June 2011 - Haskell-Cafe

Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 93, Issue 58
by Gregory Guthrie 14 Jun '11

14 Jun '11

An earlier note on students reactions to the imperative style forced on them by some Haskell libraries ("do ...") is interesting, and seems similar to an observation in a project I was developing for students; making a version of a simple lab from previous SML assignment. It uses a dictionary to do some statistics for a text analysis, but since the dictionary is read at a leaf node of the analysis algorithm, that function must be IO(), and thus the analysis using it also, ... etc. all the way up to the top. So the implication of the rules: 1) all IO must start from the top level, and there is only one IO 2) you cannot extract anything from an IO Seems to be that the whole program structure becomes a series of do... blocks, which is basically a sequential imperative looking style. The general advice of "Strive to keep as much of the program pure as possible" thus seems difficult. An option I suppose would be to read the dictionary at the top level, and then pass it all the way down to the analysis routine that uses it, but that exposes the details of how the analysis is done, and couples the top and bottom levels of the previously modular functions. While this is all logical and understandable, it is quite different than how I did this in SML; where I could encapsulate the IO in the analysis function. It was a local secret of the analysis what data it needed, and where it came from. Note that (of course...) if the dictionary was static and an internal data structure, then this would all go away. It is interesting to me (and curious at first) that one could not somehow treat a "constant" data definition file resource in a more encapsulated manner, and not have it ripple all the way up through the program because it was "impure" once converted to an external definition (=IO). My first impulse was to read the dictionary in a do... and then try to extract it and go merrily on, but that won't work - by design of course! Considering something like a properties file in Java, it thus seems like every part of a program wanting to use these, must either be passed some global definition, or become a leaf on a hierarchy of do.. blocks if it does its own IO to read the properties. Anyway, while the more precise treatment and isolation of IO in Haskell seems valuable, it also seems to have a notable impact on the lack of ability to encapsulate and decouple parts of the program, and keep things pure between them. I suppose this is because that by definition, once you have touched IO at any leaf of a program, the whole thing is impure all the way up the functional tree. I rather had the feeling expressed by Robert Harper: " Once you're in the IO monad, you're stuck there forever, and are reduced to Algol-style imperative programming." (http://existentialtype.wordpress.com/2011/05/01/of-course-ml-has-monads/) Just an observation, in case I am missing something - being fairly new to Haskell. I suppose this is just an adjustment to proper treatment of the impurity of IO, but the effect on program structure was not good. :-) ------------------------------------------- > -----Original Message----- > Subject: Haskell-Cafe Digest, Vol 93, Issue 58 > Now, I have a personal pedagogical comment. >A few months ago I gave to some 30 students an >The results? A true DISASTER! > The OpenGL bindings in Haskell are hardly "functional". > You make us sweat with generic functional patterns, laziness, exquisite typing, non-determinism monad, parsing tools, etc., > and then you throw us into this ugly imperativism !

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(no subject)
by Fernando Henrique Sanches 13 Jun '11

13 Jun '11

http://maipenarai.com/lindex02.html

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Unexpected type error
by Alexander Solla 13 Jun '11

13 Jun '11

Hi everybody, I'm having a bit of a problem with Parsec. The most relevant error message is: Couldn't match expected type `Text.Parsec.Prim.ParsecT String () Control.Monad.Identity.Identity String' against inferred type `parsec-2.1.0.1:Text.ParserCombinators.Parsec.Prim.Parser [Char]' In the expression: countMinMax ... So apparently, the module/package that defines countMinMax and the current module are using different versions of Parsec. My global GHC database has Parsec 3.1.1 installed, but each package has the constraint parsec >=2.1 && <2.2 Both modules use the exact same import line: import Text.ParserCombinators.Parsec -- Lime.Data.IPAddress import Text.ParserCombinators.Parsec -- Lime.Utility.Parsec countMinMax :: (Integral n) => n -> n -> Parser a -> Parser [a] and the expression GHC is complaining about above has type Parser String. The cabal-dev package database contains only one version of Parsec: parsec-2.1.0.1-190ec41e7316b7eb7fddf355acddd826.conf What's going on here?

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Profiling with Data.PQueue.Min
by blackcat＠pro-ns.net 13 Jun '11

13 Jun '11

I'm having trouble with getting a profiling version of Data.PQueue.Min. I've set the profiling options in ~/.cabal/config and used it to build profiling versions of other packages. When I run "cabal install --reinstall pqueue", it runs successfully and properly installs the package...but it isn't building a profiling version. Has anyone else gotten this to work? I'd appreciate some pointers. thanks Lee Short

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question about interruptable state runner
by Alexander V Vershilov 13 Jun '11

13 Jun '11

Hello. I'm writing a small tcp server with that can handle connections and answer by rules writen in a small script that can be interpreted by server. For this purpose I've written an interpreter that has type ErrorT MyError (StateT ScriptState IO) so I can call "native" IO function in that script, and define new one. I can run this script with runState (runErrorT (...)) oldState. But there is one problem: in script i should be able to call functions that will stop script interpretation and wait for some server event. To continue interpretation. Can smb give an advice what is the best way to do it? -- Best regards Alexander V Vershilov

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GHC handles badly with math formulas
by Grigory Sarnitskiy 13 Jun '11

13 Jun '11

I've noted several times that GHC doesn't evaluate formulas for Double values during compilation. That is using 4.2326514735445615 instead of (512 / 0.844)**(1/3) or 0.906179845938664 instead of (1/3) * sqrt(5 + 2 * sqrt(10 / 7)) This can lead to significant slowdown if such values are called often. It's not always convenient to use where or let to reduce the number of calculations of such constants so I wonder how to force GHC to evaluate them during compilation?

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: Parallel and Concurrent programming in Haskell, a tutorial
by Henning Thielemann 13 Jun '11

13 Jun '11

On Thu, 19 May 2011, Simon Marlow wrote: > I've put together a tutorial on Parallel and Concurrent programming in > Haskell, here: > > http://community.haskell.org/~simonmar/par-tutorial.pdf > > The main reason for writing this was that I needed some lecture notes for a > course at the CEFP summer school next month, but I hope the material is > useful to the Haskell community in general. Thank you a lot for this overview! Concerning the parallelisation of K-means: It seems to be difficult to choose a size for the pieces of points. Since the 'combine' function is associative something map-reduce-like seems to be applicable. That is, if there is a library function for a parallelised 'reduce' and I promise, that I call it with an associative function as argument, then the library could choose an appropriate size for the pieces. Is there already library support for parallelism of this style?

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Input and output of mathematical expressions
by Jacek Generowicz 13 Jun '11

13 Jun '11

Greetings Cafe, What would you recommend as a Haskell-based means of interactively reading and writing mathematical formulae? As a toy example, what might I use to write a program which presents the user with Please simplify the expression: \pi x^2 + 3\pi x^2 (Where the TeX-style expression would be presented with a greek pi and superscript twos on the xs.) The user should then have the ability to reply with something that looks like the result of TeXing 5 \pi x^2 Whatever means the user uses to enter this expression, he should be able to preview the "typeset" version of his input before submitting. Any ideas? Thanks.

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location of IEEE Viz code ?
by briand＠aracnet.com 13 Jun '11

13 Jun '11

Hi, I hate to even ask this because the answer is probably staring me in the face. This trac page http://hackage.haskell.org/trac/PolyFunViz/wiki/IEEEVisCode talks about the code being available through darcs but I can't seem to put my hands on the http address I would need to pull the code. This is all relating to the paper, "Huge Data but Small Programs: Visualization Design via Multiple Embedded DSLs". I was wondering if anyone has accessed it and/or knows where it's hiding. Thank you, Brian

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Replacing stdin from within Haskell
by Neil Davies 13 Jun '11

13 Jun '11

Hi Anyone out there got an elegant solution to being able to fork a haskell thread and replace its 'stdin' ? Why do I want this - well I'm using the pcap library and I want to uncompress data to feed into 'openOffile' (which will take "-" to designate read from stdin). Yes I can create a separate command and use the stuff in System.Process or uncompress it into a temporary file - but that all seems in-elegant. Suggestions? Cheers Neil

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