May 2009 - Haskell-Cafe

is value evaluated?
by Nikhil Patil 09 May '09

09 May '09

Hi, I am curious to know if there is a function in Haskell to find if a certain value has already been evaluated. The function I need would have the type: > (?!) :: a -> Bool And I expect it to be such that the following terminates after printing the first 101 fibonacci numbers. > fibs = 0 : 1 : zipWith (+) fibs (tail fibs) > > main = do print $ fibs !! 100 > print $ takeWhile (?!) fibs Although I guess I can imagine the following not terminating: > print $ filter (?!) fibs I would find such a function immensely useful in "printing out" my infinite lists. Thanks! nikhil

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Simulation and GHC Thread Scheduling
by Thomas DuBuisson 09 May '09

09 May '09

All, I have a simple Haskell P2P library that I've been playing with in simulations of 20 to 600 nodes. To run the simulation there is a Haskell thread (forkIO) for every node in the system, one that starts up all the nodes and prints the info (so prints aren't mangled), and one that acts as the router. Before its mentioned - I understand the best way forward would be to refactor the code into IO-less 'algorithm' sections and other sections that perform the needed IO when I'm not simulating. I know this would allow me to declare what order each node runs in and would free me from the scheduler. I'd like to do that if its practical... but! None-the-less, here I am saying that there are many interesting little simulations that could be done without refactoring and the correctness isn't altered by the order of operations (not if the nodes behave properly, the slight variation is actually a good test). What I would like to know is are there any plans for GHC to incorporate user-definable scheduler? It would be useful in numerous instance beyond this poor example; I know user scheduling was briefly mentioned in Li's paper but haven't seen or heard of any interest from others since then. Thomas

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runhaskell CLI parameters
by Vasili I. Galchin 09 May '09

09 May '09

Hello, I have forgotten the runhaskell CLI parameters ... sigh. In particular I want to a "local" build of a set of of package: runhaskell Setup.hs configure --user??? I just did a "runhaskell -?" which didn't tell me a lot! Kind regards, Vasili

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Question concerning Haskell Foundation
by Vasili I. Galchin 09 May '09

09 May '09

Hello, With Haskell Foundation, 1) Can we still publish packages on Hackage? 2) Is Hackage going away? Regards, Vasili

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Writing a compiler in Hakell
by Rouan van Dalen 09 May '09

09 May '09

Hi everyone. I am designing my own programming language. I would like to know what is the best way to go about writing my compiler in haskell. What are the tools available in haskell that can help with compiler construction? I know about Happy. Is that a good tool to use? The compiler is intended for serious use and I would like it to be very efficient, maybe competing with compilers written in C. It should also be very easy to extend as the languoge grows. Are there any good books that you can recommend on compiler construction in general and specific to haskell? On another note, how is the operator + implemented in haskell? is there a primitve (say #+) that is wrapped by the haskell operator +? Maybe something like: (+) :: a -> a -> a v1 + v2 = #+ v1 v2 Thanks in advance Rouan.

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Can HLint help spot space leaks? (was: Re: [Haskell-cafe] Generating Haskell with associated types (and kind annotations))
by Jason Dagit 09 May '09

09 May '09

On Fri, May 8, 2009 at 8:49 AM, Neil Mitchell <ndmitchell(a)gmail.com> wrote: > > I've used the project extensively in HLint > (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlint) and > it works fantastically. Hi Neil, A bit off-topic, but your post reminded me: Does HLint currently help the user find space leaks? For example, does it recommend strict folds instead of lazy folds? I looked at the FAQ but this was not listed. I don't really know how feasible this is. Thanks, Jason

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Generating Haskell with associated types (and kind annotations)
by Daniel Peebles 09 May '09

09 May '09

Hi all, I was wondering whether anyone had any suggestions on a good way to generate repetitive code with associated types and kind annotations. I'd like to use TH but as far as I understand, it doesn't support this yet (I think associated types are in HEAD but not kinds), so for now I've been using ruby with string interpolation, which feels rather ugly. It means that I have to edit the ruby script and regenerate the file in question each time I make a change, which doesn't fit in very nicely with cabal (or my patience) and so on. Ideally, I'd have something that I can write mostly like real Haskell (with "holes" to substitute values into), that can be integrated nicely into the cabal build system without using custom external preprocessors or anything like that. Any ideas? Thanks, Dan

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ANNOUNCE: hpc-strobe-0.1: Hpc-generated strobes for a running Haskell program
by Thorkil Naur 08 May '09

08 May '09

I am pleased to announce the initial release of hpc-strobe: Hpc-generated strobes for a running Haskell program. hpc-strobe is a rudimentary library that demonstrates the possibility of using Hpc (Haskell Program Coverage) to inspect the state of a running Haskell program. hpc-strobe-0.1 has been uploaded to hackage: http://hackage.haskell.org/packages/archive/hpc-strobe/0.1/hpc-strobe-0.1.t… In ordinary use of Hpc, a single so-called tix file is produced at the end of a run. The tix file records how many times each expression has been used during the run. This can be used to mark up the source code, identifying expressions that have not been used. hpc-strobe uses the basic machinery provided by Hpc to produce multiple tix files, also called strobes, representing the coverage at different times while the program is running. By subtracting such two tix files, again using Hpc machinery, a tix file representing the expressions used between the times of recording the subtracted tix files is produced. This may be used, for example, to get a better idea of what a long-running program is doing. It could also be used as a profiling tool, getting information about how many times individual expressions are used. A program is included whose strobe differences produce a crude rendering of an analog clock when they are used to mark up the source code using Hpc. Please see the attached example (gunzip the file, point you browser to it, scroll down to view the canvas function). Use of the library involves a simple change of the main function and also requires the program to be enabled for hpc. At the time of writing, this means using a fairly recent version of GHC and compiling the Haskell code with the -fhpc option. For additional details, see the README included in the package. Best regards Thorkil

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Re: [Haskell-cafe] Re: Is Haskell a Good Choice for Web Applications? (ANN: Vocabulink)
by Kalman Noel 08 May '09

08 May '09

Daniel Carrera schrieb: > I think it largely depends on the learner. Some people find vocabulary > easier, or more interesting, others not. I have a hard time learning a > lot of isolated facts (e.g. vocabulary), but I find it easier and more > enjoyable to learn a rule that I can apply many times. But I know people > who are the exact opposite. I wouldn't want to make an absolute rule. Or like a local physics prof likes to put it: “I guess you all have an idea of Ohm's law? Or wait, right, for the medics being with us, here are the three Ohm's laws: U = RI, R = U/I, and I= U/R.” Kalman

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Poking fun at haskell and other programming languages
by Andrew Wagner 08 May '09

08 May '09

http://james-iry.blogspot.com/2009/05/brief-incomplete-and-mostly-wrong.html

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