May 2013 - Haskell-Cafe

What symbol / atom/ interned-string package is currently preferred?
by Ryan Newton 09 May '13

09 May '13

Hi cafe, I use symbols all the time when manipulating code. I vacillate between using stringtable-atom, symbol, and simple-atom. Unfortunately: - Stringtable-atom is has a broken build right now (GHC 7.6). (Also I've had some crashes with it in the past.) - simple-atom has dependency upper bounds that restrict it to GHC <= 7.0 - "symbol" has a design flaw, which is that it exposes the constructor for Symbol, which reveals the (nondeterministic) unique Int IDs. I.e. it is not "SafeHaskell". I submitted a pull request to relax simple-atom, but perhaps there are other packages people use with which I am not aware? Thanks, -Ryan

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ghci: Difference in garbage collection etc. between REPL and function
by Niklas Hambüchen 09 May '13

09 May '13

I have an annoying bug in a C binding that can be triggered like this: handle <- open ... prep <- makePreparedStatement handle "INSERT ..." performGC runStatement prep close handle If I run these steps one by one in ghci, garbage ends up in my handle as expected. However, if I "let main = do ..." this whole block in order to pack it in a test case, it does not happen, neither in ghci nor ghc. What might be the special magic in ghci's REPL that allows me to trigger my bug so easily there? Thanks Niklas

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Error compiling transformers-base-0.4.1 (debian 64 bits, ghc-7.4.2)
by jean-christophe mincke 09 May '13

09 May '13

Hello Café I am running into problems when installing transformers-base-0.4.1. Has anyone an idea about what is going on? REM : this is a "brand new" debian install, I have just install ghc, cabal install cabal-install etc. Thank you very much J-C cabal install transformers-base Resolving dependencies... Downloading transformers-base-0.4.1... Configuring transformers-base-0.4.1... Preprocessing library transformers-base-0.4.1... Building transformers-base-0.4.1... [1 of 1] Compiling Control.Monad.Base ( src/Control/Monad/Base.hs, dist/build/Control/Monad/Base.o ) src/Control/Monad/Base.hs:63:10: Duplicate instance declarations: instance Applicative (L.ST s) -- Defined at src/Control/Monad/Base.hs:63:10 instance Applicative (L.ST s) -- Defined in `Control.Applicative' src/Control/Monad/Base.hs:67:10: Duplicate instance declarations: instance Applicative (S.ST s) -- Defined at src/Control/Monad/Base.hs:67:10 instance Applicative (S.ST s) -- Defined in `Control.Applicative' cabal: Error: some packages failed to install: transformers-base-0.4.1 failed during the building phase. The exception was: ExitFailure 1

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Test access resources files in cabal-dev
by Graham Berks 09 May '13

09 May '13

Hi, whats the recommended way to access a resource file for a test within cabal. I have a text file containing data that I use in a test and want to be able to access it in a root path independent manner. Thanks

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Algebra in Haskell
by Robert Goss 08 May '13

08 May '13

Dear all, There are several libraries in haskell for abstract algebra but they don't seem to cover my use case I was wondering if other people have had similar issues and if there are any packages I am missing. What I am prinicpaly interested in is operation on algebraic stuctures (homomorphisms, kernels, quotients, colimits, etc) whereas most algebra packages seem to just give operations on the elements of an algebraic structure. So I would like to be able to write something like... g = freeGroup(["x","y"]) h = freeGroup(["z"]) phi = grpMorphismOnGens([("x","z"^6) , ("y","z")]) k = grpKernel phi (gens,rels) = grpPresentation (grpQuotient g k) Thank you, Robert Goss

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Formal Methods/Functional programming job position at Intel
by Levent Erkok 08 May '13

08 May '13

Our formal methods team at Intel has a full-time position available that I think would be a good fit for functional programming and formal methods enthusiasts. I'm including the description below. Do not hesitate to contact me if you've any questions, or just want to talk about it in general. To apply for the position, please visit: http://www.intel.com/jobs/jobsearch/index.htm, and in the "advanced search" area enter the job number: 709631. Thanks, -Levent. Job Description *Formal Methods & Validation Architect* *-* *709631* Description If you're interested in products going into future super computer markets then the Intel® Many Integrated Core (Intel® MIC) Hardware Engineering Group is the place for you! We design and validate silicon chips with many Intel cores integrated inside being used in high performance computing architectures. In this position you will work as part of the pre-silicon formal methods, tools, and verification team to support a continued high quality of the future Intel many core processor products. You will work together with Intel's formal verification and validation community, the Formal Verification Center of Expertise (FV CoE) in a team of experts in formal methods and AV Validation. Your specific responsibilities will include defining formal verification (FV) test plans as well as Cluster Test Environment (CTE) based test plans for dynamic simulation validation (DV). The goal is to help optimize a combined use of FV and CTE based validation techniques (DV) and contribute to an increasing use of formal methods at Intel. Based on the test plans you will write properties in formal language, and prove the properties using our model checkers and theorem proving tools. You will also write CTE based test cases and coverage plans. Your area of strength may currently lie within either FV or DV with strong skills rooted in software development. But through your skills and work, you will become an expert in both formal methods and validation technologies. You will interact very closely with design teams, and other validation teams, as well as with Intel's internal R&D groups that continue to improve and develop formal method and verification tools. You must be able to communicate effectively with various technical groups and coordinate activities amongst those groups.

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Categories (cont.)
by Christopher Howard 08 May '13

08 May '13

I've perhaps been trying everyones patiences with my noobish CT questions, but if you'll bear with me a little longer: I happened to notice that there is in fact a Category class in Haskell base, in Control.Category: quote: -------- class Category cat where A class for categories. id and (.) must form a monoid. Methods id :: cat a a the identity morphism (.) :: cat b c -> cat a b -> cat a c morphism composition -------- However, the documentation lists only two instances of Category, functions (->) and Kleisli Monad. For instruction purposes, could someone show me an example or two of how to make instances of this class, perhaps for a few of the common types? My initial thoughts were something like so: code: -------- instance Category Integer where id = 1 (.) = (*) -- and instance Category [a] where id = [] (.) = (++) ------- But these lead to kind mis-matches. -- frigidcode.com

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Mutually recursive modules
by Roman Cheplyaka 08 May '13

08 May '13

I wonder whether it's always possible to break cycles using GHC's .hs-boot files. Consider the following schematic example: module A where import B data A f :: B -> A f = undefined B.g module B where import A data B g :: A -> B g = undefined A.f A.hs-boot must give a type signature for f, and since the signature contains 'B', it must import 'B'. Ditto for B.hs-boot — it must import 'A'. Even if we treat all imports as {-# SOURCE #-}, there is still a cycle between the hs-boot files. So, am I right in understanding that these recursive modules cannot be compiled by GHC at all? Roman

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Incrementation fails
by John 08 May '13

08 May '13

Hello All, I'm in a big trouble with incrementation of a counter in this code. It doesn't increment. Could you please tell me where the problem ist and how can I solve it? replaceBasedIdx :: String -> [String] -> String -> String replaceBasedIdx findStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText 0 replaceBasedIdxSub :: String -> [String] -> String -> Int -> String replaceBasedIdxSub findStr replaceStrList myText counter = loop myText where loop [] = [] loop myText = let (prefix, rest) = splitAt n myText in if findStr == prefix -- found an occurrence? then (replaceStrList !! (counter+1)) ++ loop rest -- yes: replace it else head myText : loop (tail myText) -- no: keep looking n = length findStr Thank you very mutch! Greetings! -- View this message in context: http://haskell.1045720.n5.nabble.com/Incrementation-fails-tp5729905.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Munich 13.5., live sequencer
by Heinrich Hördegen 08 May '13

08 May '13

Dear all, Henning Thielemann will give a demonstration of his live sequencer in Munich on the 13th of May (next Monday): http://www.youtube.com/watch?v=88jK162l6mE We begin at 19h00 at Oetingenstr. 67, 80538 Munich, room 157 (first floor, central). As everything is live, please bring your laptops along, if you want to participate interactivly. Heinrich

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