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Combining Network Descriptions in Reactive.Banana
by Alexander Foremny 10 Jul '12

10 Jul '12

Hello list, I am currently experimenting with Reactive.Banana and I am trying to write a program that simply displays a String which is supplied by the user. Thus in Main.hs I'd like to create a Behavior t String from some arbitrary sources and pass this to a library function performing the printing. However, I am not able to get the desired behavior of my program. A sample Main.hs is available as Gist [1]. The cause of error appears to be that the type t over which NetworkDescription and Behavior are parameterized are distinct! I tried several variations of the code in [1] using ExistentialQuantification over t and/or giving explicit type signatures. The error is slightly different in each case, but always boils down to the same reason. In case explicit error messages are required I can post them to the list. Any insight into this would be of great help. And additionally I'd be interested if this kind of program is possible with Reactive.Banana at all. [1] https://gist.github.com/3004430

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Linking against Sqlite3 on Windows problem
by Eugene Dzhurinsky 09 Jul '12

09 Jul '12

Hi all! I created simple application, which uses sqlite3 as it's datastore back-end. I faced no problems when building and running it on Linux, but after I tried to build it on Windows, I see weird linking error: Linking dist\build\hnotes\hnotes.exe ... C:\Documents and Settings\Admin\Application Data\cabal\sqlite-0.5.2.2\ghc-7.0.4/libHSsqlite-0.5.2.2. a(sqlite3-local.o):sqlite3-local.c:(.text+0x21): undefined reference to `sqlite3_temp_directory' C:\Documents and Settings\Admin\Application Data\cabal\sqlite-0.5.2.2\ghc-7.0.4/libHSsqlite-0.5.2.2. a(sqlite3-local.o):sqlite3-local.c:(.text+0x40): undefined reference to `sqlite3_temp_directory' collect2: v ld 1 cabal.EXE: Error: some packages failed to install: hnotes-0.1 failed during the building phase. The exception was: ExitFailure 1 What may be wrong there? I suspect that qalite3.dll has to be added to linking stage, but have no idea how to do that. Adding --extra-lib-dirs=path-to-sqlite-dll doesn't help either (perhaps because I need to update my cabal file somehow, to support this?). Thank you for the help! -- Eugene N Dzhurinsky

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ANNOUNCE: control-monad-loop 0.1
by Joey Adams 09 Jul '12

09 Jul '12

This package provides imperative-style loops supporting "continue" and "break". For example: import Control.Monad import Control.Monad.IO.Class import Control.Monad.Trans.Loop import Control.Monad.Trans.Class main :: IO () main = foreach [0..] $ \i -> foreach [0..] $ \j -> do when (j == 0) $ continue -- skip to next iteration when (j >= 5) $ exit -- exit the loop when (i >= 5) $ lift exit -- exit the outer loop by calling 'exit' in the parent monad liftIO $ print (i, j) It works by having the loop body run under the LoopT monad transformer, which provides early exit functions 'continue' and 'exit'. Functions like 'foreach' and 'while' run a LoopT callback, passing it continuations defining 'continue' and 'exit'. http://hackage.haskell.org/package/control-monad-loop

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ANNOUNCE: Sifflet visual programming language, release 2.0.0.0
by gdweber＠iue.edu 09 Jul '12

09 Jul '12

I don't usually get this excited about a release, but after nearly a year of not being able to do any work on Sifflet, I am now *Extremely Happy* to announce -- Sifflet and sifflet-lib 2.0.0.0, now available on Hackage! This version introduces a type checker and partial support for higher order functions in Sifflet, the visual, functional programming language and support system for students learning about recursion. Sifflet programmers define functions by drawing diagrams, and the Sifflet interpreter uses the diagrams to show how function calls are evaluated. Sifflet-lib is the library containing many modules of the Sifflet application. What's New ---------- July 5, 2012, Version 2.0.0.0: * Partial support for higher order functions, like map and filter. See Lesson 10 in the Sifflet Tutorial. * Sifflet now provides type checking and type inference. This should make it possible to provide exporters to languages, like Java, that require type declarations. (Unfortunately, the error messages for incorrectly typed functions are not yet friendly for novice programmers.) * Added a menu command (File / Save image ...) to save vector graphic images of functions in the Sifflet Workspace or Edit function windows. Images can be saved in SVG, Postscript, and PDF formats. * Reorganized hierarchical modules in the library to conform to the recommended practice. See the RELEASE-NOTES for details. * Several bugs are fixed, including one which crashed Sifflet when applying a function definition with an incomplete `if` tree. See ISSUES for details. * There is a new file format for saving function definitions to support higher-order function types. The new format, siffml 2.0, has a RELAX NG schema; siffml 1.0 files can still be opened in Sifflet 2.0. About Sifflet ------------- Sifflet is a visual, functional programming language intended as an aid for learning about recursion. * A picture explains Sifflet better than words: please see the screenshot showing how to evaluate 3!: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Features: - Visual editor. - Visual tracer/debugger which shows how recursive and other function calls are evaluated. To support active learning and avoid screen clutter, Sifflet displays only as much of the computation as the user requests. - Carefully crafted tutorial with 44 pictures, about 26 pages if printed. - Number, string, and list data types. - Small collection of primitive functions. - Runnable examples of compound functions. - Sifflet functions can be exported to Scheme, Python 3, and Haskell. References ---------- * Download: http://hackage.haskell.org/package/sifflet-lib http://hackage.haskell.org/package/sifflet * Home page: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Sifflet Tutorial: http://mypage.iu.edu/~gdweber/software/sifflet/doc/tutorial.html * RELEASE-NOTES: http://mypage.iu.edu/~gdweber/software/sifflet/RELEASE-NOTES * ISSUES: http://mypage.iu.edu/~gdweber/software/sifflet/ISSUES ------------------------------------------------------------------------ -- Gregory D. Weber, Ph. D. : Associate Professor of Informatics / \ Indiana University East 0 : Tel. (765) 973-8420; FAX (765) 973-8550 / \ http://mypage.iu.edu/~gdweber/ 1 []

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combining predicates, noob question
by Sebastián Krynski 09 Jul '12

09 Jul '12

As I was using predicates (a -> bool) , it appeared the need for combining them with a boolean operator (bool -> bool -> bool) in order to get a new predicate combining the previous two. So I wrote my function combinerPred (see code below). While I think this is JUST ok, i'm feeling a monad in the air. So.. where is the monad? combinerPred :: (a -> Bool) -> (a -> Bool) -> (Bool -> Bool -> Bool) -> (a -> Bool) combinerPred pred1 pred2 op = \x -> op (pred1 x) (pred2 x) thanks Sebastián

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Best way to build a GHC backend?
by Jonathan Geddes 09 Jul '12

09 Jul '12

Venerable Haskell Hackers, I love Haskell and think it should run everywhere. Now supposing I would like to build another backend for GHC, perhaps for Java Bytecode, .Net CIL, or JavaScript, What would be the best way to approach that? I can think of a few options: 1. Produce External Core with -fext-core and compile that with a completely separate compiler 2. Use the GHC apis to build a compiler that reuses a load of GHC's code, but has it's own backend 3. Add a new backend directly into GHC Any other options? While I'm on the subject, why has Haskell not been ported to the likes of the JVM, .NET CLR, or JavaScript? Are Haskell's non-strict semantics just too different from the semantics of these other platforms? SPJ is known for saying that Haskell's plan for world domination is support for many parallelism/concurrency idioms. I believe running on many platforms is just as important. From my point of view, languages that cannot run on one of the 3 aforementioned platforms will become irrelevant. (with the exception of C, of course). Thoughts? --J Arthur

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Haskell Implementors' Workshop talk proposals due in one week!
by Johan Tibell 09 Jul '12

09 Jul '12

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ Important dates Proposal Deadline: 10th July 2012 Notification: 27th July 2012 Workshop: 14th September 2012 The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2012. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to: johan.tibell(a)gmail.com We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Lennart Augustsson (Standard Chartered Bank) * Manuel M T Chakravarty (University of New South Wales) * Gregory Collins - co-chair (Google) * Simon Marlow (Microsoft Research) * David Terei (Stanford University) * Johan Tibell - co-chair (Google)

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Re: [Haskell-cafe] sample terms and interpreting program output from Tc Monad
by Andres Löh 09 Jul '12

09 Jul '12

[Sorry, I forgot to reply to the list.] Hi. > I understand these to be Rank 0 terms: > > (\(x::Int) . x) (0 :: Int) :: (forall. Int) -- value > > (\(x::Int). x) :: (forall. Int -> Int) Yes. > (\(x::a). x) :: (forall. a -> a) > > Although the program prints forall, the absence of a type variable > indicates Rank 0, correct? It's a bit unclear what you mean, and the prototype implementation you seem to be using deviates from Haskell conventions here. The prototype checker sees "a" as a type constant here, so it plays the same role as "Int" before. In particular, "a" isn't a type variable, and there's no quantification. > I understand these to be Rank 1 terms: > > (\x. x) :: (forall a. a -> a) -- This is not the same as the third > example above, right? This one identifies the type variable a, the one > above does not. Also, there's no explicit annotation, it's inferred. The implementation indeed considers them to be different. Before "a" was a concrete type, and the function was the monomorphic identity function on that type. Now, "a" is a type variable, and the function is the polymorphic identity function. [...] > I understand these to be Rank 2 terms: > > (\(x::(forall a. a)). 0) :: (forall. (forall a. a) -> Int) > > The explicit forall annotation on the bound and binding variable x > causes the program to infer a Rank 2 polytype as indicated by the "-> > Int" following the (forall a. a), while noting the absence of a type > variable following the left-most forall printed by the program, correct? Your description here is a bit strange. It's a rank-2 type because there's a rank-1 type occurring to the left of the function arrow. > > (\(x::(forall a. a -> a)). x) :: (forall b. (forall a. a -> a) -> b -> > b) > > Also Rank 2, only one arrow to the right of (forall a. a -> a) counts. It's rank 2, yes. I'm not sure what you mean here by saying that only one arrow counts. > The universal quantifier on type variable b ranges over the type > variable a, correct? No. The universal quantifier ranges over all (mono)types. > I understand this to be a Rank 3 term: > > (\(f::(forall a. a -> a)). \(x::(forall b. b)). f (f x)) :: (forall c. > (forall a. a -> a) -> (forall b. b) -> c) No, this is still rank 2. It uses two rank 1 types as function arguments. For it to be rank 3, it'd have to use a rank 2 type in the argument position of a function type. Note that the function arrow associates to the right: forall c. (forall a. a -> a) -> ((forall b. b) -> c) -- rank 2 (forall c. ((forall a. a -> a) -> (forall b. b)) -> c) -- rank 3 > The arrows to the right of the universally quantified a and b > expressions qualify this as Rank 3. No, you seem to be applying a wrong definition of rank. The correct definition is given in Section 3.1 of the paper. Here's how you can derive the type above as a sigma_2: sigma_2 ~> forall c. sigma_2 ~> forall c. sigma_1 -> sigma_2 ~> forall c. sigma_1 -> sigma_1 -> sigma_2 ~> forall c. sigma_1 -> sigma_1 -> sigma_1 ~> forall c. sigma_1 -> sigma_1 -> sigma_0 ~> forall c. sigma_1 -> sigma_1 -> c ~> forall c. (forall a. sigma_1) -> sigma_1 -> c ~> forall c. (forall a. sigma_0) -> sigma_1 -> c ~> forall c. (forall a. tau -> tau) -> sigma_1 -> c ~> forall c. (forall a. a -> a) -> sigma_1 -> c ~> forall c. (forall a. a -> a) -> (forall b. sigma_1) -> c ~> forall c. (forall a. a -> a) -> (forall b. sigma_0) -> c ~> forall c. (forall a. a -> a) -> (forall b. b) -> c > Type variable c ranges over type > variables a and b, correct? No. Each of the type variables ranges over all (mono)types. (Or do you mean what the scope of "c" is? If so, then the scope of c is the complete type signature.) Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

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Interest in typed relational algebra library?
by Paul Visschers 08 Jul '12

08 Jul '12

Hello, I've been out of the Haskell game for a bit, but now I'm back. A couple of years ago I made a small library that implements relational algebra with types so that malformed queries and other operations are caught at compile time. It is heavily based off of the internals of HaskellDB (see http://hackage.haskell.org/packages/archive/haskelldb/2.1.1/doc/html/Databa…) but types so that it can actually be used directly instead of having to use HaskellDB's query monad. Besides the joy of using relational algebra directly in your code, this also means that you can make query-optimizing code in a type-safe way, you can subquery results returned by the database directly without accessing the database again and you have more options when converting from relation algebra to SQL or another query language. The library isn't quite ready for release, but I might want to work on it a bit and then release it. Is anyone interested in such a library? Paul Visschers

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Haskell User Group Hamburg (Germany) - Meeting Thursday July 12th 2012
by Stefan Schmidt 08 Jul '12

08 Jul '12

Hello cafe, there's an upcomming meeting of the Haskell user group Hamburg next Thursday (July 12th). We'll start 19:00 at the Xing AG in Hamburg (Dammtorstraße 35, 20354 Hamburg ). Jan Ahrens will give a short talk about web development with Yesod ( http://www.yesodweb.com/) Although this announcement is written in english the talk will be given in german. If you are interested to join us, have a look at our doodle (also in german) http://www.doodle.com/5ekivcacuam9czxs Further announcements will be made via Twitter, so you may want to follow https://twitter.com/hug_hh.

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