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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 24 Jan '08

24 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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Has character changed in GHC 6.8?
by Magnus Therning 23 Jan '08

23 Jan '08

I vaguely remember that in GHC 6.6 code like this length $ map ord "a string" being able able to generate a different answer than length "a string" At the time I thought that the encoding (in my case UTF-8) was “leaking through”. After switching to GHC 6.8 the behaviour seems to have changed, and mapping 'ord' on a string results in a list of ints representing the Unicode code point rather than the encoding: > map ord "åäö" [229,228,246] Is this the case, or is there something strange going on with character encodings? I was hoping that this would mean that 'chr . ord' would basically be a no-op, but no such luck: > chr . ord $ 'å' '\229' What would I have to do to get an 'å' from '229'? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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Precedence and associativity in a pretty-printer
by Edsko de Vries 23 Jan '08

23 Jan '08

Hi, Suppose we have some algebraic datatype describing an expression language containing the usual suspects (various binary arithmetic operators such as addition, subtraction, multiplication, division, exponentiation, function abstraction and application, etc.) each with their own precendence (multiplication binds stronger than addition) and associativity (function application is left associative, exponentiation is right associative, addition is associative, etc.) Is there a nice way to pretty-print such an expression with the minimal number of brackets? I can come up with something, but I'm sure somebody thought hard about this problem before and came up with a really nice solution :) Any hints or pointers would be appreciated, Thanks, Edsko

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Darcs Patch Theory and GADTs at London Haskell User Group tonight
by Neil Bartlett 23 Jan '08

23 Jan '08

Hi all, Just a reminder about tonight's London Haskell User Group meeting, at City University from 6:30PM. Ganesh Sittampalam will be giving a talk entitled "Darcs Patch Theory and GADTs". For full details see the website: http://www.londonhug.net/2008/01/07/darcs-patch-theory-and-gadts-with-ganes… For those outside the London area or unable to come, there will be a video posted to the website later, so subscribe to it in your RSS reader now! Regards, Neil

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