June 2013 - Haskell-Cafe

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"

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ANN: HaTeX 3.6. Now with Babel, TikZ and more!
by Daniel Díaz Casanueva 06 Jul '13

06 Jul '13

Hello everyone! It is time for a new release of HaTeX, the Haskell LaTeX library. http://hackage.haskell.org/package/HaTeX-3.6 == What is HaTeX? == HaTeX contains numerous types and functions that will assist you in the process of creating a LaTeX document within Haskell. You can also include your manually created LaTeX code in HaTeX, so you can choose the more convenient tool you want to use for each case. == What's new in this release? == A lot of changes have been made since the last release! The most important changes are explained in the blog post I just wrote: http://deltadiaz.blogspot.com.es/2013/06/hatex-36-texy-class-babel-fontenc-… However, the new feature I am more excited with is the TikZ script generator. Using a very intuitive interface, you will be able to generate high quality graphics for LaTeX documents. It is still far from be complete, but next releases will fill the gaps that you may find right now. Just to give you a taste, consider the following code: myFigure :: Figure myFigure = Scale 2 $ Figures [ RectangleFilled (0,0) 1 1 , Colored Green $ RectangleFilled (-1,1) 1 1 , Colored Red $ RectangleFilled ( 0,2) 1 1 , Colored Blue $ RectangleFilled ( 1,1) 1 1 ] With this simple construction you already have the following picture ready to be inserted in your LaTeX file output. http://daniel-diaz.github.com/projects/hatex/tikzsimple.png In the examples directory contained in the source distribution there is also an example (tikz.hs) that includes the plotting of a Haskell function (sine). However, the interface it uses may be less comfortable than the one shown in the example above. If you are a HaTeX user, don't miss this update, because it also contains some very important bug fixes. Any feedback gladly accepted. Good luck, Daniel Díaz.

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ghci & ghc -> JS (Emscripten)
by B B 03 Jul '13

03 Jul '13

Hi! Does anybody tried, or is there anywhere a project, of online ghc or ghci (compiled to JavaScript with Emscripten)?

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Re: [Haskell-cafe] some questions about Template Haskell
by oleg＠okmij.org 02 Jul '13

02 Jul '13

TP wrote: > pr :: Name -> ExpQ > pr n = [| putStrLn $ (nameBase n) ++ " = " ++ show $(varE n) |] The example is indeed problematic. Let's consider a simpler one: > foo :: Int -> ExpQ > foo n = [|n + 1|] The function f, when applied to an Int (some bit pattern in a machine register), produces _code_. It helps to think of the code as a text string with the source code. That text string cannot include the binary value that is n. That binary value has to be converted to the numeric text string, and inserted in the code. That conversion is called `lifting' (or quoting). The function foo is accepted because Int is a liftable type, the instance of Lift. And Name isn't. BTW, the value from the heap of the running program inserted into the generated code is called `cross-stage persistent'. The constraint Lift is implicitly generated by TH when it comes across a cross-stage persistent identifier. You can read more about it at http://okmij.org/ftp/ML/MetaOCaml.html#CSP Incidentally, MetaOCaml would've accepted your example, for now. There are good reasons to make the behavior match that of Haskell.

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Upgrading GHC 7.6.3 global packages to latest versions
by Rouan van Dalen 02 Jul '13

02 Jul '13

Hi, I am trying to install the latest version of some hackage packages I use often in GHC 7.6.3. However, I am having some problems with packages like [containers] and [time], which are shipped with GHC as global packages. Now when I try to install the latest version of the [time] package, I have 2 time packages, 1 in the global package db (the older version), and 1 in the user package db (the newer version). Now I would like subsequent installed packages to always use the latest version of the [time] package. But if i try to install the [plugins] package, cabal says: #In order, the following would be installed: #ghc-paths-0.1.0.9 (new package) #random-1.0.1.1 (reinstall) changes: time-1.4.1 -> 1.4.0.1 #plugins-1.5.3.0 (new package) So I guess I have 2 options here: 1. unregister all the global packages so that I can install the latest versions of all the global packages. 2. force cabal to only look at latest packages in user package db (not sure if this is possible). Can anyone please point me in the right direction here. I have read about [cabal-dev], but I am not sure if it can be used to solve my problem. Regards --Rouan

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question about indentation conventions
by Richard Cobbe 01 Jul '13

01 Jul '13

I hope I'm not starting a holy war with this, but I'm curious about an aspect of coding style that's been bugging me for a while, and I'm not finding much discussion of this question on the web or in the mailing list archives. Two questions: 1) Are there wide-spread conventions in the Haskell community for how to indent an application expression that's split across multiple lines? For how to indent an expression that uses infix operators? Or does everyone pretty much do their own thing? 2) If there is such a convention, how do I make Emacs's haskell-mode do it? By default, in most cases Emacs's haskell-mode with turn-on-haskell-indentation does function firstArgument (second argument) thirdArgument Personally, I'd prefer some indentation on the 2nd and 3rd lines to indicate that they're continuing an expression begun on a previous line. I can use parens around the entire application to force haskell-mode to indent subsequent lines (and of course this is necessary in some contexts, like a 'do' expression), but haskell-mode only indents that by a single space: do (function firstArgument (second argument) thirdArgument) nextAction I'd find a larger indent---even just 2 spaces---to be more readable. My inclination to indent the second and following lines of a multi-line application expression is informed by my long experience in Scheme, Racket, and Lisp, whose S-expressions lend themselves to fairly straightforward (and automatable!) indentation conventions. If the Haskell community does things differently, though, I'm happy to adapt. This is the sort of thing that one picks up from the community, as in other languages. I don't, however, have a whole lot of contact with that community outside this list -- thus the post, despite the dangers inherent in discussing subjective stuff like this that people often feel strongly about. Thanks, Richard

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Re: [Haskell-cafe] Subclass or no subclass?
by Patrick Browne 01 Jul '13

01 Jul '13

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some questions about Template Haskell
by TP 29 Jun '13

29 Jun '13

Hi everybody, I am trying to learn Template Haskell, and I have two independent questions. 1/ First, the following code (which is not in its final version, but it is a test) does not compile: ------------------- {-# LANGUAGE TemplateHaskell #-} module Pr where import Language.Haskell.TH pr :: Name -> ExpQ pr n = [| putStrLn $ (nameBase n) ++ " = " ++ show $(varE n) |] ------------------- I obtain: ------------------- No instance for (Lift Name) arising from a use of `n' Possible fix: add an instance declaration for (Lift Name) In the first argument of `nameBase', namely `n' ------------------- Why? Indeed, there is no typeclass constraint on n in the definition of nameBase: ghci> :t nameBase nameBase :: Name -> String Contrary to lift for example: ghci> :t lift lift :: Lift t => t -> Q Exp 2/ If I define in a module: j = 3 and then define in another module: ------------------- h x = $([|j|]) main = do print $ h undefined ------------------- I obtain "3" as expected. However, I do not achieve to make this system work with an infix declaration: infix $([| j |]) + I obtain: parse error on input `$(' What is the problem? Thanks in advance, TP

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Subclass or no subclass?
by Patrick Browne 29 Jun '13

29 Jun '13

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Improving GHC's LLVM Backend
by Alex Ford 29 Jun '13

29 Jun '13

Hello, I'm interested in improving the LLVM backend of GHC by using the existing Haskell LLVM bindings to the C API, as suggested by option 1 in the LLVM FAQ: http://llvm.org/docs/FAQ.html#i-d-like-to-write-a-self-hosting-llvm-compile… At the moment, the backend uses the option 2: emitting LLVM assembly code which is then assembled to LLVM bitcode, optimised, and compiled to machine code by standard LLVM tools. Changing this to use the Haskell FFI bindings to the LLVM C API would allow better tracking of any changes to the LLVM IR, and hopefully a reasonable speed increase by avoiding the overhead of first emitting LLVM Assembly to be assembled. In his bachelor's thesis, David Terei considered this approach when initially writing the LLVM backend, but dismissed it due to the existing Haskell bindings being too large and high level to use with GHC. I have not personally looked at the high level Haskell bindings in much depth, but the low level (llvm-base) bindings appear to be somewhat incomplete with respect to LLVM 3.3. Therefore, I plan to extend the FFI bindings to cover more or all of the C API, and then to modify the GHC LLVM backend to use these bindings to call into the LLVM libraries directly. http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/Backends/LLVMc… a link to David's thesis, as well as documentation of the LLVM backend. I'd really appreciate it if anyone who knows about the Haskell LLVM bindings or about the GHC LLVM backend could give any advice regarding what sort of work needs to be done with the LLVM bindings to make them more appropriate for use within GHC, or how to approach modifying the existing LLVM backend. Many thanks, Alex

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