January 2013 - Haskell-Cafe

Weird problem: Couldn't read cabal file "hashable/1.2.0.0/hashable.cabal"
by Eugene Kirpichov 07 Jan '13

07 Jan '13

Hi, A friend of mine is trying to install my "timeplotters" (see email signature). He's getting a weird error: one of the tools installs fine but the other one fails with the error "Couldn't read cabal file "hashable/ 1.2.0.0/hashable.cabal""; curiously, so does running "cabal install cabal-install". Even more curiously, hashable 1.2.0.0 does not appear anywhere in the list of dependencies, even indirect. Look: $ cabal -v3 install cabal-install searching for ghc in path. found ghc at /usr/bin/ghc ("/usr/bin/ghc",["--numeric-version"]) /usr/bin/ghc is version 7.0.3 looking for tool "ghc-pkg" near compiler in /usr/bin found ghc-pkg in /usr/bin/ghc-pkg ("/usr/bin/ghc-pkg",["--version"]) /usr/bin/ghc-pkg is version 7.0.3 ("/usr/bin/ghc",["--supported-languages"]) ("/usr/bin/ghc",["--info"]) Reading installed packages... ("/usr/bin/ghc-pkg",["dump","--global","-v2"]) ("/usr/bin/ghc-pkg",["dump","--user","-v2"]) ("/usr/bin/ghc",["--print-libdir"]) Reading available packages... Resolving dependencies... cabal: Couldn't read cabal file "hashable/1.2.0.0/hashable.cabal" He's using haskell platform on Ubuntu 12.04. $ ghc —version The Glorious Glasgow Haskell Compilation System, version 7.0.3 $ cabal —version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library He *did* try erasing ~/.cabal, ~/.ghc and /usr/lib/ghc and then reinstalling haskell-platform package - it didn't change anything. He doesn't remember ever having Haskell installed before, but that can't be completely ruled out. I just tried installing a fresh Ubuntu 12.04 onto a VM and I did not have this problem that he has. What additional information can I ask him for? Maybe somebody already encountered this problem? (Actually, I have a whole strace cabal install cabal-install, but it's 33Mb - I can upload it somewhere if it could be helpful). -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov http://jkff.info/software/timeplotters - my performance visualization tools

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ANN: New version of pretty-show, now with Html support
by Iavor Diatchki 06 Jan '13

06 Jan '13

Hello, I've uploaded a new version of the pretty-show package (1.5) to Hackage, which provides some useful tools for inspecting Haskell data-structures. I find it quite useful while debugging. The package provides a binary and a library: the binary, named `ppsh` can take as an input a Haskell value in standard `Show` notation, and can: * Either render it to a more human readable version by adding extra white-space and newlines, thus ensuring that the result is still a valid Haskell value, or * It can render the value as an interactive Html page, which is new in this version. The library provides similar functionality but in a programatically. Also, it has some support for working with non-Showable values, by using the `PrettyVal` class, which may be derived generically. As an example, I am attaching a picture of a part of a data-structure generated with the `language-c-quote` library, by using the standard `Show` instance. I hope you find this useful, -Iavor

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ln, e
by Christopher Howard 05 Jan '13

05 Jan '13

Hi. Are natural log and Euler's constant defined somewhere in base, or a convenience math module somewhere? I'm having trouble finding them with hayoo or system documentation. -- frigidcode.com

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Compliments on the "IO inside" tutorial
by Andriy Drozdyuk 04 Jan '13

04 Jan '13

I just wanted to express my gratitude to whoever wrote the "IO inside" tutorial: http://www.haskell.org/haskellwiki/IO_inside It is one of the best pieces of writing in Haskell I've ever come across: not only is it clear and very intuitive, but it also manages to strike a perfect balance between "programmers" and "beginner haskell programmers". It certainly ties together a lot of other stuff one reads in books and category theory - and actually makes sense! My sincere gratitude, and keep up great job. I can only hope to write something as clear and useful as this. My only complaint is that it is really wasting away inside the Haskell Wiki - which gives it a rather bad "reputation", since a lot of wiki pages are of much lower quality. This should be nicely formatted in a PDF file or a even part of a book! Make it pretty and get people to notice it!!! Now i must sleep, -Andriy Drozdyuk

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Object Oriented programming for Functional Programmers
by Daniel Díaz Casanueva 04 Jan '13

04 Jan '13

Hello, Haskell Cafe folks. My programming life (which has started about 3-4 years ago) has always been in the functional paradigm. Eventually, I had to program in Pascal and Prolog for my University (where I learned Haskell). I also did some PHP, SQL and HTML while building some web sites, languages that I taught to myself. I have never had any contact with JavaScript though. But all these languages were in my life as secondary languages, being Haskell my predominant preference. Haskell was the first programming language I learned, and subsequent languages never seemed so natural and worthwhile to me. In fact, every time I had to use another language, I created a combinator library in Haskell to write it (this was the reason that brought me to start with the HaTeX library). Of course, this practice wasn't always the best approach. But, why I am writing this to you, haskellers? Well, my curiosity is bringing me to learn a new general purpose programming language. Haskellers are frequently comparing Object-Oriented languages with Haskell itself, but I have never programmed in any OO-language! (perhaps this is an uncommon case) I thought it could be good to me (as a programmer) to learn C/C++. Many interesting courses (most of them) use these languages and I feel like limited for being a Haskell programmer. It looks like I have to learn imperative programming (with side effects all over around) in some point of my programming life. So my questions for you all are: * Is it really worthwhile for me to learn OO-programming? * If so, where should I start? There are plenty of "functional programming for OO programmers" but I have never seen "OO programming for functional programmers". * Is it true that learning other programming languages leads to a better use of your favorite programming language? * Will I learn new programming strategies that I can use back in the Haskell world? Thanks in advance for your kind responses, Daniel Díaz.

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Fwd: Foreign.Erlang help
by Junior White 04 Jan '13

04 Jan '13

Hi Alexandar, Thanks for your help! I give up Foreign.Erlang and try usual tcp server in Haskell instead. ---------- Forwarded message ---------- From: Junior White <efiish(a)gmail.com> Date: Fri, Jan 4, 2013 at 3:18 PM Subject: Re: [Haskell-cafe] Foreign.Erlang help To: Alexander Alexeev <mail(a)eax.me> Hi Alexandar, Thanks for your help! I'll give up Foreign.Erlang and try usual tcp server in Haskell instead. On Tue, Dec 25, 2012 at 4:08 PM, Alexander Alexeev <mail(a)eax.me> wrote: > First of all, you can write usual TCP server in Haskell and use it from > Erlang. It would be handy to use some widely used protocol, like Memcached > or Redis. > > If in some reason you want to use Foreign.Erlang, make sure that you are > using the same cookie and the same node name type (short or full) in Erlang > node and Haskell application. > > > On 12/24/2012 08:46 PM, Junior White wrote: > > Hi all, > I have a game server programming in erlang, but now i want to write some > game logic in haskell, how to write a haskell node for erlang? > > ps:I have read the introduce of Foreign.Erlang,I use haskell send a > message to a erlang echo server, but the server not receive anything. > > > _______________________________________________ > Haskell-Cafe mailing listHaskell-Cafe@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -- > Best regards, > Alexander Alexeevhttp://eax.me/ > >

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Re: [Haskell-cafe] Mancunian Haskellers out there?
by James Jeffries 03 Jan '13

03 Jan '13

Hi there There isn't currently a haskell user groupu in Manchester, however there is a functional programming group where a lot of the member s are haskellers. http://www.lambdalounge.org.uk/ https://groups.google.com/forum/?fromgroups#!forum/lambda-lounge-manchester It is at Madlab in the Northern Quarter and there is usually a trip to the pub afterwards. Thanks James On 30/12/12 14:38, Alfredo Di Napoli wrote: Morning Cafe, I've been living in Manchester for 1 month now and I'm wondering if some on you are from the Greater Manchester area, so that we could chat about out beloved language in front of a glass of ale / tea :P Happy new year to everyone! A. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org <Haskell-Cafe(a)haskell.org>> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] [Haskell] Importing Repa library
by Henning Thielemann 03 Jan '13

03 Jan '13

On Thu, 3 Jan 2013, Емануела Моллова wrote: > Hello! :) I think you should better post your question to haskell-cafe(a)haskell.org. > Then I put this script > > import qualified Data.Array.Repa as R > :m +Data.Array.Repa > Z > > into the file "file.hs", This is GHCi syntax, but it is not a valid Haskell module as indicated by the .hs filename extension. > opened WinGHCi and loaded the file, and then evaluated it, but what I get is: > > Could not find module `Data.Array.Repa' > Perhaps you meant > Data.Array.Base (from array-0.4.0.0) > Data.Array.IO (from array-0.4.0.0) > Data.Array.ST (from array-0.4.0.0) > Use -v to see a list of the files searched for. > Failed, modules loaded: none. I wonder why it loads file.hs at all. I would expect that it gives a syntax error. > Also ghc-pkg list repa says: > > WARNING: cache is out of date: C:/Program Files/Haskell Platform/2012.4.0.0\lib\package.conf.d\package.cache > use 'ghc-pkg recache' to fix. C:/Program Files/Haskell Platform/2012.4.0.0\lib\package.conf.d: > C:\Users\Faery\AppData\Roaming\ghc\i386mingw32-7.4.2\package.conf.d: Since 'repa' is not listed, it was not installed successfully. This would be consistent with the "Could not find module" message of GHCi. You may post the output of the 'cabal install repa' run.

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Proving programs
by Christopher Howard 03 Jan '13

03 Jan '13

I'm working through a video lecture describing how to prove programs correct, by first translating the program into a control flow representation and then using propositional logic. In the control flow section, the speaker described how the program should be understood in terms of an input vector (X, the inputs to the program), a program vector (Y, the storage variables), and an output vector (Z, the outputs of the program), with X mapping into Y, Y being affected by execution, and X and Y mapping into Z. However, only part way into the video, two practical questions come to mind: 1. Does this approach need to be adjusted for a functional language, in which computation is (at least idealistically) distinct from control flow? 2. How do we approach this for programs that have an input loop (or recursion)? E.g., I have an application that reads one line for stdin, modifies said line, outputs to stdout, and repeats this process until EOF? Should I be thinking of every iteration as a separate program? -- frigidcode.com

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Safe 'chr' function?
by Myles C. Maxfield 03 Jan '13

03 Jan '13

Hello, I'm working on a general text-processing library [1] and one of my quickcheck tests is designed to make sure that my library doesn't throw exceptions (it returns an Either type on failure). However, there are some inputs that cause me to pass bogus values to the 'chr' function (such as 1208914), which causes it to throw an exception. Is there a version of that function that is safe? (I'm hoping for something like Int -> Maybe Char). Alternatively, is there a way to know ahead of time whether or not an Int will cause 'chr' to throw an exception? Thanks, Myles C. Maxfield [1] http://hackage.haskell.org/package/punycode

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