightweight threads that (unlike in Python) can use multiple cpu's. Coming from Python, I personally appreciate this. Using STM to handle concurrency issues often greatly simplifies multithreaded code.

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OT: Isorecursive types and type abstraction
by Edsko de Vries 24 Jan '08

24 Jan '08

Hi, This is rather off-topic but the audience of this list may be the right one; if there is a more appropriate venue for this question, please let me know. Most descriptions of recursive types state that iso-recursive types (with explicit 'fold' and 'unfold' operators) are easy to typecheck, and that equi-recursive types are much more difficult. My question regards using iso-recursive types (explicitly, not implicitly using algebraic data types) together with type abstraction and application. The usual typing rules for fold and unfold are e :: Fix X. t ----------------------------- Unfold unfold e :: [X := Fix X. t] t e :: [X := Fix X. t] t ----------------------------- Fold fold e : Fix X. t This works ok for the following type: ListInt = Fix L. 1 + Int * L (where "1" is the unit type). If x :: ListInt then unfold x :: 1 + Int * ListInt using the Unfold typing rule. However, this breaks when using type abstraction and application. Consider the polymorphic version of list: List = Fix L. /\A. 1 + A * L A Now if we have x :: List Int we can no longer type unfold x because x does not have type (Fix ..), but ((Fix ..) Int) instead. Of course, we can unroll List Int by first unrolling List, and then re-applying the unrolled type to Int to get (/\A. 1 + A * (Fix L. /\A. 1 * L A) A) Int which can be simplified to 1 + Int * List Int but this is not usually mentioned (that I could find; in particular, TAPL does not mention it) and I'm worried that there are subtleties here that I'm missing--nor do I have an exact definition of what this 'extended' unrolling rule should do. Any hints or pointers to relevant literature would be appreciated! Edsko PS. One way to simplify the problem is to redefine List as List = /\A. Fix L. 1 + A * L so that List Int can easily be simplified to the right form (Fix ..); but that can only be done for regular datatypes. For example, the nested type Perfect = Fix L. /\A. A + Perfect (A, A) cannot be so rewritten because the argument to Perfect in the recursive call is different.

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ghc and Red Hat 5 .rpm
by Galchin Vasili 24 Jan '08

24 Jan '08

Hello, I have searching on http://haskell.org/ghc/download_ghc_682.html .. but can't find a ghc .rpm so that I can install ghc. ? Regards, Vasili

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Fwd: Arrow without `>>>'
by Valery V. Vorotyntsev 24 Jan '08

24 Jan '08

The problem has been resolved. Kudos to Ross Paterson. ---------- Forwarded message ---------- From: Valery V. Vorotyntsev <valery.vv(a)gmail.com> Date: Jan 24, 2008 3:34 PM Subject: Re: Arrow without `>>>' To: libraries(a)haskell.org On 1/24/08, Ross Paterson <ross(a)soi.city.ac.uk> wrote: > > The right question: how to build `arrows' in such circumstances? > > To build with the darcs version of the base library (built with GHC), > you'll need the darcs version of the arrows library, which I think is > right now: > > darcs get http://darcs.haskell.org/packages/arrows Yes, after pulling the latest change from repo installation went smoothly. Thank you very much, Ross!

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An idea - Feasibility and effort
by Vimal 23 Jan '08

23 Jan '08

Hi Haskellers, This semester, I am having a software engineering course. I always wanted to do something significant in Haskell. This is probably an opportunity that I can utilize. I have the following ideas, with inspiration from various sources. Please let me know your views/ideas on them. If you feel you are experienced enough, do warn me if the idea might not be feasible to implement! I will have about half a month for getting the Software Requirement Specification ready, and ~ 1 month of time for coding. Idea 1: A Traceroute visualizer. I saw the video on the London Haskell Group on how to create a game in Haskell, and loved the "rotating earth" part of the game. (Space Invaders or something like that). Combine the earth (with a Google earth touch) with a traceroute backend and animate packet routing via different cities live on the screen. I like this idea. More of a fun project than a useful one. Probably Haskell isnt a good language to implement it, but as they say, one should never think about moulding a project to suit the language you code in. Idea 2: A module that generates optimum (in some sense) set of instructions for a given (pure, functional) expression. So, this basically involves concepts like * avoiding recalculating common subexpressions (expression-tree matching?), * register allocation (assuming a finite supply of registers, modeling as graph colouring), * Term rewriting I am just quoting some terms there, I may be wrong. but this might be a project that Haskell might help me beautifully model and execute. I would like to have some opinion on the complexity of the project ideas that I have in mind. Thanks, -- Vimal Indian Institute of Technology Madras Chennai India

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Abstracting ByteStrings
by Chad Scherrer 23 Jan '08

23 Jan '08

A lazy ByteString is an alternative to a String=[Char], where sacrificing some degree of laziness through "chunks" gives much greater performance in many applications. If I remember correctly, we could as well create an IntString, DoubleString, etc by filling the chunk arrays with different types. Now, this might only work for primitive types, but couldn't we extend it by taking a similar approach to NDP? For example, if we have (lazy) ByteString = [Chunk Char], then we could represent, say [Chunk (a,b)] as ([Chunk a], [Chunk b]), etc Is this reasonable? Do you think it would work? Or is all of this subsumed by fusion anyway? -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Ann: Drawing with Cairo: Getting Started
by Hans van Thiel 23 Jan '08

23 Jan '08

Hello All, There is now an appendix to the Gtk2Hs tutorial on getting started with Cairo drawing. It's not actually about drawing, but on how to get drawings on the screen and how to write them to .png, .pdf, .ps. or .svg files. Though this is very easy, the 'recipes' are not self-evident, so this mini-tutorial could save prospective users some time. See: http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/app1.xhtml or: http://j-van-thiel.speedlinq.nl/gtk2hs/app1.html I'd like to add, for Spanish readers, that the Gtk2Hs tutorial itself has been translated into Spanish. See: http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/es-index.xhtml or: http://j-van-thiel.speedlinq.nl/gtk2hs/es-index.html Alex Tarkovsky's Glade tutorial http://haskell.org/gtk2hs/docs/tutorial/glade/ has also been translated into Spanish: http://j-van-thiel.speedlinq.nl/glade/es-index.html The translator of both texts prefers to be known as Laszlo Keuschnig. In case anyone wonders, this is not me, though I wish.. Best Regards, Hans van Thiel

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