- Haskell-Cafe - Haskell.org

ANN: hflags-0.1, a command line flags library similar to Google's gflags
by Gergely Risko 30 Apr '12

30 Apr '12

Hello, I would like to announce the release of HFlags, a command line flags library that makes it very easy to define and use flags. The API is similar to Google's gflags, here is a very simple example program: -=- #!/usr/bin/env runhaskell {-# LANGUAGE TemplateHaskell #-} import HFlags defineFlag "name" "Indiana Jones" "Who to greet." defineFlag "r:repeat" (3 + 4 :: Int) "Number of times to repeat the message." main = do remainingArgs <- $(initHFlags "Simple program v0.1") sequence_ $ replicate flags_repeat greet where greet = putStrLn $ "Hello " ++ flags_name ++ ", very nice to meet you!" -=- As you can see, we have TemplateHaskell functions to help with the definition (and initialization) of the flags, and the values themselves are pure, no need for being in the IO monad to use the value of the flags. Also, the initHFlags function automagically gathers all the flags defined anywhere in the whole program, so if a library defines flags, you don't have to mention it in the main, but the user will still be able to set those flags too. More details behind this design and more example can be found in the following post: http://blog.risko.hu/2012/04/ann-hflags-0.html Comments and criticism is welcome. Code: http://github.com/errge/hflags Examples: https://github.com/errge/hflags/tree/master/examples Hackage: http://hackage.haskell.org/package/hflags Best regards, Gergely Risko

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Hong Kong Haskell hackathon - call for participation
by Matthias Fischmann 30 Apr '12

30 Apr '12

Hi everybody, if you are in or around Hong Kong in May and want to play, please check out this: http://www.haskell.org/haskellwiki/HkHac2012 Otherwise, any wider circulation or suggestion of channels / university people / companies that I should invite directly is welcome. cheers, Matthias

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Printing call site for partial functions
by Michael Snoyman 30 Apr '12

30 Apr '12

I had a bug in a site of mine[1] for a few weeks, where it would just print: Prelude.head: empty list It took a long time to track down the problem, as it came from some other library I was depending on. Eventually I tracked it down, reported it, and the problem was fixed the next day. The weak link in this chain was identifying what package was to blame. I noticed the other day that the docs for undefined[2] state: > It is expected that compilers will recognize this and insert error messages which are more appropriate to the context in which undefined appears. I'm not sure if the current output of `Prelude.undefined` is meant to fit the criterion of "appropriate to the context", but in my opinion it does not. I'd like to propose a bit of a strawman to improve the situation for not just `undefined`, but any arbitrary partial function (including `head`). What we really want is to define some function like: headContext :: String -> [a] -> a headContext context [] = error $ "Prelude.head: empty list (" ++ context ++ ")" headContext _ (x:_) = x And then have the compiler automatically include (optional) package name, module name, and line number where `headContext` was called. How about we borrow a bit from rewrite rules, and have a pragma such as: {-# WITH_CONTEXT head headContext #-} If the compiler supports the feature, and is able to provide context in the given location, it will call `headContext` with an appropriate `String`. Otherwise, normal `head` would be called. I'm sure there are many better ways to approach the problem, and I can't speak to the complexity of implementation within GHC. I *can* say, however, that this would have saved me a lot of time in the example I gave above, and I'd bet many Haskellers have similar stories. This could be a huge debugging win across the board. Michael [1] It was actually yesodweb.com, and resulted in empty Javascript files being generated, thus disabling some features of the site. [2] http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Prelude.h…

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ANN: unfoldable-0.4.0
by Sjoerd Visscher 30 Apr '12

30 Apr '12

I am pleased to announce the 5th version of the unfoldable package. (This is the first announcement, you didn't miss anything.) http://hackage.haskell.org/package/unfoldable-0.4.0 Just as there's a Foldable class, there should also be an Unfoldable class. This package provides one: class Unfoldable t where unfold :: Unfolder f => f a -> f (t a) Writing instances of Unfoldable is similar to writing Traversable instances. For example, given a data type data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) a suitable instance would be instance Unfoldable Tree where unfold fa = choose [ pure Empty , Leaf <$> fa , Node <$> unfold fa <*> fa <*> unfold fa ] The choose function comes from the Unfolder class: class Applicative f => Unfolder f where choose :: [f x] -> f x (If f is an Alternative instance, choose is simply Data.Foldable.asum.) Different unfolders provide different ways of generating values, for example: - Random values - Enumeration of all values (depth-first or breadth-first) - Convert from a list - An implementation of QuickCheck's arbitrary should also be possible (still working on that) Some examples can be found in the examples directory in the github repo: https://github.com/sjoerdvisscher/unfoldable Ideas and comments are welcome! greetings, Sjoerd

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Can we write a parser with both name resolution and error handling by using Happy?
by Tsuyoshi Ito 30 Apr '12

30 Apr '12

Hello, I have a question about Happy, the parser generator. Section 2.1 of Happy User Guide explains a neat trick to perform the name resolution inside a parser by turning the semantic values of nonterminal symbols into Reader monads. http://www.haskell.org/happy/doc/html/sec-using.html#sec-other-datatypes (The example uses bare functions instead of Reader monads, but this difference is insignificant.) A compilable Calc.y file is available from https://gist.github.com/2554028. The important part of the example is the following: Exp : let var '=' Exp in Exp { \p -> $6 (($2, $4 p) : p) } | Exp1 { $1 } Exp1 : ... Term : ... Factor : int { \p -> $1 } | var { \p -> case lookup $1 p of Nothing -> error "no var" Just i -> i } | '(' Exp ')' { $2 } The example code calls `error` on the failure in the name resolution. Although this is fine as an example, it is not good in an actual parser because that means that I have to run the parser in IO monad to catch a parse error. I would like to handle this error just like a parse error by using a monadic parser as explained in Section 2.5.1. http://www.haskell.org/happy/doc/html/sec-monads.html#sec-exception However, I cannot make this work. I tried the following approaches: 1. Just combine these two techniques. That is, write `%monad { E } { thenE } { returnE }` in .y file, where `E t` is equivalent to `Either String t` as defined in Section 2.5.1, and make the semantic value a Reader monad. This does not achieve what I want: now Happy expects that the semantic expression of the form {% ... } will have type `E (Env -> t)` (or `E (Reader Env t)`), but I want to write an expression of type `Env -> E t` (or `ReaderT Env E t`) because whether name resolusion succeeds or not depends on the environment. See https://gist.github.com/2554041 for a code (which does not compile). 2. Write `%monad { P } { thenP } { returnP }`, where `P t` is equivalent to `Env -> E t` or `ReaderT Env E t` and `thenP` and `returnP` are defined accordingly. Now I cannot write the semantic expression for the production rule `Exp : let var '=' Exp in Exp`, because the semantic values of children ($2, $4, and $6) accessible in the `{ ... }` or `{% ... }` part is no longer Reader monads but plain values, meaning that I cannot evaluate a subexpression ($6) using a different environment from the current one. Moreover, this feels wrong because not every symbol requires the environment to evaluate. See https://gist.github.com/2554044 for a code (which does not run because it contains `undefined`). 3. Maybe I cannot use the `%monad` support for this purpose, so shall I give up using it and just use `Env -> E t` as a type of semantic values? This is cumbersome at best because now in each production rule, I have to handle the case where the semantic values of some of the children are `Nothing` (after giving an environment). But more importantly, this does not work because Happy expects the `parseError` function to have type `forall a. [Token] -> a` (which is understandable), which means that it cannot return anything like `Nothing`, defeating the purpose of the use of a monadic parser for error handling. See https://gist.github.com/2554122 for a code (which does not run). >From this, it does not seem that name resolution can be performed inside a parser if we need any error handling better than calling `error`. Is this really the case? Because the combination of parsing and name resolution is very natural (which is why the example in Section 2.1 is so appealing), it is hard to believe that it cannot be done nicely. Am I missing something? In case it matters, I am using Haskell Platform 2011.4.0.0 on Windows. (Just in case, I am aware that we can split parsing and name resolution, in which case a parser generates an abstract syntax tree (AST) with unresolved names and a name resolver converts it to an AST with resolved names. But is that a recommended way to use Happy? If so, the example in Section 2.1 is misleading at best.) Best regards, Tsuyoshi

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Announce: HaTeX 3.3
by Daniel Díaz Casanueva 30 Apr '12

30 Apr '12

Greetings Haskellers! A new version of HaTeX is ready to use in Hackage: http://hackage.haskell.org/package/HaTeX-3.3 == Description == HaTeX is a heavily maintained LaTeX code parser/generator. Write LaTeX documents with all the advantages you already have in Haskell (recursion, type system, high order functions, ...), create a LaTeX backend for your own program, make analysis of LaTeX code through its Abstract Syntax Tree (AST), find another way to pretty-printing values, ... == Release notes == Please, to read the release notes go to the following blog post: http://deltadiaz.blogspot.com.es/2012/04/hatex-33-release-notes.html Thanks and good luck, Daniel Díaz.

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Cabal try to install a package I already have
by Daniel Díaz Casanueva 29 Apr '12

29 Apr '12

Hi Cafe, today I opened the command prompt and typed "cabal install ftphs". Then I read: > Resolving dependencies... > Configuring network-2.3.0.5... And I started to feel confused. Because I already have installed that package with that version! I continued reading: > cabal: The package has a './configure' script. This requires a Unix > compatibility toolchain such as MinGW+MSYS or Cygwin. Yes, I knew this would happen! (thought I don't like it with a so common-used library) But I can't understand why cabal is trying to install network-2.3.0.5. If I type "ghc-pkg list" I can see precisely network-2.3.0.5 is installed. In fact, 2.3.0.5 is the version that comes with the Haskell Platform I recently installed (2011.4.0.0) as you can see here: http://code.haskell.org/haskell-platform/haskell-platform.cabal. Could someone, please, explain me this? I will be very glad. I was working with ghc-7.4 but I needed the network package so I switched to the Haskell Platform, because install network in Windows is painful. Yes, I read the Mitchell guide (found in http://neilmitchell.blogspot.com.es/2010/12/installing-haskell-network-libr…) but once I installed the library I was having compile time error with "unknown symbol" errors. I felt very odd so I switched this machine to the Haskell Platform. I hope someone could help me. Thanks in advance, Daniel Díaz.

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Embed Haskell
by Rosario Borda 29 Apr '12

29 Apr '12

Hi All, Is there a simple method for embed haskell in other languages, like latex? An example in quasiquotation notation: \documentclass{article} \title{Hello World} \begin{document} \maketitle Hello world! and reversed: [haskell: putStrLn reverse "Hello, World!"] \end{document} Many thanks, :) Rosario

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Automatic discovery of tests
by Graham Berks 28 Apr '12

28 Apr '12

Hi, trying to learn haskell and the associated build env. I come from a java/scala world so trying to understand the how to invoke tests. Have a cabal file that invokes a main function to execute a test. But wondering if there is any automatic discovery of tests as per junit. I have seen test-framework-th but to me that implies putting a testGroupGenerator into each module. Am i correct about that and if so is there an example of calling multiple modules tests ? Or is there a better way just to invoke specific functions prefixed with case_ prop_ etc in the entire src/test directory of the cabal build ? Thanks

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Update of haskell cafe on google groups
by Graham Berks 28 Apr '12

28 Apr '12

Has this stopped working ? Seems a bit out of date ? Thanks

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: planar-graph-1.0
by Ivan Lazar Miljenovic 28 Apr '12

28 Apr '12

On 28 April 2012 11:34, Christian Höner zu Siederdissen <choener(a)tbi.univie.ac.at> wrote: > Hi, > > one minor correction: >> This probably won't be of many use to people > for bioinformatics I'd disagree ;-) I'll take a look at your library. > > (for example, RNA /secondary/ structure forms a planar graph) I stand corrected! -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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ANNOUNCE: planar-graph-1.0
by Ivan Lazar Miljenovic 28 Apr '12

28 Apr '12

I uploaded this [1] yesterday, posted the blog article [2] about it... but forgot to send a message to the lists! [1]: http://hackage.haskell.org/package/planar-graph [2]: http://ivanmiljenovic.wordpress.com/2012/04/27/announcing-planar-graph/ planar-graph is an implementation of, strangely enough, planar graphs (that is, a graph that contains an embedding on a surface, can be drawn with no edge crossings and has a specific ordering of edges). It handles graphs on planes and spheres, but I'm not sure about other surfaces (and there seems to be little demand for such). This probably won't be of many use to people, but as I described in the blog post, I've been using this as a test bed for graph library design (specifically usage of abstract node/edge identifiers, using half-edges and the serialisation/encoding setup). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] [Haskell] ANN: Portackage - a hackage portal
by Andrew Seniuk 28 Apr '12

28 Apr '12

Hi Derek, Thanks for your comments. Yes, I am probably going to focus on making it work as a web service and stay away from platform-specific mobile apps. Not been thinking mobile much at all, since the quantities of data are taxing even to my laptop, and also I'm inclined to believe people are mostly at a laptop or desktop when coding Haskell. However, I'm planning to try server-side sorting and filtering, and compare performance. No DB is used on the server yet, but it would be simple enough I suppose -- all this is quite new territory for me. Also my shared hosting resource limits may become a problem if this gets a lot of use. On the other hand, was planning to use persistence (cookies i guess?) to save the data on the client, so that patches based on Hackage RSS could be applied incrementally. Also, I think the sorting could be faster by sending per-column lists of indices. But maybe it is just the browser rendering which is taking all the time? I'll take this opportunity to announce a few improvements: - shift-click a row to see the Hackage page (bare click does nothing) - per-column filtering widgets Presently working on getting the "last updated" column, this information is not in a .cabal file but I'm hoping it is in the TAR metadata... The link again: http://www.fremissant.net/portackage A.

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Uploading a new hsc2hs
by Thomas DuBuisson 27 Apr '12

27 Apr '12

Warning: I, not the maintainer of hsc2hs, will be uploading a trivial fix for hsc2hs to hackage (new build deps). Even after public attempts to contact anyone in charge of hsc2hs (last January) there still has been no word. Speak now or forever hold your peace. Cheers, Thomas P.S. I still think c2hs is the right philosophy even if it's too verbose.

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Annotations in abstract syntax tree
by j.romildo＠gmail.com 27 Apr '12

27 Apr '12

Hello. I need to annotate abstract syntax tree with additional information in a compiler. Using the Annotations package[1] I have written the following small program: import Annotations.F.Annotated import Annotations.F.Fixpoints data ExprF r = Num Double | Var String | Add r r | Sub r r | Mul r r | Div r r deriving (Eq,Show) type BareExpr = Fix ExprF e :: BareExpr e = In (Mul (In (Num 5)) (In (Add (In (Var "x")) (In (Num 8))))) type ValExpr = Fix (Ann Double ExprF) type Memory = [(String,Double)] eval :: Memory -> BareExpr -> ValExpr eval _ (In (Num x)) = In (Ann x (Num x)) eval m (In (Var x)) = let y = case lookup x m of Just k -> k Nothing -> 0 in In (Ann y (Var x)) eval m (In (Add x y)) = op m (+) Add x y eval m (In (Sub x y)) = op m (-) Sub x y eval m (In (Mul x y)) = op m (*) Mul x y eval m (In (Div x y)) = op m (/) Div x y op m f k x y = let x'@(In (Ann v1 _)) = eval m x y'@(In (Ann v2 _)) = eval m y in In (Ann (f v1 v2) (k x' y')) With these definitions we can represent simple arithmetic expressions and we can also evaluate them, annotating each node in the abstract syntax tree with its corresponding value. Now I want to add statements to this toy language. One statement may be a print statement containing an expression whose value is to be printed, an assign statement containing an identifier and an expression, or a compound statement containing two statements to be executed in sequence. How the data types for statements can be defined? How a function to execute an statement anotating its node with the corresponding state (memory plus output) after its execution can be defined? Without annotations the type of statements could be: data Stm = PrintStm Expr | AssignStm String Expr | CompoundStm Stm Stm How to enable annotations in this case? Note that Stm uses both Expr and Stm. [1] http://hackage.haskell.org/package/Annotations Romildo

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Automatic discovery of tests
by G 27 Apr '12

27 Apr '12

Hi, trying to learn haskell and the associated build env. I come from a java/scala world so trying to understand the how to invoke tests. Have a cabal file that invokes a main function to execute a test. But wondering if there is any automatic discovery of tests as per junit. I have seen test-framework-th but to me that implies putting a testGroupGenerator into each module. Am i correct about that and if so is there an example of calling multiple modules tests ? Or is there a better way just to invoke specific functions prefixed with case_ prop_ etc in the entire src/test directory of the cabal build ? Thanks

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Re: [Haskell-cafe] [Haskell] ANN: Portackage - a hackage portal
by Andrew Seniuk 27 Apr '12

27 Apr '12

Excuse the delay, I have no home internet. > On Fri, Apr 27, 2012 at 1:16 AM, <briand(a)aracnet.com> wrote: > Great idea. Maybe the format can start as compactly as possible, > i.e. 1 entry per line to make it easy to browse as many entries > as possible in one page. > > It looks as though disabling just the dependencies would go a > long way to accomplish that. Yep, and in fact the version I uploaded while reading your message has the deps column elided (and one or two others). Enforcing single-line rows is a but a few tweaks away. The thing is, the module namespace was the main motivator for this project, so I'm also seeing whether I can get away with a single view, package-oriented as presently, and still give good focus to the modules names. Soon I hope to include "accordion" views of the module namespace, with cookie-based (?) persistent state so user's collapse/expand tree state is preserved. > I'd like to vote to make the synopsis to be the second column. > > will each column type have an enable/disable ? I like the idea (although that one wasn't in the upload). Hopefully when I explore the JavaScript UI tools a bit more, the columns (key structure) will be totally configurable by drag-and-drop, with columns sizable, and keys selected from a listbox. (?) stuff like that > it's probably best if a click on a specific target, say the actual > name of the package, sends you to the hackage entry instead of a > click anywhere on the row. I find myself clicking on the page > by accident and it sending me off to some random hackage entry. :) That's a feature! But seriously, you're right, and as a bonus the links can be specific to the columns (package -> hackage, modules -> API pages, homepages -> hps, maintainer -> sendto [1]). (A 2-finger or Mod- click anywhere on a row might still be nice, but a bare tap is admittedly way too accident prone with touch input.) > I really like the idea - makes your hackage surfing > much more enjoyable ! > > Brian And thank-you for sharing your thoughts! I'm committed to improving it, until the ideas run out. > On Fri, Apr 27, 2012 at 4:12 AM, Christoph Breitkopf <chbreitkopf(a)googlemail.com> wrote: > this already looks great. So here's my wish-list, in case you have > too much time on yours hands ;-) Thanks! Yes I suppose I do have too much time on my hands, but the pain of that is conterbalanced by all the excitement. > Quality/popularity indicators (I'm aware that some of these are > real work and better provided by hackage itself): > - used by how many other packages (number of edges to that package > in the dependency graph of all packages) > - haddock coverage, test coverage, etc. > - like, +1, whatnot button > - link to issue tracker, if there is one If there's interest, this could become integrated with hackage at some point in the future. Next thing is to get a "last upload" field which can replace the version field. The codependency graph :) would be a nice metric to add, I'll look into it. I remember seeing a package that does this someplace on hackage. The +1 button is a good one! In short I'll try to implement all these ideas, thanks to you both for the encouraging comments. -Andrew [1] I omitted the maintainer column, thinking its inclusion might be harmful, making the emails so easy for a bot to collect.

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ANN: Portackage - a hackage portal
by Andrew Seniuk 27 Apr '12

27 Apr '12

Hello, This is to announce a provisional webpage to summarise hackage.haskell.org. http://www.fremissant.net/portackage It is a lot of data, so will be a bit slow to load. Columns can be sorted by clicking the headers, but this will also take some seconds on a typical client. The initial sort is by module name. Clicking a table row links to the project page on hackage. Although I believe this is a complete list of all packages/modules in hackage, it is probably still buggy, so if you see anything amiss please let me know. Also, a subsequent version in the near future will update automatically based on the http://hackage.haskell.org/packages/archive/recent.rss RSS feed, but for now it's based on a recent snapshot. Haskell code which prepares the page will be released after a few more days testing and refinement. Some previous work [1] provided a list of all modules, but is not being maintained, and I was unable to rearouse the author's interest. Feature requests and suggestions are welcome! Kind Reg'ds, Andrew Seniuk [1] http://folk.ntnu.no/hammar/explore-hackage/

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Re: [Haskell-cafe] [Haskell] ANN: Portackage - a hackage portal
by Andrew Seniuk 26 Apr '12

26 Apr '12

Jonathan, On Thu, Apr 26, 2012 at 4:13 PM, Jonathan Daugherty <jtd(a)galois.com> wrote: >> Although I believe this is a complete list of all packages/modules in >> hackage, it is probably still buggy, so if you see anything amiss >> please let me know. > > It might not be showing the most recent releases of everything. As an > example, it shows a much older version of my vty-ui library (1.0) and > its metadata, instead of the latest version, 1.5. As a result the > version, module list, description, homepage, etc., are not current. Thank-you, I'd realised this issue but forgot to address it. It is due to a false assumption on my part that the .cabal entries in the 00-index.tar file were in increasing version order. I'll address this problem today. Cheers, Andrew

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ANN: signed-multiset-0.1
by Stefan Holdermans 26 Apr '12

26 Apr '12

I am pleased to announce the first release of signed-multiset, which implements an abstract datatype for multisets with negative membership. The package can be obtained from http://hackage.haskell.org/package/signed-multiset-0.1 As always, feedback is welcome. Cheers, Stefan signed-multiset-0.1 ------------------- Multisets (or bags) are sets in which elements may occur more than once. The number of times an element occurs in a multiset is called its multiplicity. This package provides an efficient implementation of so-called signed multisets, which generalise multisets by allowing for negative membership. That is, elements in a signed multiset can have negative multiplicities. See also: Wayne D. Blizard. Negative membership. Notre Dame Journal of Formal Logic/, 31(3):346--368, 1990.

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JSON library suggestions?
by Jeff Shaw 26 Apr '12

26 Apr '12

Hello, Up until now I've been using Aeson, but I've found that its number type isn't going to work for me. I need to use decimal numbers while avoiding conversions from and to Double, which Aeson doesn't allow. There are quite a few more JSON libraries for Haskell, which all appear to use Rational for numbers, so I'm wondering if anyone can recommend one. Thanks, Jeff

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Haskell Weekly News: Issue 224
by Daniel Santa Cruz 26 Apr '12

26 Apr '12

Welcome to issue 224 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 15 to 21, 2012. Announcements Some of these announcements are a bit behind, for which I apologize. Better late than never... The long awaited version of cabal-install is out! [1] http://goo.gl/LlO7s Janis Voigtlander is looking for contributors for the May 2012 edition of the Haskell Comunities and Activities Report. If you have anything Haskell related that you have been working on, make sure to send Janis a note. The deadline is May 1st. [2] http://goo.gl/AW3gR Jeremy O'Donoghue announced the release of wxHaskell 0.90. "his release represents a significant milestone for us as it includes support for wxWidgets 2.9.x." [3] http://goo.gl/vTYTp Janis Voigtlander also issued a call for papers for the Haskell Symposium 2012, Copenhagen, Denmark on 13th September, 2012, directly after ICFP. [4] http://goo.gl/fR2hB Eric Kow released the 9th edition of the Parallel Haskell Digest. [5] http://goo.gl/gBIhK Quotes of the Week * TSC: The type families might have little type children * mm_freak: i'd love to have an embeddable Agda type checker and evaluator mm_freak: that would make a great scripting language for haskell programs * shachaf: You can't spell "-funsafe" without "fun" and "safe". * elliott: By the way, I find the inclusion of a package named "colour" in a section titled "Color" offensive :) * elliott: I don't think Haskell can ever hope to enter the enterprise if we can't model non-standard colourblind observers with it. * danharaj:oh hey this looks nice. Mathnerd314: every open-source math library under the sun glued together into a Python framework danharaj: nevermind o_o * elliott: What do we do with people who paste code blocks on IRC? We yell at them. * acowley: I am a bull in a math shop * elliott: I like how edwardk's answer to every question always goes "most abstract solution to the problem possible --> microoptimisation details". elliott: I swear I've seen him go from category theory to unboxed types and unsafeCoerce in a single line of IRC. * shachaf: It is a sobering thought that by the time Galois was my age, he had been dead for almost two months. Top Reddit Stories * Why do Monads Matter? (or it's Monad tutorial season again) Domain: cdsmith.wordpress.com, Score: 64, Comments: 27 On Reddit: [6] http://goo.gl/DCJYV Original: [7] http://goo.gl/8glhF * Ur/Web records: can we apply lessons from them to Haskell? Domain: blog.ezyang.com, Score: 53, Comments: On Reddit: [8] http://goo.gl/9m3Sw Original: [9] http://goo.gl/dNcET * Blank Canvas - a new Haskell graphics library based on HTML5 and <canvas> Domain: ittc.ku.edu, Score: 42, Comments: 10 On Reddit: [10] http://goo.gl/H3SMB Original: [11] http://goo.gl/if7kG * Client Side Yesod, an FRP-inspired approach Domain: yesodweb.com, Score: 41, Comments: 44 On Reddit: [12] http://goo.gl/gECTq Original: [13] http://goo.gl/Rn0LS * Understanding Haskell Kinds Domain: softwaresimply.blogspot.com, Score: 38, Comments: 18 On Reddit: [14] http://goo.gl/XxiwQ Original: [15] http://goo.gl/xaqDM * Adding SIMD Support to Data Parallel Haskell Domain: ghc-simd.blogspot.co.uk, Score: 35, Comments: 8 On Reddit: [16] http://goo.gl/R8M9L Original: [17] http://goo.gl/ZeJ7e * Working With HTML In Haskell Domain: adit.io, Score: 31, Comments: 24 On Reddit: [18] http://goo.gl/4NMBc Original: [19] http://goo.gl/EOvM1 * Descending the level ladder Domain: monoidal.blogspot.com, Score: 26, Comments: 13 On Reddit: [20] http://goo.gl/9IYZL Original: [21] http://goo.gl/TJm0D * Why is Haskell so large? Domain: self.haskell, Score: 25, Comments: 29 On Reddit: [22] http://goo.gl/KQnbj Original: [23] http://goo.gl/KQnbj * Parallel Haskell Digest 9: GSoC, STM, SIMD, and the Downfall of Imperative Programming? Domain: well-typed.com, Score: 25, Comments: 1 On Reddit: [24] http://goo.gl/Z5bF3 Original: [25] http://goo.gl/eLnVs * blaze-html 0.5 released Domain: jaspervdj.be, Score: 25, Comments: 3 On Reddit: [26] http://goo.gl/7XtMD Original: [27] http://goo.gl/RkbR7 * Four Tips for New Haskell Programmers Domain: softwaresimply.blogspot.com, Score: 22, Comments: 72 On Reddit: [28] http://goo.gl/GQ4O1 Original: [29] http://goo.gl/Ye57L * Check for interest - an argument/option parser using type level strings Domain: self.haskell, Score: 22, Comments: 13 On Reddit: [30] http://goo.gl/eOtE5 Original: [31] http://goo.gl/eOtE5 * New Cabal feature: cabal bench Domain: blog.johantibell.com, Score: 22, Comments: 3 On Reddit: [32] http://goo.gl/ClkjJ Original: [33] http://goo.gl/sA3aD Top StackOverflow Questions * The Pause monad votes: 20, answers: 5 Read on SO: [34] http://goo.gl/DNIo1 * To what extent are macros “functions in reverse?” votes: 15, answers: 6 Read on SO: [35] http://goo.gl/ZoEht * Curry-Howard isomorphism votes: 13, answers: 3 Read on SO: [36] http://goo.gl/1pzKd * How does Djinn work? votes: 12, answers: 3 Read on SO: [37] http://goo.gl/RnEny * How is co-recursion handled? votes: 12, answers: 2 Read on SO: [38] http://goo.gl/DXtj1 * Distinction between typeclasses MonadPlus, Alternative, and Monoid? votes: 11, answers: 1 Read on SO: [39] http://goo.gl/4LZd7 * Why can't Existential Types use record syntax? votes: 11, answers: 2 Read on SO: [40] http://goo.gl/4Gn96 * Given a Haskell type signature, is it possible to generate the code automatically? votes: 11, answers: 4 Read on SO: [41] http://goo.gl/7Xff3 * Is it possible to force a type to a class in Haskell? votes: 10, answers: 6 Read on SO: [42] http://goo.gl/SeFpb * Haskell concurrency - is forkIO really nondeterministic? votes: 10, answers: 2 Read on SO: [43] http://goo.gl/z2q9Y * Where to find programming exercises for applicative functors? votes: 9, answers: 4 Read on SO: [44] http://goo.gl/CXOYU Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19239 2. http://www.haskell.org/pipermail/haskell/2012-April/023244.html 3. http://permalink.gmane.org/gmane.comp.lang.haskell.wxhaskell.general/1178 4. http://permalink.gmane.org/gmane.comp.science.types.announce/2949 5. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/97851 6. http://cdsmith.wordpress.com/2012/04/18/why-do-monads-matter/ 7. http://www.reddit.com/r/haskell/comments/sfvbu/why_do_monads_matter_or_its_… 8. http://blog.ezyang.com/2012/04/how-urweb-records-work-and-what-it-might-mea… 9. http://www.reddit.com/r/haskell/comments/sjoh5/urweb_records_can_we_apply_l… 10. http://ittc.ku.edu/csdl/fpg/Tools/BlankCanvas 11. http://www.reddit.com/r/haskell/comments/seppr/blank_canvas_a_new_haskell_g… 12. http://www.yesodweb.com/blog/2012/04/client-side 13. http://www.reddit.com/r/haskell/comments/sm72n/client_side_yesod_an_frpinsp… 14. http://softwaresimply.blogspot.com/2012/04/less-travelled-monad-tutorial-pa… 15. http://www.reddit.com/r/haskell/comments/scagx/understanding_haskell_kinds/ 16. http://ghc-simd.blogspot.co.uk/2012/04/adding-simd-support-to-data-parallel… 17. http://www.reddit.com/r/haskell/comments/sfxck/adding_simd_support_to_data_… 18. http://adit.io/posts/2012-04-14-working_with_HTML_in_haskell.html 19. http://www.reddit.com/r/haskell/comments/sbxhb/working_with_html_in_haskell/ 20. http://monoidal.blogspot.com/2012/04/descending-level-ladder.html 21. http://www.reddit.com/r/haskell/comments/scq1q/descending_the_level_ladder/ 22. http://www.reddit.com/r/haskell/comments/sb80y/why_is_haskell_so_large/ 23. http://www.reddit.com/r/haskell/comments/sb80y/why_is_haskell_so_large/ 24. http://www.well-typed.com/blog/65 25. http://www.reddit.com/r/haskell/comments/sjbgv/parallel_haskell_digest_9_gs… 26. http://jaspervdj.be/posts/2012-04-21-blaze-html-0.5.html 27. http://www.reddit.com/r/haskell/comments/sl6ps/blazehtml_05_released/ 28. http://softwaresimply.blogspot.com/2012/04/four-tips-for-new-haskell-progra… 29. http://www.reddit.com/r/haskell/comments/sarkz/four_tips_for_new_haskell_pr… 30. http://www.reddit.com/r/haskell/comments/sbpmz/check_for_interest_an_argume… 31. http://www.reddit.com/r/haskell/comments/sbpmz/check_for_interest_an_argume… 32. http://blog.johantibell.com/2012/04/cabal-bench.html 33. http://www.reddit.com/r/haskell/comments/sfhv3/new_cabal_feature_cabal_benc… 34. http://stackoverflow.com/questions/10236953/the-pause-monad 35. http://stackoverflow.com/questions/10249110/to-what-extent-are-macros-funct… 36. http://stackoverflow.com/questions/10212660/curry-howard-isomorphism 37. http://stackoverflow.com/questions/10217931/how-does-djinn-work 38. http://stackoverflow.com/questions/10257326/how-is-co-recursion-handled 39. http://stackoverflow.com/questions/10167879/distinction-between-typeclasses… 40. http://stackoverflow.com/questions/10192663/why-cant-existential-types-use-… 41. http://stackoverflow.com/questions/10205793/given-a-haskell-type-signature-… 42. http://stackoverflow.com/questions/10211664/is-it-possible-to-force-a-type-… 43. http://stackoverflow.com/questions/10247555/haskell-concurrency-is-forkio-r… 44. http://stackoverflow.com/questions/10239630/where-to-find-programming-exerc…

1 0

"desactivate" my Show instance implementations temporarily
by TP 25 Apr '12

25 Apr '12

Hello, I have a module where I have made several types as instances of the Show typeclass. For debugging purpose, I would like to use the default implementation for Show (the one obtained when using "deriving", which shows all the constructors). Is there some option to do that, or have I to comment all the Show instances of my code, and add "Show" in "deriving (...)" for each of my types? If this is the only possibility, is there some script around here to do that automatically? Thanks in advance, TP

5 6

Multi-site haddock documentation with proper links?
by Ryan Newton 25 Apr '12

25 Apr '12

Hello cafe, For various reasons, some packages don't build documentation on hackage: http://hackage.haskell.org/package/accelerate Therefore I want to locally install documentation for a set of packages like this and host them on a separate website. I want all of these ~ten packages' haddock documentation to be properly interlinked with eachother, but also to link to Hackage for types and classes defined in other modules. Is this possible? Hackage haddocks are all interlinked, but that is simply because hackage is one giant local install, right? If it's not possible (and it seems not) do any haddock devs have pointers on how to implement this? Thanks, -Ryan P.S. Someone recommended to me the following simple hack -- just use sed to rewrite the links after haddock generates the html. I think I'll do that for the time being unless someone has a better suggestion.

2 3

atomicModifyMutVar and casMutVar
by Thomas Bereknyei 25 Apr '12

25 Apr '12

I have seen some push to take advantage of the new primop casMutVar, yet I see that atomicModifyMutVar hasn't been used yet either. For example, modifySTRef is just a read followed by a write. I've been unable to create a version that does not allocate with something as simple as modifySTRef ref (+1). There is a package Data.Ref.Unboxed that allows ST to use an unboxed STURef, but even though I can see the Core using Int#, any use of modifySTURef, or any version I've been able to make will excessively box/rebox. Should I instead be using something like modifySTURef ref (GHC.Prim.+#) or can this be automatically done by the compiler?

1 0

static linking with ghc?
by Johannes Waldmann 25 Apr '12

25 Apr '12

Hi. I want to produce a statically linked executable. I am trying 'ghc --make -fforce-recomp -static -optl-static Main' but it gives lots of errors like (.text+0xfa): undefined reference to `pthread_mutex_unlock' collect2: ld returned 1 exit status A similar thing is mentioned here (see Caveat) http://www.haskell.org/haskellwiki/Web/Literature/Static_linking The ghc user guide talks a great length about shared libs http://www.haskell.org/ghc/docs/latest/html/users_guide/using-shared-libs.h… but I don't see anything on how to switch this off. - J.W.

4 4

I am having trouble with the type declaration for creating an identity matrix.
by KC 25 Apr '12

25 Apr '12

initIdentityMat :: Int -> ST s (STUArray s (Int,Int) ((Int, Int), Double)) initIdentityMat m = newListArray ((1,m),(1,m)) ([((i,j), if i == j then 1.0 else 0.0) | i <- [1..m], j <- [1..m]] :: [((Int,Int), Double)]) Doesn't seem to compile, nor do minor variations of the type declaration. -- -- Regards, KC

3 3

Is protocol-buffers package maintainer reachable?
by Paul Graphov 25 Apr '12

25 Apr '12

Hello Cafe! I am using protocol-buffers and hprotoc packages but they fail to compile with recent GHC due to trivial errors. Hackage names Christopher Edward Kuklewicz as their maintainer. I've sent him patches more than a month ago but neiter they were applied nor I got any response. It's quite inconvenient to keep patched versions all the time. Does anybody know if he is still interested in maintaining those packages? Is it possible to contact him? And what should I do if he is unreachable? Thanks! Links: http://hackage.haskell.org/package/protocol-buffers http://hackage.haskell.org/package/hprotoc

8 8

Offer to mirror Hackage
by Dan Knapp 24 Apr '12

24 Apr '12

With Hackage down, now seemed like a good time to push this issue again. It's such an important site to us that it's really rather a shame there are no mirrors of it. I have a personal-and-business server in a data center in Newark, with a fair chunk of bandwidth, which I'd like to offer for a permanent mirror. Is there interest in this? Who do I need to talk to for it to happen? Strategy-wise, I think the best approach is round-robin DNS, since that's transparent to the end user - everything would still appear at the URL it's at now, but behind-the-scenes magic would let things keep working when one or the other site is down. I haven't personally set up such a system before but I'm willing to take on the burden of figuring it out. So I have a better idea of what I'm signing up for, can anyone tell me how much disk space and how much bandwidth per month Hackage uses? I have a fair chunk of both, as I say, but I'd like to know in advance to ensure that things go smoothly. As for what I'd want in return for this, really nothing. I wouldn't say no to an unobtrusive mention somewhere on the site, but I'd be happy just knowing I'd given something back to the Haskell community, which has given a lot to me. -- Dan Knapp "An infallible method of conciliating a tiger is to allow oneself to be devoured." (Konrad Adenauer)

21 53

SOE code on Mac OS X Lion
by N. Raghavendra 24 Apr '12

24 Apr '12

I am trying to use code from the book `The Haskell School of Expression' on an iMac with Mac OS X Lion. I have installed the latest version of Haskell Platform (2011.4.0.0 64bit). I then followed step 1 on the Web page http://www.cs.yale.edu/homes/hudak/SOE/software1.htm, and did `cabal install GLFW'. There were no errors in the installation. Regarding step 2 in the instructions, I have downloaded SOE-20120121.zip, but don't know how to proceed further. The Web page says, "Note for OS X users: running graphics applications from GHCi is no longer supported. Instead, one has to compile a graphics program using GHC in order to run it (see example/GMIExamples.lhs for an example)." Also, SOE/src/Code.html says, "For MacOS X, unfortunately SOE cannot be run from under GHCi interactively due to problems with the Graphics library it uses. SOE programs must be compiled in order to run OS X. This means you have to export the main function from a particular module (by adding it to the module export list) and compile with command line: ghc -main-is=...." I am only trying to learn Haskell from SOE, and don't understand how to compile the SOE software with GHC. What precisely is the command that I should invoke in the Terminal for the compilation? I'd appreciate any help on compiling and using the SOE code. Thanks and regards, Raghavendra. -- N. Raghavendra <raghu(a)mri.ernet.in> | http://www.retrotexts.net/ Harish-Chandra Research Institute | http://www.mri.ernet.in/

1 0

Announce: Hackager - new version of hackage-test
by David Terei 23 Apr '12

23 Apr '12

Hi all, I've updated the old hackage-test tool and renamed to hackager. http://hackage.haskell.org/package/hackager Hackager is a tool to automate the compiling of all packages on Hackage. It builds each package on hackage in isolation and records the results. The purpose being to catch regressions caused by changes to GHC (and Cabal although this was not the motivation). Two runs of Hackager can be compared, so the first run is done with a known version of GHC and the next run with a new, experimental version of GHC... ect. The improvements to Hackager over hackage-test are: * Parallelized the build process. Can now specify how many packages to build in parallel, which cuts total run time down greatly (e.g 2 days -> 5 hours) * hackage-test and hackage-report are now one tool, 'hackager' that works as a mutli-command tool. * Proper option handling * Fixed some stability issues The new homepage for development can be found here: https://github.com/dterei/Hackager Cheers, David

2 1

The last statement in a 'do' construct must be an expression: a <- readArray arr (1, 1)
by KC 23 Apr '12

23 Apr '12

I'm getting the above error message and I cannot figure out why? buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC

2 2

HaL-7, 2012-07-13, Call for submissions
by Henning Thielemann 23 Apr '12

23 Apr '12

Call for submissions and Save the date for our local Haskell Workshop in Halle/Saale, Germany. Tutorials, talks, demonstrations ... everything welcome. Workshop language is German (mainly), and English (by request). Submission deadline: May, 21, Workshop date: July, 13 Workshop homepage: http://iba-cg.de/hal7.html The complete call in German: ------------------------------------------------- Aufruf zum Einreichen von Beiträgen und Hinweis zum Vormerken des Termins Was: Haskell-Treffen HaL-7 Wann: Freitag, 13.07.2012 Wo: Institut für Informatik an der Martin-Luther-Universität in Halle an der Saale Wir suchen Vorträge zu Haskell im Besonderen und der funktionalen Programmierung im Allgemeinen, zum Beispiel zu den Themen * Neues von Sprache, Bibliotheken, Werkzeugen, * Anwendungen von Kunst bis Industrie, * Lehre an Schulen und Hochschulen, gerne aber auch zu anderen Themen. Die Beiträge können präsentiert werden als * Tutorium (60 .. 90 min) * Vortrag (30 min) * Demonstration, künstlerische Aufführung Die Veranstaltungssprache ist Deutsch, in begründeten Ausnahmen Englisch. Presentations will be given in German but we can switch to English if requested. Bitte reichen Sie Kurzfassungen der Beiträge ein (2 bis 4 Seiten), die dem Programmkomitee eine Einschätzung ermöglichen. Die Kurzfassung soll mit einer Zusammenfassung (10 Zeilen) beginnen und einem Literaturverzeichnis enden. Teilnehmer des Workshops sind Interessenten (keine Erfahrung mit Haskell/FP), Anfänger (wenig Erfahrung) und Experten. Wir bitten die Vortragenden, die Zielgruppe des Beitrags anzugeben und die nötigen Vorkenntnisse zu beschreiben. Bei Tutorien sollen Teilnehmer auf eigenen Rechnern arbeiten. Bitte beschreiben Sie dazu die vorher zu installierende Software. Schicken Sie Beitragsvorschläge als PDF-Dokument bis zum 21.05.2012 per Mail an hal-committee at iba-cg punkt de oder an ein Mitglied des Programmkomitees. Programmkomitee * Henning Thielemann - Univ. Halle (Vorsitzender), * Petra Hofstedt - BTU Cottbus, * Alf Richter - iba CG Leipzig, * Uwe Schmidt - FH Wedel, * Janis Voigtländer - Univ. Bonn, * Johannes Waldmann - HTWK Leipzig. Mit besten Grüßen Henning Thielemann

1 0

(no subject)
by KC 23 Apr '12

23 Apr '12

buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC

1 0

Is it only one data structure per ST monad?
by KC 23 Apr '12

23 Apr '12

Is it only one data structure per ST monad? -- -- Regards, KC

3 2

Need feedback on my EDSL attempt for writing test scripts
by C K Kashyap 23 Apr '12

23 Apr '12

Dear cafe, Recently, I decided to use Haskell to drive the testing of a C++ DLL that we develop. After getting the FFI etc working, I thought it might be a good idea to expose an EDSL for the testers to alter the flow of invocations of the functions in the DLL. I've tried to demonstrate the approach I am contemplating here - http://hpaste.org/67495 The outcome is that I'll have a set of "Instructions" for the testers can arrange to create a sequence of calls. > data Command = Void | Init | Get3Numbers Int Int Int| GetName String | PrintName | PrintSum | Close | PrintMessage String > deriving (Show) > mainloop :: StateT (ScriptState Command) IO () > mainloop = do > liftIO $ putStrLn "Hello World" > executeCommand Init > executeCommand Init > executeCommand $ PrintMessage "Enter name" > executeCommand $ GetName "abcd" > return () I'd like to ensure that some level of validation done. For example, if Init is called twice, the tester should get to know about it. Similarly, GeName should not be called unless PrintMessage has been called. > executeCommand :: Command -> StateT (ScriptState Command) IO () > executeCommand Init = do > (ScriptState c) <- get > case c of > Void -> liftIO $ putStrLn (show c) > _ -> liftIO $ putStrLn "Init already called" > put (ScriptState Init) > return () > executeCommand (GetName x) = do > (ScriptState c) <- get > case c of > PrintMessage _ -> do { str <- liftIO $ getLine; put (ScriptState (GetName str)); return ()} > _ -> liftIO $ putStrLn "PrintMessage not called" > executeCommand (PrintMessage m) = do > liftIO $ putStrLn m > put (ScriptState (PrintMessage m)) I'd appreciate it very much if you could give me some feedback on my approach. I get this feeling that I am wrapping up the whole program inside a State monad - does this mean that I am giving up the functional goodies. Regards, Kashyap

1 0

No email verification token from trac.haskell.org
by Niklas Broberg 23 Apr '12

23 Apr '12

Hi all, I have problems with my trac for haskell-src-exts. When I log in, I am asked to verify my email address, but no verification token is ever sent to my email address. And before I verify my address, I'm not allowed to do anything on the trac. Help? :-) /Niklas

2 2

ANNOUNCE: hsx-0.10.1, hjscript-0.6.0, hsp-0.7.0
by Niklas Broberg 23 Apr '12

23 Apr '12

Hi all, I'm pleased to announce the release of the latest versions of the libraries in the Haskell Server Pages suite. Haskell Server Pages enables literal XML syntax to be used mixed with Haskell code (like e.g. PHP), in a sane way (unlike said PHP). In principle the functionality could be used together with any Haskell web framework, but is (currently) best integrated in and supported by Happstack. For this release I owe much to Jeremy Shaw, both for inspiration and for actual patches. There are some grand plans in the pipeline for some more extensive changes to the suite, to shape it up to be worthy of the current exciting state-of-the-art of Haskell web development. Stay tuned! Cheers, /Niklas Specific changes in this release: * In the XMLGen class, the following associated types have been renamed to avoid frequent name clashes, and to better match the intention of those types: XML => XMLType, Child => ChildType, Attribute => AttributeType. * Literal CDATA strings are now given an explicit type signature in the post-processed code. This to avoid ambiguities when OverloadedStrings is enabled. * class XMLGenerator m now has EmbedAsChild m () as another premise. * HSX.Transform now exports a method transformExp :: Exp -> Exp, intended to be used with quasi-quotation. * trhsx no longer performs fixity resolution before transforming, allowing the tool to be used on files using operators with (locally) unknown fixity. * hsp and hjscript updated to work with hsx-0.10.

1 0

Data-Type Serialization
by Lyndon Maydwell 23 Apr '12

23 Apr '12

Hi Café. I'm pondering over an old chestnut that I'd forgotten about until recently. I'd like to be able to express schemas for general recursive datatypes (excluding functions and possibly encoding bounds) and have an interactive constructor for that datatype generated automatically. Originally this idea came about as an XML/JS framework for creating advanced form inputs on the client-side, however, I'm interested in how this could be implemented idiomatically in Haskell. I'm guessing that the best way to go about this would be to capture the declaration in template Haskell, having a passthrough for the declaration itself, and also generating a representation that can be used for introspection and the creation of the interactive component. Ideally I'd like to integrate this concept into one of the web-frameworks such as Yesod so that complicated data-types can be constructed interactively on the client-side using (generated) javascript in order to get instantaneous feedback about validity of fields, etc, and avoid page reloads and even ajax communication. The details of the web-side of things would probably be quite messy, but if the Haskell side can be made general enough then this can be considered a separate problem :-) Has anyone had experience with trying to solve this, or a similar issue? What approach did you take? Did you use Template-Haskell, or something else? Thanks!

2 1

Munich Haskell Meeting
by Heinrich Hördegen 23 Apr '12

23 Apr '12

Dear all, we will meet again for our monthly Haskell Meeting in Munich on the 26th of April, at 19h30 at Cafe Puck. If you plan to join, please go to the site an click the button in order to reserve enough tables: http://www.haskell-munich.de/dates Also, consider joining us for our local Hackathon which will be held on Saturday, the 12th of May. See the details on the site. Until then, I wish all of you a good time, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

1 0

Re: [Haskell-cafe] default instance for IsString
by Markus Läll 22 Apr '12

22 Apr '12

Isn't this a special case already? I.e you already can default a string literal to anything you want, the only thing needed is a language pragma (and more obvious docs). The only "problem" is, that it's not part of the Haskell2010 report, and thus not supported in other compilers (or is it?). On Sun, Apr 22, 2012 at 8:51 PM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > On Sun, Apr 22, 2012 at 10:37 AM, Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: >> I do not think this is a bug. Since type classes are open, GHC does >> not do any reasoning of the form "X is the only instance in scope, so >> I will pick that one". Other instances could be added at any time >> (perhaps in other modules). In this particular instance, GHC has no >> reason to choose the Text instance other than the fact that it is the >> only instance in scope -- that is, type inference is not enough to >> determine that the Text instance should be chosen. >> >> However, I do agree that it would be nice to have a mechanism for >> specifying default instances for arbitrary (user-defined) type >> classes. > > Couldn't we make a special case for IsString, like we do for Num, > given it's special syntactic association with OverloadedStrings? > > -- Johan > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users -- Markus Läll

1 0

Re: [Haskell-cafe] default instance for IsString
by Markus Läll 22 Apr '12

22 Apr '12

Hi Greg You *can* have what you want -- enable extended defaulting and set Text as default by > {-# LANGUAGE ExtendedDefaultRules #-} > default (Integer, Double, T.Text) -- keep the default defaults At the moment you only by chance have one instance of NoDefault in scope, which is why it shouldn't work at the moment. (ExtendedDefaultRules is required because the standard defaulting rules apply only when defaulting numeric literals.) On Sun, Apr 22, 2012 at 7:55 AM, Greg Weber <greg(a)gregweber.info> wrote: > This is a better demonstration of the issue. I am going to open a GHC > bug report, as I can't see how this behavior is desirable. > > > {-# LANGUAGE OverloadedStrings #-} > import Data.Text as T > > class NoDefault a where noDefault :: a -> Text > instance NoDefault T.Text where noDefault = id > > main = print (noDefault "Hello!") > > default.hs:7:15: > Ambiguous type variable `a0' in the constraints: > (NoDefault a0) arising from a use of `noDefault' > at default.hs:7:15-23 > (Data.String.IsString a0) arising from the literal `"Hello!"' > at default.hs:7:25-32 > Probable fix: add a type signature that fixes these type variable(s) > In the first argument of `print', namely `(noDefault "Hello!")' > In the expression: print (noDefault "Hello!") > In an equation for `main': main = print (noDefault "Hello!") > > > On Sat, Apr 21, 2012 at 7:51 PM, Greg Weber <greg(a)gregweber.info> wrote: >> my actual use case looks more like this: >> >> {-# LANGUAGE OverloadedStrings #-} >> {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} >> >> import Data.Text as T >> >> class ShowT a where >> showT :: a -> String >> >> instance ShowT T.Text where >> showT = show >> >> instance ShowT String where >> showT = show >> >> main = print (showT "Hello!") >> >> Ambiguous type variable `a0' in the constraints: >> (ShowT a0) arising from a use of `showT' at default.hs:16:15-19 >> (Data.String.IsString a0) arising from the literal `"Hello!"' >> >> >> So I actually want to define a default instance for a typeclass I >> define that uses isString instances. >> >> >> >> On Sat, Apr 21, 2012 at 6:24 PM, Daniel Peebles <pumpkingod(a)gmail.com> wrote: >>> I think it'll be hard to do that without putting Text in base, which I'm not >>> sure anyone wants to do. >>> >>> Dan >>> >>> On Sat, Apr 21, 2012 at 8:20 PM, Greg Weber <greg(a)gregweber.info> wrote: >>>> >>>> I would like to default IsString to use the Text instance to avoid >>>> ambiguous type errors. >>>> I see defaulting capability is available for Num. Is there any way to >>>> do this for IsString? >>>> >>>> Thanks, >>>> Greg Weber >>>> >>>> _______________________________________________ >>>> Glasgow-haskell-users mailing list >>>> Glasgow-haskell-users(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >>> >>> > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users -- Markus Läll

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http-enumerator: users?
by Michael Snoyman 22 Apr '12

22 Apr '12

Hi all, I'm wondering if there are still active users out there of http-enumerator. Four months ago I released http-conduit, and since then, my development efforts have gone almost exclusively there. At this point, I'm not longer actively using http-enumerator on any of my projects, and while I've backported any security fixes to http-enumerator, it is otherwise not be updated. I see three possibilities for the future of http-enumerator: 1. Deprecate it in favor of http-conduit 2. Someone else takes over as maintainer 3. Turn it into a frontend for http-conduit, exposing an enumerator interface. This will likely, though not necessarily, result in API breakage I'm curious how others feel on this. If I don't hear anything back on the topic, I'll assume that no one is interested in http-enumerator, and thus will opt for option (1). Michael

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Call for Papers - Haskell Symposium 2012 - six weeks to go
by Janis Voigtländer 21 Apr '12

21 Apr '12

======================================================================== "Haskell 2012" ACM SIGPLAN Haskell Symposium 2012 Copenhagen, Denmark 13th September, 2012 (directly after ICFP) CALL FOR PAPERS http://www.haskell.org/haskell-symposium/2012/ ======================================================================== The ACM SIGPLAN Haskell Symposium 2012 will be co-located with the 2012 International Conference on Functional Programming (ICFP), in Copenhagen, Denmark. The purpose of the Haskell Symposium is to discuss experiences with Haskell and future developments for the language. Topics of interest include, but are not limited to: * Language Design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal treatments of the semantics of the present language or future extensions, type systems, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management as well as foreign function and component interfaces; * Tools, in the form of profilers, tracers, debuggers, pre-processors, testing tools, and suchlike; * Applications, using Haskell for scientific and symbolic computing, database, multimedia, telecom and web applications, and so forth; * Functional Pearls, being elegant, instructive examples of using Haskell; * Experience Reports, general practice and experience with Haskell, e.g., in an education or industry context. Papers in the latter three categories need not necessarily report original research results; they may instead, for example, report practical experience that will be useful to others, reusable programming idioms, or elegant new ways of approaching a problem. (Links with more advice appear on the symposium web page.) The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a program! Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work (also for other languages where appropriate). In addition, we solicit proposals for system demonstrations, based on running (perhaps prototype) software rather than necessarily on novel research results. Such short demo proposals should explain why a demonstration would be of interest to the Haskell community. Travel Support: =============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC programme, see its web page (http://www.sigplan.org/PAC.htm) Proceedings: ============ There will be formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. Authors must transfer copyright to ACM upon acceptance (for government work, to the extent transferable), but retain various rights (http://www.acm.org/publications/policies/copyright_policy) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Accepted demo proposals, assessed for relevance by the PC, will be published on the symposium web page, but not formally published in the proceedings. Submission Details: =================== * Abstract Submission: Thu 31st May 2012 * Submission Deadline: Sun 3rd June 2012, 11:00 am, UTC * Author Notification: Wed 27th June 2012 * Final Papers Due : Tue 10th July 2012 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (9pt format, more details appear on the symposium web page). The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 pages. Each paper submission must adhere to SIGPLAN's republication policy, as explained on the web. Demo proposals are limited to 2-page abstracts, in the same format. "Functional Pearls", "Experience Reports", and "Demo Proposals" should be marked as such with those words in the title at time of submission. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. Submission is via EasyChair: https://www.easychair.org/conferences/?conf=haskell2012 Programme Committee: ==================== * Amal Ahmed, Northeastern University * Jost Berthold, University of Copenhagen * Nils Anders Danielsson, University of Gothenburg * Iavor Diatchki, Galois Inc. * Jeremy Gibbons, University of Oxford * Jurriaan Hage, Utrecht University * Zhenjiang Hu, National Institute of Informatics Tokyo * Daan Leijen, Microsoft Research * Ben Lippmeier, University of New South Wales * Simon Peyton Jones, Microsoft Research * Colin Runciman, University of York * Eijiro Sumii, Tohoku University * Janis Voigtländer (chair), University of Bonn * Brent Yorgey, University of Pennsylvania Links: ====== * http://www.haskell.org/haskell-symposium (the permanent web page of the Haskell Symposium) * http://www.haskell.org/haskell-symposium/2012 (the 2012 Haskell Symposium web page) * http://www.icfpconference.org/icfp2012 (the ICFP 2012 web page) -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Problem Installing ad (Automatic Differentiation) Package
by Dominic Steinitz 21 Apr '12

21 Apr '12

Hi, I was trying to install ad (Automatic Differentiation) but got the following error. I guess the answer is to upgrade my ghc (perhaps someone could confirm) but I am using the Haskell Platform and would rather stick with that. I see from http://hackage.haskell.org/platform/ that the next release is due out in May. Will that be early May? in which case I may well wait a week or will it be late May? in which case I might bite the bullet and upgrade my ghc. Thanks, Dominic Building template-haskell-2.7.0.0... [1 of 7] Compiling Language.Haskell.TH.Syntax.Internals ( Language/Haskell/TH/Syntax/Internals.hs, dist/build/Language/Haskell/TH/Syntax/Internals.o ) [2 of 7] Compiling Language.Haskell.TH.Syntax ( Language/Haskell/TH/Syntax.hs, dist/build/Language/Haskell/TH/Syntax.o ) [3 of 7] Compiling Language.Haskell.TH.PprLib ( Language/Haskell/TH/PprLib.hs, dist/build/Language/Haskell/TH/PprLib.o ) Language/Haskell/TH/PprLib.hs:55:10: Illegal instance declaration for `Show Doc' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `Show Doc' Updating documentation index /Users/Test/Library/Haskell/doc/index.html cabal: Error: some packages failed to install: ad-1.3.0.1 depends on template-haskell-2.7.0.0 which failed to install. template-haskell-2.7.0.0 failed during the building phase. The exception was: ExitFailure 1 The Glorious Glasgow Haskell Compilation System, version 7.0.3

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Parameterize constraints of existentially quantified types
by Bas van Dijk 21 Apr '12

21 Apr '12

Hi, I just found out that with the new ConstraintKinds extension we can parameterize the constraint of an existentially quantified type: {-# LANGUAGE KindSignatures, ConstraintKinds, ExistentialQuantification #-} import GHC.Exts data Some (c :: * -> Constraint) = forall a. c a => Some a This could be used to define SomeException for example: import Control.Exception (Exception) type SomeException = Some Exception Are there any other use cases? Bas

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[ANN] blaze-html-0.5
by Jasper Van der Jeugt 21 Apr '12

21 Apr '12

Hello all, I've just released a new version of blaze-html on Hackage. It has some backward-incompatible changes, so feel free to contact me if you run into any trouble. Summary of changes: - Split into blaze-markup and blaze-html - Easy creation of custom HTML elements - Very simple HTML tree manipulation For more details, see: http://jaspervdj.be/posts/2012-04-21-blaze-html-0.5.html Cheers, Jasper

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Google Summer of Code - Lock-free data structures
by Florian Hartwig 21 Apr '12

21 Apr '12

Hi everyone, I would like to do the GSoC project outlined in http://hackage.haskell.org/trac/summer-of-code/ticket/1608 One of Haskell's great strengths is its support for all kinds of concurrent and parallel programmming, so I think that the Haskell ecosystem would benefit from having a wider range of efficient concurrent data structures. Also, more selfishly, I remember reading the article in CACM last summer and thinking that the whole idea of updating data structures directly using atomic compare-and-swap was really cool, so I would love to have an excuse to play around with it. Some (initial) ideas for lock-free data structures worth implementing: 1. A lock-free priority queue, e.g. using the approach based on skip-lists explained in [2] 2. Stacks, see [1] and [3] 3. Hash tables [4] but if anyone has other suggestions, I would obviously happy to hear them. GSoC stretches over 13 weeks. I would estimate that implementing a data structure, writing tests, benchmarks, documentation etc. should not take more than 3 weeks (it is supposed to be full-time work, after all), which means that I could implement 4 of them in the time available and still have some slack. About me: My name is Florian Hartwig, I'm a fifth year (Master's) student in Computing Science at the University of Glasgow. I've been using Haskell for a bit more than two years now (both for university courses and my recreational programming) and I'm currently using it for my Master's project, so I won't have to spend any time learning the Haskell basics. I don't have any other plans for the summer that might conflict with the project. I'd be thankful for any thoughts, questions and suggestions. Cheers, Florian [1] http://cacm.acm.org/magazines/2011/3/105308-data-structures-in-the-multicor… [2] http://www.sciencedirect.com/science/article/pii/S0743731504002333 [3] http://dl.acm.org/citation.cfm?id=1007944 [4] http://dl.acm.org/citation.cfm?id=564881

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TLS support for haskell-xmpp
by John Goerzen 20 Apr '12

20 Apr '12

Dmitry & others, Attached is a diff implementing TLS support for haskell-xmpp, and correcting a build failure. The support is rough but it seems to work so far. -- John

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Fwd: Now Accepting Applications for Mentoring Organizations for GSoC 2012
by Johan Tibell 20 Apr '12

20 Apr '12

Hi all, Anyone interested in acting as an admin for haskell.org this year? I'm afraid I won't have time. It's not that much work (filling in some information, sending out some emails, making sure things happen in time.) ---------- Forwarded message ---------- From: Carol Smith <carols(a)google.com> Date: Mon, Feb 27, 2012 at 11:47 AM Subject: Now Accepting Applications for Mentoring Organizations for GSoC 2012 To: Google Summer of Code Announce < google-summer-of-code-announce(a)googlegroups.com> Hi all, We're pleased to announce the applications for mentoring organizations for GoogleSummer of Code 2012 are now being accepted [1]. Please go Melange [2] to apply on behalf of your organization. Please note that the application period [3] closes on 9 March at 23:00 UTC. We will not accept any late applications for any reason. [1] - http://google-opensource.blogspot.com/2012/02/mentoring-organization-applic… [2] - http://www.google-melange.com [3] - http://www.google-melange.com/gsoc/events/google/gsoc2012 Cheers, Carol -- You received this message because you are subscribed to the Google Groups "Google Summer of Code Announce" group. To post to this group, send email to google-summer-of-code-announce(a)googlegroups.com. To unsubscribe from this group, send email to google-summer-of-code-announce+unsubscribe(a)googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-summer-of-code-announce?hl=en.

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