- Haskell-Cafe - Haskell.org

partially indeterminate?
by Christopher Howard 16 Jul '13

16 Jul '13

Hi. For learning, I was doing the "phone words" problem, where a function translates the digits of a phone number into all possible words. I am trying to connect this idea to the idea of list comprehensions / list monads (sort of the same thing, yes?) I know it is easy to do this: w = do two <- "ABC" three <- "DEF" four <- "GHI" -- and the other digits [[two, three, two, four]] -- for example But what if you don't know in advance what the digits will be? I'm not sure how to insert that deterministic component into this idea. So far, I have a framework like so: p dx = do undefined where m = map (\d -> case d of 2 -> "ABC" 3 -> "DEF" 4 -> "GHI" 5 -> "JKL" 6 -> "MNO" 7 -> "PRS" 8 -> "TUV" 9 -> "WXY" otherwise -> show d) dx I would appreciate any guidance.

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Re: [Haskell-cafe] Non-recursive let [Was: GHC bug? Let with guards loops]
by oleg＠okmij.org 16 Jul '13

16 Jul '13

I'd like to emphasize that there is a precedent to non-recursive let in the world of (relatively pure) lazy functional programming. The programming language Clean has such non-recursive let and uses it and the shadowing extensively. They consider shadowing a virtue, for uniquely typed data. Richard A. O'Keefe wrote > >> let (x,s) = foo 1 [] in > >> let (y,s) = bar x s in > >> let (z,s) = baz x y s in ... > I really wish you wouldn't do that. > ... > I find that that when the same name gets reused like > that I get very confused indeed about which one I am > looking at right now. > ... > If each instance of the variable is labelled with a > sequence number, I don't get confused because each > variable has a different name and I can *see* which > one this is. > > Yes, sequence numbering variable states is a chore for > the person writing the code, but it's a boon for the > person reading the code. Let me point out the latest Report on the programming language Clean http://clean.cs.ru.nl/download/doc/CleanLangRep.2.2.pdf specifically PDF pages 38-40 (Sec 3.5.4 Let-Before Expression). Let me quote the relevant part: Many of the functions for input and output in the CLEAN I/O library are state transition functions. Such a state is often passed from one function to another in a single threaded way (see Chapter 9) to force a specific order of evaluation. This is certainly the case when the state is of unique type. The threading parameter has to be renamed to distinguish its different versions. The following example shows a typical example: Use of state transition functions. The uniquely typed state file is passed from one function to another involving a number of renamings: file, file1, file2) readchars:: *File -> ([Char], *File) readchars file | not ok = ([],file1) | otherwise = ([char:chars], file2) where (ok,char,file1) = freadc file (chars,file2) = readchars file1 This explicit renaming of threaded parameters not only looks very ugly, these kind of definitions are sometimes also hard to read as well (in which order do things happen? which state is passed in which situation?). We have to admit: an imperative style of programming is much easier to read when things have to happen in a certain order such as is the case when doing I/O. That is why we have introduced let-before expressions. It seems the designers of Clean have the opposite view on the explicit renaming (that is, sequential numbering of unique variables). Let-before expressions have a special scope rule to obtain an imperative programming look. The variables in the left- hand side of these definitions do not appear in the scope of the right-hand side of that definition, but they do appear in the scope of the other definitions that follow (including the root expression, excluding local definitions in where blocks. Notice that a variable defined in a let-before expression cannot be used in a where expression. The reverse is true however: definitions in the where expression can be used in the let before expression. Use of let before expressions, short notation, re-using names taking use of the special scope of the let before) readchars:: *File -> ([Char], *File) readchars file # (ok,char,file) = freadc file | not ok = ([],file) # (chars,file) = readchars file = ([char:chars], file) The code uses the same name 'file' all throughout, shadowing it appropriately. Clean programmers truly do all IO in this style, see the examples in http://clean.cs.ru.nl/download/supported/ObjectIO.1.2/doc/tutorial.pdf [To be sure I do not advocate using Clean notation '#' for non-recursive let in Haskell. Clean is well-known for its somewhat Spartan notation.] State monad is frequently mentioned as an alternative. But monads are a poor alternative to uniqueness typing. Granted, if a function has one unique argument, e.g., World, then it is equivalent to the ST (or IO) monad. However, a function may have several unique arguments. For example, Arrays in Clean are uniquely typed so they can be updated destructively. A function may have several argument arrays. Operations on one array have to be serialized (which is what uniqueness typing accomplishes) but the relative order among operations on distinct arrays may be left unspecified, for the compiler to determine. Monads, typical of imperative programs, overspecify the order. For example, do x <- readSTRef ref1 y <- readSTRef ref2 writeSTRef ref2 (x+y) the write to ref2 must happen after reading ref2, but ref1 could be read either before or after ref2. (Assuming ref2 and ref1 are distinct -- the uniqueness typing will make sure of it.) Alas, in a monad we cannot leave the order of reading ref1 and ref2 unspecified. We must choose the order, even if it is irrelevant. Overspecification inhibits the possible optimizations. Why Clean is relatively unknown? Well, why is Amiga?

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[GSoC] Interactive-diagrams GSoC progress report
by Daniel F 16 Jul '13

16 Jul '13

Hello everyone, I am participating in Haskell.org's Google Summer of Code, working on the interactive-diagrams [1] project. The mid-term evaluations are approaching and I am doing some writeup on the work I did so far: http://parenz.wordpress.com/2013/07/15/interactive-diagrams-gsoc-progress-r… A beta-version of the site is also available to the public: http://paste.hskll.org Please note that this is not a final product and you may experience cryptic error messages, some downtime. The security is pretty tight though :) Any suggestions/comments are welcome. [1] https://google-melange.appspot.com/gsoc/proposal/review/google/gsoc2013/dif… -- Sincerely yours, -- Daniil Frumin

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Homotopy Type Theory IRC Channel
by Darin Morrison 15 Jul '13

15 Jul '13

Hello, I know there are many haskellers interested in dependent type theory so I wanted to mention that there is now an IRC channel on freenode for discussing Homotopy Type Theory (HoTT), e.g., the recent book, possible implementations, dependent type theory in general, etc. The channel is ##hott (NOTE the *two* hashes!) and you can connect directly through freenode's web interface if you don't already have a client: https://webchat.freenode.net/?channels=%23%23hott Currently we have around 50 members but the more the merrier! If you don't know much about IRC but would like to learn more: * http://www.irchelp.org — how IRC works, software clients, FAQs, and other info * http://freenode.net — info about the freenode IRC network specifically Cheers, Darin Morrison

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Re: [Haskell-cafe] What have happened to haskell.org?
by Daniel F 15 Jul '13

15 Jul '13

The web site is migrating. IRC says: Topic for #haskell: haskell.org in the middle of migration; expect turbulence; use www.haskell.org On Mon, Jul 15, 2013 at 6:19 PM, Kirill Zaborsky <qrilka(a)gmail.com> wrote: > On URL http://haskell.org/ I get starting Apache page and 404 on > http://haskell.org/hoogle/ > URL with starting www - http://www.haskell.org/ seems to be working but > http://www.haskell.org/hoogle/ responds with some ELF file. > > Kind regards, > Kirill Zaborsky > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Sincerely yours, -- Daniil -- Sincerely yours, -- Daniil

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What have happened to haskell.org?
by Kirill Zaborsky 15 Jul '13

15 Jul '13

On URL http://haskell.org/ I get starting Apache page and 404 on http://haskell.org/hoogle/ URL with starting www - http://www.haskell.org/ seems to be working but http://www.haskell.org/hoogle/ responds with some ELF file. Kind regards, Kirill Zaborsky

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Compiling haskell-platform-2013.2.0.0 fails
by Roger Mason 15 Jul '13

15 Jul '13

Hello, I'm attempting to compile haskell-platform on an Arch Linux system. It fails very soon after invoking 'make': Configuring HUnit-1.2.5.2... Setup: Use of GHC's environment variable GHC_PACKAGE_PATH is incompatible with Cabal. Use the flag --package-db to specify a package database (it can be used multiple times). If it makes a difference, I'm using GHC 7.6.3 from the Arch package repo. The envar GHC_PACKAGE_PATH mentioned in the error message is unset. Thanks for any help, Roger This electronic communication is governed by the terms and conditions at http://www.mun.ca/cc/policies/electronic_communications_disclaimer_2012.php

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Hoogle website not working?
by Ivan Perez 15 Jul '13

15 Jul '13

Hi cafe, I'm seeing a binary executable file every time I try to access haskell.org/hoogle. I don't know if the maintainers are aware of this. Cheers. Ivan

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Workshop on Termination (deadline: July 22)
by Johannes Waldmann 15 Jul '13

15 Jul '13

Dear all. Still one week to go: submit your results and questions on all things related to termination and bounded resource consumption (of user programs, of type checkers, etc.) to the Workshop on Termination. We are specifically encouraging contributions from the Haskell (and Agda, and Idris) community. - Johannes. ################################################################# 13th International Workshop on Termination (WST) ( full CfP see http://www.imn.htwk-leipzig.de/WST2013/ ) Centro Residenziale Universitario di Bertinoro (near Bologna, Italy). submission: July 22, 2013 notification: July 25, 2013 final version: August 10, 2013 workshop: August 29 - 31, 2013 The Workshop on Termination traditionally brings together, in an informal setting, researchers interested in all aspects of termination, whether this interest be practical or theoretical, primary or derived. The workshop also provides a ground for cross-fertilisation of ideas from term rewriting and from the different programming language communities. The friendly atmosphere enables fruitful exchanges leading to joint research and subsequent publications.

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OpenGL extensions survey results
by Brian Lewis 15 Jul '13

15 Jul '13

Hi, here are the results of the recent OpenGL extensions survey: https://github.com/bsl/opengl-extensions-survey If I did anything dumb, please send me mail or a pull request. Thanks to everyone who contributed!

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Database conectivity in Haskell. HDBC-3 or something.
by Alexey Uimanov 15 Jul '13

15 Jul '13

Hello, haskellers. I has made some rework of HDBC, if you are interested follow the link http://s9gf4ult.github.io/hdbc/Database-HDBC.html So, why did I made this ? Because I did not found good enough (inside and outside) common interface for database connectivity. What I mean is flexible and simple interface, supporting arbitrary precision values and providing convenient and minimalistic interface for just query execution. In ''roadmap'' in the documentation you can see, that I think, that Haskell must have such interface which must become the conveient base for other high-level interfaces, such as Persistent and HaskellDB. Other languages, like Python goes this way, so why we must not ? I dont know will this patches accepted to main HDBC repository or not. I will fork HDBC under another name if necessary. The patches are: https://github.com/s9gf4ult/hdbc https://github.com/s9gf4ult/hdbc-postgresql There is just PostgreSQL driver for now, but I plan to write some other drivers as well. Any help and pull requests are welcome. My question is: what do you think about all of this? Do you think this is good idea or this is not necessary for now? I just want to notify the community about this and maybe find support. Discuss.

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Do combinatorial algorithms have a matroid strucutre XOR non-matroid structure?
by KC 15 Jul '13

15 Jul '13

I ask this on this mailing list because there are quite a few mathematically oriented people here. Casey -- -- Regards, KC

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Re: [Haskell-cafe] Deriving with generics without values
by Roman Cheplyaka 14 Jul '13

14 Jul '13

Hi, (Redirecting this back to cafe to keep it discoverable — hope you don't mind.) * JP Moresmau <jpmoresmau(a)gmail.com> [2013-07-14 16:02:56+0200] > Hello, sorry to bother you after you've been kind enough to answer me on > the list! I've looked a the smallcheck code but I don't see how to apply it > to my issue. First of all, I can't find the definition of >>- that you use > for example in <~>. I suppose I'm doing something silly... That comes from LogicT. >>- is almost the same as >>=, and <~> is almost the same as <*>. Byt that's not relevant to the generic aspect of the code. > But then, if I understand the code, the instances for GSerial rebuild > a type definition that mimicks the one passed as type argument, using > the final values (under the K1s) to generate a series. But in my case, > I want to be able to extract information from the original type, I'm > not just interested in the final value. What I want to be able is to > extract for example the constructor name and do something with it. I > don't see how to achieve that with your system. Yes, it doesn't do everything that you want to do, but it shows the idea. The information you need is all there — for example, to get the constructor name, you need to get hold of the C1 type (which is a synonym for M1 C), and then call the conName method. Lest you get lost in all this, it is useful to visualize the generic representation by running your code through -ddump-deriv. Here's an example: % ghci -XDeriveGeneric -ddump-deriv -dsuppress-module-prefixes Prelude GHC.Generics> data T a = A { a :: a } | B | C deriving Generic ==================== Derived instances ==================== Derived instances: instance Generic (T a_ao7) where from (A g1_aph) = M1 (L1 (M1 (M1 (K1 g1_aph)))) from B = M1 (R1 (L1 (M1 U1))) from C = M1 (R1 (R1 (M1 U1))) to (M1 (L1 (M1 (M1 (K1 g1_api))))) = A g1_api to (M1 (R1 (L1 (M1 U1)))) = B to (M1 (R1 (R1 (M1 U1)))) = C instance Datatype D1T where datatypeName _ = "T" moduleName _ = ":Interactive" instance Constructor C1_0T where conName _ = "A" conIsRecord _ = True instance Constructor C1_1T where conName _ = "B" instance Constructor C1_2T where conName _ = "C" instance Selector S1_0_0T where selName _ = "a" Generic representation: Generated datatypes for meta-information: D1T C1_0T C1_1T C1_2T S1_0_0T Representation types: type Rep (T a_ao7) = D1 D1T (C1 C1_0T (S1 S1_0_0T (Rec0 a_ao7)) :+: (C1 C1_1T U1 :+: C1 C1_2T U1)) This should give you an idea about how the structure you're dealing with looks like, and where the important information resides. > I've also look at Aeson with the generic JSON parsing, but that's > using SYB which is again different, and SYB doesn't seem to provide > the types of the constructor fields, which I would need too... It provides the types of the constructor fields through the Typeable class, which may or may not be sufficient for your needs... Anyway, you're right in that it is a completely different approach. Roman > On Fri, Jul 12, 2013 at 10:57 AM, Roman Cheplyaka <roma(a)ro-che.info> wrote: > > > Well, in your case, you need not 'from', but 'to', in order to convert > > from a generic representation to yours. > > > > Take a look at how a similar task is done in SmallCheck: > > > > https://github.com/feuerbach/smallcheck/blob/master/Test/SmallCheck/Series.… > > > > https://github.com/feuerbach/smallcheck/blob/master/Test/SmallCheck/Series.… > > > > Roman > > > > * JP Moresmau <jpmoresmau(a)gmail.com> [2013-07-12 10:45:39+0200] > > > Hello all, > > > My problem is the following: I have my own data types, and I'd like to > > > derive automatically instances of some type class from them. I've started > > > looking at GHC.Generics, which offer tools to do exactly that. However, > > > some functions of my typeclass do not take my data type as a parameter, > > but > > > as a result. Basically: > > > class MyClass where > > > fromString :: String -> a > > > > > > data MyData=MkMyData { > > > myField ::Int > > > } deriving (Generic) > > > > > > and I want to automatically generate the instance instance MyClass > > MyData, > > > using default methods, etc. > > > The GHC Generic class does say that it uses a from function that convert > > > from the datatype to its representation: from :: a -> > > > Rep< > > http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-G… > > > > > > a > > > xfrom :: a -> Rep< > > http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-G… > > > > > > a > > > x > > > But I don't have a "a" to start from! I see from the related papers that > > > the automatically generated code from from actually does pattern matches > > on > > > constructors, so I need a value, undefined won't work. However I see the > > > GHC.Generics also provide :+: (Sums: encode choice between constructors). > > > If I have to provide an value, then the choice between constructor has > > been > > > done! The examples about generics on > > > http://www.haskell.org/haskellwiki/GHC.Generics do provide an example of > > > defining the instance for :+: but I don't understand how we can get > > there. > > > If I have a class method that takes a value as a parameter, and I pass > > > undefined to it, the code will crash, since it can't pattern match on > > > undefined. > > > > > > Can somebody shed some light on this? Am I using the wrong tool for the > > > job? How can I achieve what I want? I want the full type representation > > > with sums, but without a value to start from. > > > > > > Thanks a million! > > > > > > JP > > > -- > > > JP Moresmau > > > http://jpmoresmau.blogspot.com/ > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-Cafe(a)haskell.org > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > -- > JP Moresmau > http://jpmoresmau.blogspot.com/

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OS X ghci problem
by Brian Lewis 14 Jul '13

14 Jul '13

Here's a problem variations of which have been plaguing the Haskell community for as long as I can remember. To see it for yourself: 1.) Be running OS X 2.) install GLFW-b-1.0.0 (you may need to cabal update) [1] 3.) ghci -package GLFW-b 4.) import Graphics.UI.GLFW as GLFW 5.) GLFW.init Security camera footage of the crime in progress: https://gist.github.com/dagit/5980438/raw/2e92a63135008921040478566e56ac7c0… One of the first things GLFW.init does on OS X is ... = [[NSAutoreleasePool alloc] init]; That causes -[NSAutoreleasePool init]: unrecognized selector sent to instance ... which seems to mean that NSAutoreleasePool doesn't understand the "init" message/method. But [[NSAutoreleasePool alloc] init] is really basic stuff. What is going on? [1]: http://hackage.haskell.org/package/GLFW-b

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build failure on Hackage
by Brian Lewis 14 Jul '13

14 Jul '13

I maintain a library that, on Linux, needs libXxf86vm to build. The server where Hackage runs doesn't have that library, so the build fails. I think this is reasonable -- that box can't possibly have all the libraries various packages might need. But the build failure seems to cause the Haddock docs to not be generated. Why? My understanding is, if someone specifies my library as a build dependency, my library's build failure will also cause the docs for their package not to be built. I think my library will kill the Hackage Haddock docs for any other package that depends on it, due to a build error that's not even meaningful. What can I do?

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getting haddock to cooperate with cpp
by Evan Laforge 13 Jul '13

13 Jul '13

So haddock ignores {-# LANGUAGE CPP #-}, which makes it crash on any file that uses it. But if you pass --optghc=-cpp, it runs CPP on everything, which makes it crash on any file that uses string gaps, or happens to contain a /*. /* is rare and easily fixed, but not string gaps. It looks like a workaround would be to manually inspect the files for LANGUAGE CPP and run two haddock passes, but then I would have to get the two passes to cooperate creating a single TOC and index. Isn't there some way to run haddock on files that use CPP? In the broader scheme, it seems perverse to be using CPP in the first place. I use it to configure imports and exports, e.g. to swap out a driver backend on different OSes, and to export more symbols when testing. Would it make sense to have a haskell version of CPP that provides only these features (e.g. just #ifdef, #else, #endif, and #define) and leaves out the problematic C comments and backslash expectations?

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ANNOUNCE: MFlow 3.0
by Alberto G. Corona 13 Jul '13

13 Jul '13

The third version of MFlow is out. http://hackage.haskell.org/package/MFlow MFlow is an all-heterodox web application framework, but very haskellish. Now MFlow support restful URLs. It is the first stateful web framework to my knowledge that supports it. The type safe routes are implicitly expressed as normal monadic code within a navigation monad. The application look as a normal imperative console application, but the navigation monad goes back and forth to match the path of the URL. The user has control of the state, that can roll-back or not when the navigation goes back depending on the application needs. The state is in the form of normal Haskell variables In a monadic computation, with the weird addition of backtracking. The menu of the application below is implemented as an imperative-like syntax, but the application navigate forward and backward to synchronize with the requests of the web browser: http://mflowdemo.herokuapp.com/ This version support in-page flows. What is that? look at this example: http://mflowdemo.herokuapp.com/noscript/fviewmonad These flows are implemented as formlets with a monad instance, and callbacks which change the look. I call them "widgets": http://haskell-web.blogspot.com.es/2013/06/the-promising-land-of-monadic-fo… Each page may have many of these active widgets, each one running their own flow. These widgets refresh themselves trough Ajax if they are enclosed in the primitive "autoRefresh". If there is no Ajax or JavaScript available, they gracefully degrade by refreshing the entire page: http://mflowdemo.herokuapp.com/noscript/combination http://haskell-web.blogspot.com.es/2013/06/and-finally-widget-auto-refreshi… The page flows and the multiflow idea was inspired in Seaside<http://www.seaside.st/>, a great Smalltalk web framework and adapted to the pure recursive nature of Haskell and the formlets. It also support some JQuery widgets integrated: modal and not modal dialogs, datePicker and other active widgets that handle other widgets. It also support the older features: persistent state, WAI, blaze-html and others integration, server process timeouts, Ajax, requirements, content management, caching of widget rendering and all the other previous stuff. I wish to thank some people for their feedback. Specially Adrian May for his feedback and interest -- Alberto.

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Quick-check: how to generate arbitrary complex data?
by martin 13 Jul '13

13 Jul '13

Hello all, I have a type ("Mail") which consists of hash and a list, where the hash keeps some redundant data of the list for faster access. I can add and remove elements to values of this type using custom functions, called push and pop. Now I wanted to write some quick checks, but I have no clue how to generate arbitrary values of this type. It will certainly no suffice to write arbitrary instances for the underlying types (Int and Char), because the hash and the list need to be synchronized. Currently "Mail" it is only a type synonym. I suppose as a prerequisite I need to wrap it into a type constructor. But then what?

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Correct way to "catch all exceptions"
by Michael Snoyman 12 Jul '13

12 Jul '13

There's a pattern that arises fairly often: catching every exception thrown by code. The naive approach is to do something like: result <- try someCode case result of Left (e :: SomeException) -> putStrLn $ "It failed: " ++ show e Right realValue -> useRealValue This seems perfectly valid, except that it catches a number of exceptions which seemingly should *not* be caught. In particular, it catches the async exceptions used by both killThread and timeout. I think it's fair to say that there's not going to be a single function that solves all cases correctly, but it is a common enough situation that people need to write code that resumes work in the case of an exception that I think we need to either have some guidelines for the right approach here, or perhaps even a utility function along the lines of: shouldBeCaught :: SomeException -> Bool One first stab at such a function would be to return `False` for AsyncException and Timeout, and `True` for everything else, but I'm not convinced that this is sufficient. Are there any thoughts on the right approach to take here?

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Reify type
by Jose A. Lopes 12 Jul '13

12 Jul '13

Hello everyone, Is there a way to automatically reify a type ? In other words, to do the following: reifyType (LitT ...) = ConT ''LitT ... I am using Template Haskell and I want the generated code to have access to Type datatypes that were available to the Template Haskell code. Cheers, Jose

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Deriving with generics without values
by JP Moresmau 12 Jul '13

12 Jul '13

Hello all, My problem is the following: I have my own data types, and I'd like to derive automatically instances of some type class from them. I've started looking at GHC.Generics, which offer tools to do exactly that. However, some functions of my typeclass do not take my data type as a parameter, but as a result. Basically: class MyClass where fromString :: String -> a data MyData=MkMyData { myField ::Int } deriving (Generic) and I want to automatically generate the instance instance MyClass MyData, using default methods, etc. The GHC Generic class does say that it uses a from function that convert from the datatype to its representation: from :: a -> Rep<http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-G…> a xfrom :: a -> Rep<http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-G…> a x But I don't have a "a" to start from! I see from the related papers that the automatically generated code from from actually does pattern matches on constructors, so I need a value, undefined won't work. However I see the GHC.Generics also provide :+: (Sums: encode choice between constructors). If I have to provide an value, then the choice between constructor has been done! The examples about generics on http://www.haskell.org/haskellwiki/GHC.Generics do provide an example of defining the instance for :+: but I don't understand how we can get there. If I have a class method that takes a value as a parameter, and I pass undefined to it, the code will crash, since it can't pattern match on undefined. Can somebody shed some light on this? Am I using the wrong tool for the job? How can I achieve what I want? I want the full type representation with sums, but without a value to start from. Thanks a million! JP -- JP Moresmau http://jpmoresmau.blogspot.com/

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Comparing functions
by Vlatko Basic 12 Jul '13

12 Jul '13

Hello Cafe, I have data CmpFunction a = CF (a -> a -> Bool) that contains comparing functions, like ==, <, > ..., and I'm trying to declare the Show instance for it like this instance Show (CmpFunction a) where show (CF (==)) = "== " -- no good show f = case f of -- no good also CBF (==) -> "==" _ -> "Other" but compiler complains for both with This binding for `==' shadows the existing binding imported from `Prelude' at src/Main.hs:6:8-11 (and originally defined in `ghc-prim:GHC.Classes') Is it possible at all to compare two functions or how to solve this problem, to show some string for a specific function? br, vlatko

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How to use cabal-dev ghci and multiple targets/build-depends from cabal file?
by Kirill Zaborsky 12 Jul '13

12 Jul '13

Currently I'm creating a small library and I wanted to create tests for it. So I have a library section in cabal file and also a test-suite section. Everything goes well but when I tried to load file with tests into ghci (actually it was a REPL in emacs) I received errors stating that GHCi can not find e.g. Test.Framework module. Is there any way to use multiple build-depends in GHCi? KInd regards, Kirill Zaborsky

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Netwire bouncing ball
by Just 12 Jul '13

12 Jul '13

Hello, I'm trying to get a grasp of netwire by implementing a bouncing ball simulation and I'm failing. The ball starts from the ground with a given velocity and when hitting the ground the wire inhibits successfully. Now I'm kinda stuck. How can I make the ball bounce? Here is the code: {-# LANGUAGE Arrows #-} module Main where import Control.Wire import Prelude hiding ((.), id) import Control.Concurrent type Pos = Double type Vel = Double type ObjState = (Pos, Vel) testApp :: Pos -> Vel -> WireP () ObjState testApp p0 v0 = proc _ -> do v <- integral_ v0 -< -9.81 p <- integral1_ p0 -< v when (>= 0) -< p returnA -< (p, v) main :: IO () main = loop' (testApp 0 30) clockSession where loop' w' session' = do threadDelay 1000000 (mx, w, session) <- stepSessionP w' session' () case mx of Left ex -> putStrLn ("Inhibited: " ++ show ex) Right x -> putStrLn ("Produced: " ++ show x) loop' w session Thanks in advance!

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There was someone here going to use REPA and a functional way to find eigenvalues :)
by KC 12 Jul '13

12 Jul '13

Have you succeeded? In general, is there a functional way to do matrix manipulations? Linear algebra? Linear programming? Integer Programming? Numerical Analysis? Casey -- -- Regards, KC

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Re: [Haskell-cafe] Non-recursive let
by Doug McIlroy 12 Jul '13

12 Jul '13

By analogy to ML, which has "let" and "let rec", where the latter corresponds to Haskell's "let", one is led to "let nonrec". I would definitely not like "shadow", for it means that new variable does NOT cast a shadow on its definining expression. I fear also that "let nonrec" by any name would introduce another attractive nuisance, just as insidious as the one it is intended to correct. For example x = ... let nonrec { x = someFunction x xsq = x^2} in ... won't do what was probably intended. In my own code, this idiom is more likely than the one that sparked the discussion. Doug

2 1

Well-Typed are hiring: Haskell developer
by Duncan Coutts 11 Jul '13

11 Jul '13

Fellow Haskellers, We are looking to hire a Haskell expert to work with us at Well-Typed as a Haskell developer. This is an exciting opportunity for someone who is passionate about Haskell and who is keen to improve and promote Haskell in a professional context. The role is quite general and could cover any of the projects and activities that we are involved in as a company. The tasks may involve: * working on the Haskell compiler, libraries and tools; * Haskell application development; * working directly with clients to solve their problems; * teaching Haskell, and developing training materials. At the moment, we are particularly hoping to find someone with an interest in supporting the development and maintenance of GHC. Therefore, some knowledge or interest in compiler internals, operating systems, the foreign function interface (FFI), and/or deployment issues would be welcome. Well-Typed has a variety of clients. For some we do proprietary Haskell development and consulting. For others, much of the work involves open-source development and cooperating with the rest of the Haskell community: the commercial, open-source and academic users. Our ideal candidate has excellent knowledge of Haskell, whether from industry, academia, or personal interest. Familiarity with other languages, low-level programming, and good software engineering practices are also useful. Good organisation and ability to manage your own time, and reliably meet deadlines, is important. You should also have good communication skills. Being interested or having experience in teaching Haskell (or other technical topics) is a bonus. Experience of consulting, or running a business, is also a bonus. You are likely to have a bachelor's degree or higher in computer science or a related field, although this isn't a requirement. The offer is initially for a one-year full time contract. We are also happy to receive applications for part-time work. The annual salary is from GBP 34,800 or pro rata for part-time or flexible work. We also operate a bonus scheme. We offer flexible hours and work from home. Living in England is not required. We may be able to offer either employment or sub-contracting, depending on the jurisdiction in which you live. If you are interested, please apply via info(a)well-typed.com. Tell us why you are interested and why you would be a good fit for the job, and attach your CV. Please also indicate how soon you might be able to start. We are more than happy to answer informal enquiries. Contact Duncan Coutts, Ian Lynagh or Andres Löh for further information, either by email or IRC. To ensure we can properly consider your application, please get it to us by July 25th, 2013, though we may be able to consider applications received later. -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

1 0

Music / MIDI help
by Mark Lentczner 11 Jul '13

11 Jul '13

I'm a little lost in the bewildering array of music packages for Haskell, and need some help. I'm looking to recreate one of my algorithmic music compositions from the 1980s. I can easily code the logic in Haskell. I'm looking for a the right set of packages and SW so that I can: a) generate short sequences and play them immediately, preferrably in ghci, -- but 'runHaskell Foo.hs | barPlayer' would be acceptable 2) generate MIDI files I'm on OS X. So far what I've found is: Haskore, the midi package, and the jack package - and then I'd need some MIDI software synth for the Mac, and Jack based patcher.... Or perhaps I want SuperCollider, and the Haskell bindings - but that seems rather low level for my needs here (I don't really need to patch together my instruments, and I don't want to have re-write the whole timing framework from scratch.) So - What's a quick easy path here? - Mark

4 3

Problem with pipes interpretation of Lazy MapM program.
by Lyndon Maydwell 11 Jul '13

11 Jul '13

Hi café. I've come up with a little version of 'uniq' that should take into account md5 sums of the file changes... In essence, this: main :: IO () main = getContents >>= mapM check . lines -- PROBLEM!!!! >>= mapM_ (putStrLn . (" --> " ++ )) . strip check :: String -> IO (String, ABCD) check s = (s,) . md5 . Str <$> readFile s strip :: (Ord a, Eq b) => [(a,b)] -> [a] strip = concat . uncurry (zipWith look) . (id &&& maps) look :: (Ord a, Eq b) => (a,b) -> M.Map a b -> [a] look (k,v) m | M.lookup k m == Just v = [] | otherwise = [k] maps :: Ord a => [(a,b)] -> [M.Map a b] maps = scanl (flip (uncurry M.insert)) M.empty Unfortunately mapM isn't lazy, so this doesn't work. I thought this would be a good opportunity to try out the Pipes library for a simple real-world task, but I've come up against some issues with using 'zip' and 'scan' like functions when translating the code. This is what I've got so far, but I'm not sure how to proceed: main :: IO () main = runProxy $ stdinS >-> pipe >-> stdoutD pipe :: () -> ProxyFast () String () String IO () pipe = mapMD check >-> mapScan -- zip, check, output go here >-> mapD ((" --> " ++) . show) mapScan :: () -> ProxyFast () (String, ABCD) () (M.Map String ABCD) IO b mapScan = scanlp (uncurry M.insert) (M.empty) check :: String -> IO (String, ABCD) check s = (s,) . md5 . Str <$> readFile s -- Utils scanlp :: (Monad (p () t a b1 m), Monad m, Functor (p () t a b1 m), Proxy p) => (t -> b1 -> b1) -> b1 -> () -> p () t a b1 m b scanlp f a b = do void $ respond a v <- request () scanlp f (f v a) b There doesn't seem to be any easy zipLike functions, and having to write my own scan function seems odd. Can someone point me in the right direction for this? Thanks! - Lyndon

1 0

Re: [Haskell-cafe] Non-recursive let [Was: GHC bug? Let with guards loops]
by oleg＠okmij.org 11 Jul '13

11 Jul '13

Alberto G. Corona wrote: > I think that a non-non recursive let could be not compatible with the pure > nature of Haskell. I have seen this sentiment before. It is quite a mis-understanding. In fact, the opposite is true. One may say that Haskell is not quite pure _because_ it has recursive let. Let's take pure the simply-typed lambda-calculus, or System F, or System Fomega. Or the Calculus of Inductive Constructions. These calculi are pure in the sense that the result of evaluation of each expression does not depend on the evaluation strategy. One can use call-by-name, call-by-need, call-by-value, pick the next redex at random or some other evaluation strategy -- and the result will be just the same. Although the simply-typed lambda-calculus is quite limited in its expressiveness, already System F is quite powerful (e.g., allowing for the list library), to say nothing of CIC. In all these systems, the non-recursive let let x = e1 in e2 is merely the syntactic sugar for (\x. e2) e1 OTH, the recursive let is not expressible. (Incidentally, although System F and above express self-application (\x.x x), a fix-point combinator is not typeable.) Adding the recursive let introduces general recursion and hence the dependence on the evaluation strategy. There are a few people who say non-termination is an effect. The language with non-termination is no longer pure.

1 0

Non-recursive let [Was: GHC bug? Let with guards loops]
by oleg＠okmij.org 11 Jul '13

11 Jul '13

Andreas wrote: > The greater evil is that Haskell does not have a non-recursive let. > This is source of many non-termination bugs, including this one here. > let should be non-recursive by default, and for recursion we could have > the good old "let rec". Hear, hear! In OCaml, I can (and often do) write let (x,s) = foo 1 [] in let (y,s) = bar x s in let (z,s) = baz x y s in ... In Haskell I'll have to uniquely number the s's: let (x,s1) = foo 1 [] in let (y,s2) = bar x s1 in let (z,s3) = baz x y s2 in ... and re-number them if I insert a new statement. BASIC comes to mind. I tried to lobby Simon Peyton-Jones for the non-recursive let a couple of years ago. He said, write a proposal. It's still being written... Perhaps you might want to write it now. In the meanwhile, there is a very ugly workaround: test = runIdentity $ do (x,s) <- return $ foo 1 [] (y,s) <- return $ bar x s (z,s) <- return $ baz x y s return (z,s) After all, bind is non-recursive let.

7 13

Non-recursive let [Was: GHC bug? Let with guards loops]
by oleg＠okmij.org 11 Jul '13

11 Jul '13

> If you would like to write > > let (x,s) = foo 1 [] in > let (y,s) = bar x s in > let (z,s) = baz x y s in > > instead, use a state monad. Incidentally I did write almost exactly this code once. Ironically, it was meant as a lead-on to the State monad. But there have been other cases where State monad was better avoided. For instance, functions like foo and bar are already written and they are not in the state monad. For example, foo may take a non-empty Set and return the minimal element and the set without the minimal element. There are several such handy functions in Data.Set and Data.Map. Injecting such functions into a Set monad for the sake of three lines seems overkill. Also, in the code above s's don't have to have the same type. I particularly like repeated lets when I am writing the code to apply transformations to it. Being explicit with state passing improves the confidence. It is simpler to reason with the pure code.

3 2

Is the following implemented by a sparse matrix representation? type Graph n w = Array (n, n) (Maybe w)
by KC 10 Jul '13

10 Jul '13

type Graph n w = Array (n,n) (Maybe w) -- -- Regards, KC

3 3

Re: [Haskell-cafe] Parsec error message not making any sense
by Fredrik Karlsson 10 Jul '13

10 Jul '13

Hi, Sorry, that was a careless extraction of code - I should have made sure that it was complete. Please, have a look again. When downloading and running the gist ( https://gist.github.com/dargosch/5955045) , I still get the error: Main> let testFile = "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" *Main> parseFromFile textgridfile testFile Left "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" (line 35, column 5): unexpected "t" expecting "intervals [" on the attached testfile. The "tier" parser works once, but then I get an error that I cant understand, given the input. How come the parser finds the "unexpected "t"" when the expected thing is what is in the input at that point? Thankful for any help I can get on this. On Tue, Jul 9, 2013 at 10:22 PM, Fredrik Karlsson <dargosch(a)gmail.com>wrote: > Hi, > > Sorry, that was a careless extraction of code - I should have made sure > that it was complete. > Please, have a look again. When downloading and running the gist, I still > get the error: > > Main> let testFile = > "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" > *Main> parseFromFile textgridfile testFile > Left > "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" > (line 35, column 5): > unexpected "t" > expecting "intervals [" > > on the attached testfile. The "tier" parser works once, but then I get an > error that I cant understand, given the input. > How come the parser finds the "unexpected "t"" when the expected thing is > what is in the input at that point? > > Thankful for any help I can get on this. > > > /Fredrik > > > On Tue, Jul 9, 2013 at 9:37 AM, Roman Cheplyaka <roma(a)ro-che.info> wrote: > >> Please check your code. >> >> I had two problems with it: mixed tabs and spaces, and undefined >> 'quotedChar'. After defining quotedChar = anyChar, I get a different >> error message from yours: >> >> *Main> parseFromFile textgridfile "testdata.TextGrid" >> Left "testdata.TextGrid" (line 137, column 1): >> unexpected end of input >> expecting quote at end of cell >> >> Roman >> >> * Fredrik Karlsson <dargosch(a)gmail.com> [2013-07-09 08:07:24+0200] >> > Hi Roman, >> > >> > I'm using parsec-3.1.3 >> > >> > I put the code in a gist here - sorry about that. >> > >> > https://gist.github.com/dargosch/5955045 >> > >> > Fredrik >> > >> > >> > >> > >> > On Tue, Jul 9, 2013 at 12:08 AM, Roman Cheplyaka <roma(a)ro-che.info> >> wrote: >> > >> > > Hi Fredrik, >> > > >> > > First, do you use the latest parsec version (3.1.3)? If not, can you >> try >> > > the same with 3.1.3? >> > > >> > > Second, please upload your code to hpaste.org or a similar service >> and >> > > give us the link. It's not much fun to extract code from an html >> email. >> > > >> > > Roman >> > > >> > > * Fredrik Karlsson <dargosch(a)gmail.com> [2013-07-08 23:54:17+0200] >> > > > Dear list, >> > > > >> > > > I have a Parsec parser that fails and gives the following error >> message: >> > > > >> > > > *Main> parseFromFile textgridfile testFile >> > > > Left >> > > > >> > > >> "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" >> > > > (line 35, column 5): >> > > > unexpected "t" >> > > > expecting "intervals [" >> > > > >> > > > Now, this is perfectly understandable, but line 35, col 5 in the >> file >> > > being >> > > > parsed looks like the supplies image - there is no 't' there. >> > > > >> > > > Any ideas on what is going on? >> > > > >> > > > The parser I am using is: >> > > > >> > > > data VariableLine = VariableLine String String deriving Show >> > > > data TierType = IntervalTier | PointTier deriving Show >> > > > >> > > > data Tier = Tier String deriving Show >> > > > data LabelFile = LabelFile Double Double deriving Show >> > > > >> > > > data Label = Label String TierType Double Double String deriving >> Show >> > > > >> > > > >> > > > haskelldef = makeTokenParser haskellDef >> > > > >> > > > >> > > > textgridfile :: Parser (LabelFile, [[Label]]) >> > > > textgridfile = do >> > > > h <- header >> > > > ll <- many1 tier >> > > > return $ (h,ll) >> > > > >> > > > header :: Parser LabelFile >> > > > header = do >> > > > string headTS1 >> > > > start <- try (float haskelldef) >> > > > <|> (fmap fromInteger $ integer haskelldef ) >> > > > string "xmax = " >> > > > end <- try (float haskelldef) >> > > > <|> (fmap fromInteger $ integer haskelldef ) >> > > > string "tiers? <exists> \n" >> > > > string "size = " >> > > > integer haskelldef >> > > > string "item []:" >> > > > whiteSpace haskelldef >> > > > return $ LabelFile start end >> > > > >> > > > tier :: Parser [Label] >> > > > tier = do >> > > > whiteSpace haskelldef >> > > > string "item [" >> > > > integer haskelldef >> > > > string "]:" >> > > > whiteSpace haskelldef >> > > > try (string "class = \"IntervalTier\"") >> > > > <|> string "class = \"TextTier\"" >> > > > whiteSpace haskelldef >> > > > string "name = " >> > > > char '"' >> > > > name <- many quotedChar >> > > > char '"' <?> "quote at end of cell" >> > > > whiteSpace haskelldef >> > > > string "xmin = " >> > > > try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) >> > > > whiteSpace haskelldef >> > > > string "xmax = " >> > > > try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) >> > > > string "intervals: size = " <|> string "points: size = " >> > > > integer haskelldef >> > > > whiteSpace haskelldef >> > > > labelList <- many1 (interval name) >> > > > return $ labelList >> > > > interval :: String -> Parser Label >> > > > interval tierName = do >> > > > whiteSpace haskelldef >> > > > string "intervals [" >> > > > integer haskelldef >> > > > string "]:" >> > > > whiteSpace haskelldef >> > > > string "xmin = " >> > > > start <- try (float haskelldef) >> > > > <|> (fmap fromInteger $ integer haskelldef ) >> > > > whiteSpace haskelldef >> > > > string "xmax = " >> > > > end <- try (float haskelldef) >> > > > <|> (fmap fromInteger $ integer haskelldef ) >> > > > whiteSpace haskelldef >> > > > string "text = " >> > > > char '"' >> > > > text <- many quotedChar >> > > > char '"' <?> "quote at end of cell" >> > > > return $ Label tierName IntervalTier start end text >> > > > >> > > > which fails on the attached input file. >> > > > >> > > > I can't see how 't' is found?? What am I doing wrong? >> > > > >> > > > /Fredrik >> > > > >> > > > >> > > > >> > > > -- >> > > > "Life is like a trumpet - if you don't put anything into it, you >> don't >> > > get >> > > > anything out of it." >> > > >> > > >> > > >> > > > _______________________________________________ >> > > > Haskell-Cafe mailing list >> > > > Haskell-Cafe(a)haskell.org >> > > > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > >> > > >> > >> > >> > -- >> > "Life is like a trumpet - if you don't put anything into it, you don't >> get >> > anything out of it." >> >> >> > > > -- > "Life is like a trumpet - if you don't put anything into it, you don't get > anything out of it." > -- "Life is like a trumpet - if you don't put anything into it, you don't get anything out of it."

2 1

Engineering position at Vector Fabrics
by Stefan Holdermans 10 Jul '13

10 Jul '13

Vector Fabrics has another position open for a functional programmer. See below and http://www.vectorfabrics.com/company/career/functional_programmer for details. This time, we particularly welcome applications from experienced developers, the position providing a fair share of opportunities for growing into the rôle of an architect. Interested? Please contact us at jobs <AT> vectorfabrics.com. Cheers, Stefan ---------------------------------------------------------------------- Vector Fabrics is growing the team: we are looking for a top-notch programmer to extend our program-analysis and parallelization products. You design and implement algorithms to assist programmers in creating parallel designs from sequential C or C++ programs. You work with our international team of world-class computer scientists and experts in the Haskell / OCaml functional programming languages. Your work is at the forefront of technology, giving you the opportunity to publish your work in major conferences and directly cooperate with processor design companies and domain-specific application vendors. As we are a startup company, you will quickly have a major impact on our products and get to know all aspects of product creation. You will be part of a strongly committed development team and contribute to our agile development process and automated test suites. Interested? Send your CV, GitHub account or other proof of what you can do to jobs(a)vectorfabrics.com. RESPONSIBILITIES * Design and implement software optimization (e.g. parallelization) algorithms for CPUs and GPUs; * Thoroughly test your code, create automated test suites; * Contribute to our agile development planning and process; * Analyze complex customer applications for optimization opportunities and translate this to new analysis algorithms. PROFILE * Your friends and colleagues describe you as a superb programmer; your programming ability is way above average; * Demonstrable experience in design and implementation of complex software applications; prior experience in functional programming languages is preferred; * You continuously surprise us with your creative yet pragmatic solutions for complex software problems; * You are strongly committed to deliver working software as early as possible; * You work against very high quality standards. Refactoring is your bread and butter, pair-programming is how you prefer to review your code; * Whatever technologies, languages, or development environments you've been using, we expect you have mastered them in depth, and we expect that you will be able to master any technology, language, or development environment that we need in the future; * Excellent command of written and spoken English. EDUCATION MSc, MEng, or PhD in Computer Science or significant relevant experience. ABOUT VECTOR FABRICS Vector Fabrics is a high-tech software company, developing tools for embedded multicore programming. Its technology and expertise is getting widespread recognition in the industry as being innovative and unique in their ability to address heterogeneous multicore application-specific silicon platforms. Due to the advanced nature of its tools, Vector Fabrics operates at the forefront of the next generation of embedded platforms for diverse markets ranging from supercomputers to automotive to cell phones. Vector Fabrics puts absolute priority on hiring top class individuals in key positions. Vector Fabrics’ team profile is exceptional and its ambition is to hire only individuals that match or surpass that profile. The company pays top salary and offers a challenging, engaging and stimulating work environment with a high degree of responsibility.

1 0

Array, Vector, Bytestring
by silvio 10 Jul '13

10 Jul '13

Hi everyone, Every time I want to use an array in Haskell, I find myself having to look up in the doc how they are used, which exactly are the modules I have to import ... and I am a bit tired of staring at type signatures for 10 minutes to figure out how these arrays work every time I use them (It's even worse when you have to write the signatures). I wonder how other people perceive this issue and what possible solutions could be. Eg. look at the type signature for changing a single entry in an array: * vector package: write :: PrimMonad m => MVector (PrimState m) a -> Int -> a -> m () * array package: writeArray :: (MArray a e m, Ix i) => a i e -> i -> e -> m () * bytestring package: not available * a reasonable type signature would be: write :: MVector a -> Int -> a -> ST s a # To many different implementations Why do we need so many different implementations of the same thing? In the ghc libraries alone we have a vector, array and bytestring package all of which do the same thing, as demonstrated for instance by the vector-bytestring package. To make matters worse, the haskell 2010 standard has includes a watered down version of array. I personally prefer the vector package but if you want to use libraries for strings, I am forced back to use bytesting. And now that Array was added to haskell 2010, that will start to pop up more and more. The multitude of already existing libraries is also why I am writing this to haskell-cafe and not spinning yet another implementation. # Index I don't really see a reason for having an index of a type other than Int and that starts somewhere else than at 0. While there might be corner cases where such a thing might be useful, it is only confusing and irritating for the (presumably) rest of us. # Storable vs Unboxed Is there really a difference between Storable and Unboxed arrays and if so can't this be fixed in the complier rather than having to expose this problem to the programmer? # ST s vs IO This is probably the hardest to resolve issue. The easiest solution is probably to just have a module for each of them as in the array package. I find the PrimState a bit complicated and circuitous. The ideal solution would be to have type IO a = ST RealWorld# a in the next haskell standard. If I understand it correctly this would let you declare the types as follows but and still use it in an IO context. write :: MVector a -> Int -> a -> ST s a Would this work? I know it would probably screw up all instances instance ... IO where On the other hand, this is not only an issue with arrays but all procedurally accelerated data structures like hash-tables. Silvio

12 24

Promoting Haskell via Youtube movies
by Mihai Maruseac 10 Jul '13

10 Jul '13

Hi, I was wondering if we have similar movies for Haskell as https://www.youtube.com/watch?v=kLO1djacsfg and https://www.youtube.com/watch?v=E3418SeWZfQ exist for Java. I indend to give them to some people to make them intrigued by the language and start learning it / looking for it. -- MM "All we have to decide is what we do with the time that is given to us"

9 9

Possible extension to Haskell overloading behavior
by Chris Smith 10 Jul '13

10 Jul '13

So I've been thinking about something, and I'm curious whether anyone (in particular, people involved with GHC) think this is a worthwhile idea. I'd like to implement an extension to GHC to offer a different behavior for literals with polymorphic types. The current behavior is something like: 1. Give the literal a polymorphic type, like (Integral a => a) 2. Type check the whole program, possibly giving the term a more constrained type. 3. If the type is still ambiguous, apply defaulting rules. I'd like to add the option to do this instead. 1. Take the polymorphic type, and immediately apply defaulting rules to get a monomorphic type. 2. Type check the program with the monomorphic type. Mostly, this would reduce the set of valid programs, since the type is chosen before considering whether it meets all the relevant constraints. So what's the purpose? To simplify type errors for programmers who don't understand type classes. What I have in mind is domain-specific dialects of Haskell that replace the Prelude and are aimed at less technical audiences - in my case, children around 10 to 13 years old; but I think the ideas apply elsewhere, too. Type classes are (debatably) the one feature of Haskell that tends to be tricky for non-technical audiences, and yet pops up in very simple programs (and more importantly, their error messages) even when the programmer wasn't aware of it's existence, because of its role in overloaded literals. In some cases, I think it's a good trade to remove overloaded literals, in exchange for simpler error messages. This leaves new programmers learning a very small, simple language, and not staring so much at cryptic error messages. At the same time, it's not really changing the language, except for the need to explicitly use type classes (via conversion functions like fromInteger) rather than get them thrown in implicitly. With GHC's extended defaulting rules that apply for OverloadedStrings, this could also be used to treat all string literals as Text, too, which might make some people happy, too. Of course, the disadvantage is that for numeric types, you would lose the convenience of overloaded operators, since this is only a sensible thing to do if you're replacing the Prelude with one that doesn't use type classes. But in at least my intended use, I prefer to have a single Number type anyway (and a single Text type that's not sometimes called [Char]). In the past, explaining these things has eaten up far too much time that I'd rather have spent on more general skills and creative activities.

2 7

GHC bug? Let with guards loops
by Andreas Abel 09 Jul '13

09 Jul '13

Hi, is this a known bug or feature of GHC (7.4.1, 7.6.3)?: I got a looping behavior in one of my programs and could not explain why. When I rewrote an irrefutable let with guards to use a case instead, the loop disappeared. Cut-down: works = case Just 1 of { Just x | x > 0 -> x } loops = let Just x | x > 0 = Just 1 in x works returns 1, loops loops. If x is unused on the rhs, the non-termination disappears. works' = let Just x | x > 0 = Just 1 in 42 Is this intended by the Haskell semantics or is this a bug? I would have assumed that non-recursive let and single-branch case are interchangeable, but apparently, not... Cheers, Andreas -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

4 7

Understanding version differences
by Patrick Browne 09 Jul '13

09 Jul '13

2 2

Boulder Haskell Programmers, Next Meeting July 17
by Peter Jones 09 Jul '13

09 Jul '13

The Boulder Haskell Programmer Group is getting together on July 17 at 6:30pm. All experience levels are welcome. Agenda: * 6:30--7:00: Social/beer time. * 7:00--7:30: What do we want from this group? * 7:30--8:30: Introduction to Haskell Types. * 8:30--9:00: Whatever! http://www.meetup.com/Boulder-Haskell-Programmers/events/125427022/ -- Peter Jones, Founder, Devalot.com Defending the honor of good code

1 0

Open Source project suitable for 2-3 persons this fall?
by Anders Bech Mellson 09 Jul '13

09 Jul '13

Is there any project that needs working this fall which could be used as a university project? I am in the university (M.Sc. in software development), so I am mainly looking for project ideas (preferably concrete ones). We are 2-3 students that have ~10 hours pr week for 3 months to work on a project. Is there a listing somewhere with project ideas for contributing to the Haskell community? Thanks in advance, Anders

6 5

ERP/CRM solution with Haskell ?
by Loïc Maury 09 Jul '13

09 Jul '13

Hello all, I don't know if it's the good mailing list to post, if not my apologize for that. I am actually looking for an ERP/CRM solution. Is there a solution (like dolibarr or openerp), but in Haskell to do that ? Thank you Loïc

1 0

Harder Question - Chapter 4 - 4.12 - haskell the craft of functional programming - Second Edition
by Manoel Menezes 09 Jul '13

09 Jul '13

Hi everybody! I am trying to solve the question for a long time: [*4.12 Harder] Find out the maximum number of pieces we can get by making a given* *number of flat (that is planar) cuts through a solid block. It is not the same* *answer as we calculated for straight-line cuts of a flat piece of paper.* I find out that this function has the following results: f 0 = 1 f 1 = 2 f 2 = 4 f 3 = 8 That is, from 0 to 3, the flat cuts all the pieces in two other pieces, so the number of pieces is doubled. But, starting from f 4, the flat can not cuts all the pieces, in case of f 4, the flat can cut 6 out of the 8 pieces, resulting in 12 pieces plus 2 pieces 2 = 14 pieces. But I can not reach a general case. Can anybody help me to find out the solution? Thank you very much! Manoel Menezes. __________________________________ Manoel Messias da Silva Menezes Jr M.Sc.in Computer Science Federal University of Pernambuco System Analyst - Petrobras

2 1

Probabilistic functional programming with Baysig/BayesHive
by Tom Nielsen 09 Jul '13

09 Jul '13

Dear cafe, I would like to announce that the Baysig programming language and the BayesHive analytics environment (http://bayeshive.com) are now available for beta testers. Baysig is a new probabilistic, functional and typed programming language that attempts to realise the vision of "fully Bayesian computing". That is, in Baysig almost all the work in data processing consists of building a probabilistic model of the incoming data. Almost everything else -- optimal decisions, categorisation, measuring hidden parameters or states, forecasting, testing hypothesis -- becomes trivial. This paradigm can in principle be applied to a large number of domains, although for the moment we are focusing on models that are based on continuous parameters. It will therefore be of interest to users of statistics and dynamical systems models, including in finance, physics and life sciences. To analyse data in Baysig, you write a program in the random-number supply monad that generates simulated data. A special construct, "estimate", then applies Bayes' theorem to this program and returns the probability distribution of the model parameters given observed data. The "estimate" procedure is difficult to implement in Haskell or similar languages, which encouraged us to develop a new language. However, in many respects Baysig should feel like Haskell, and we hope that Baysig will encourage the Haskell community to experiment with statistical modelling. We have built a web-based environment to help users, including those with little-to-no programming experience, use Baysig, at bayeshive.com. This web application allows you to upload data from spreadsheets or timeseries, and to build statistical models with a point-and-click web interface which ends up generating Baysig code. Code and the results of running it are collected in shareable and editable literate programming documents. We would appreciate any feedback from the Haskell community before releasing our platform to unsuspecting statisticians and researchers. Almost everything is written in Haskell, including the Baysig compiler. The BayesHive website is written using Yesod, with which we are mostly happy. We also use the Stan package (mc-stan.org) and the web front-end is written using AngularJS. For the moment the Baysig language is only available through the BayesHive web interface, but that will change. If you want to run Baysig on your own computer, please send me an email at tomn(a)openbrain.org. Finally, both the BayesHive web application and the Baysig language implementation are still prototypes and very much work-in-progress. We promise to work hard at fixing the bugs you find! Links: BayesHive, including a few videos: http://bayeshive.com Baysig quick tour ("QuickBAYSIG"): http://bayeshive.com/helppage/Baysig%20quick%20tour:%20fundamentals More documentation: http://bayeshive.com/help Regards, Tom Nielsen OpenBrain Ltd.

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GHC trac
by Marios Titas 09 Jul '13

09 Jul '13

I just noticed that the GHC trac was moved to http://ghc.haskell.org/trac/ghc/ and I cannot login with my old credentials. Were the old accounts transferred to the new trac? Also, the registration page [1] does not work. [1] http://ghc.haskell.org/trac/ghc/register

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Parsec error message not making any sense
by Fredrik Karlsson 09 Jul '13

09 Jul '13

Dear list, I have a Parsec parser that fails and gives the following error message: *Main> parseFromFile textgridfile testFile Left "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" (line 35, column 5): unexpected "t" expecting "intervals [" Now, this is perfectly understandable, but line 35, col 5 in the file being parsed looks like the supplies image - there is no 't' there. Any ideas on what is going on? The parser I am using is: data VariableLine = VariableLine String String deriving Show data TierType = IntervalTier | PointTier deriving Show data Tier = Tier String deriving Show data LabelFile = LabelFile Double Double deriving Show data Label = Label String TierType Double Double String deriving Show haskelldef = makeTokenParser haskellDef textgridfile :: Parser (LabelFile, [[Label]]) textgridfile = do h <- header ll <- many1 tier return $ (h,ll) header :: Parser LabelFile header = do string headTS1 start <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) string "xmax = " end <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) string "tiers? <exists> \n" string "size = " integer haskelldef string "item []:" whiteSpace haskelldef return $ LabelFile start end tier :: Parser [Label] tier = do whiteSpace haskelldef string "item [" integer haskelldef string "]:" whiteSpace haskelldef try (string "class = \"IntervalTier\"") <|> string "class = \"TextTier\"" whiteSpace haskelldef string "name = " char '"' name <- many quotedChar char '"' <?> "quote at end of cell" whiteSpace haskelldef string "xmin = " try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) whiteSpace haskelldef string "xmax = " try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) string "intervals: size = " <|> string "points: size = " integer haskelldef whiteSpace haskelldef labelList <- many1 (interval name) return $ labelList interval :: String -> Parser Label interval tierName = do whiteSpace haskelldef string "intervals [" integer haskelldef string "]:" whiteSpace haskelldef string "xmin = " start <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) whiteSpace haskelldef string "xmax = " end <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) whiteSpace haskelldef string "text = " char '"' text <- many quotedChar char '"' <?> "quote at end of cell" return $ Label tierName IntervalTier start end text which fails on the attached input file. I can't see how 't' is found?? What am I doing wrong? /Fredrik -- "Life is like a trumpet - if you don't put anything into it, you don't get anything out of it."

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ANNOUNCE: Ajhc Haskell Compiler 0.8.0.7 Release
by Kiwamu Okabe 09 Jul '13

09 Jul '13

We are happy to announce Ajhc 0.8.0.7. You can program interrupt handler with Haskell language on this release. But not yet collect (big) patch sets, the changes will be merged to jhc. You can get Ajhc using "cabal install ajhc" command. The usage is found at Ajhc's project web site http://ajhc.metasepi.org/. The source code at https://github.com/ajhc/ajhc/tags. Welcome sending any bugs or your ideas to https://github.com/ajhc/ajhc/issues. ## An example of interrupt handler written with Haskell <https://github.com/ajhc/demo-cortex-m3/tree/master/stm32f3-discovery> The demo for Cortex-M4 has main context and intrrupt context. The main context waits time expire with polling counter. <https://github.com/ajhc/demo-cortex-m3/blob/master/stm32f3-discovery/hs_src…> The interrupt context is called from clock exception, and decrement counter. <https://github.com/ajhc/demo-cortex-m3/blob/master/stm32f3-discovery/hs_src…> ## Other changes * Guard StablePtr critical section. * Add _JHC_JGC_SAVING_MALLOC_HEAP option for getting smaller malloc heap. * Link forkIO to forkOS. Enjoy! :) - - - Metasepi team

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Linux users needed for OpenGL extensions survey
by Brian Lewis 09 Jul '13

09 Jul '13

Hi, I'm doing a survey to find out how well various OpenGL extensions are supported, to know where to focus efforts on Haskell game software. If you run Linux on your desktop and want to help, here's how: 1.) Save the information like this: $ glxinfo > SOMENAME.glxinfo ... where SOMENAME is your name, or nickname, or the computer's name, whatever you like. If you don't have glxinfo, you might need to install mesa-utils on Ubuntu, or mesa-demos on Arch. 2.) Ensure the file contains information about OpenGL extensions supported by your graphics card. 3.) Attach it to me *off list*. If you have questions, please ask me *off list*. If there's interest, I'll make the survey results available. Thanks!

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Announcing postgresql-libpq-0.8.2.3
by Leon Smith 09 Jul '13

09 Jul '13

I just fixed a fairly serious performance problem with postgresql-libpq's binding to PQescapeStringConn; in was exhibiting a non-linear slowdown when more strings are escaped and retained. https://github.com/lpsmith/postgresql-libpq/commit/adf32ff26cdeca0a12fa5965… If you are using postgresql-libpq's escapeStringConn, or a library that uses it (e.g. postgresql-simple, or persistent-postgresql), I do recommend upgrading. You may or may not see a performance improvement, depending on your particular use case, but if you do it can be quite substantial. It's not entirely clear to me what the root cause really is, but it certainly appears as though it's related to the (direct) use of mallocBytes, which was replaced with (indirect) calls to mallocForeignPtrBytes / mallocPlainForeignPtrBytes (through the bytestring package). In this case, it resulted in an asymptotic improvement in time complexity of some algorithms. Best, Leon

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