April 2008 - Haskell-Cafe

C++ interface with Haskell
by Miguel Lordelo 20 Apr '08

20 Apr '08

Hi all, Well...somehow I'm a beginner in Haskell. But actually my interest in Haskell will increase if it is possible to call a haskell function in C++. Something like GreenCard ( http://www.haskell.org/greencard/ ) simplifying the task of interfacing Haskell programs to external libraries (usually). But is there also a task to interface a foreign language with Haskell, but calling Haskell functions. Or c2hs which is an interface generator that simplifies the development of Haskell bindings to C libraries. I want to know this, because in my company some guys are doing some testing with Frotran and MatLab and I want to show them the power of haskell and the software which we are using is implemented in C++ (there is the reason to make Haskel -> C++). I read somewhere that the only way for C++ calling a haskell function is to create a binding between Haskell and C and from C to C++, but a easy "Hello World" example was not there. Unfortunatelly I couldn't found anything usefull, like an complete example, or how to compile the code from haskell to C to C++. Can sombody help me, please :P Chears, Miguel Lordelo.

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Problems with "Bananas"
by Creighton Hogg 20 Apr '08

20 Apr '08

Hi, This isn't about Haskell per se, but I was reading the old Meijer et al. paper "Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire" & I think there's a notational pun that's really confusing me. On page 12 we have the CataEval equation (|phi|) . in = phi . (|phi|)_L Now, the subscript L & the following example of cons lists implies that L is a functor in this equation, yet the line immediately after this equation says that "(CataEval) states how to evaluate an application of (|phi|) to an arbitrary element of L...." so then that makes it sound like the L here is the fixed point of some functor F, not the functor itself. I'm sure I'm just being dumb, but this is really bugging me. Thanks, Creighton Hogg

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Strange message from GHC
by Chris Smith 19 Apr '08

19 Apr '08

I'm running into this in some code I wrote. What does it mean? It says to look at -fspec-constr-count, but that flag doesn't seem to be in the GHC documentation. This isn't critical; the code still seems to work fine. It just makes the build uglier. Thanks. Message below. SpecConstr: too many specialisations for one function (see -fspec-constr- count): Function: main:Expr.simplifyTerm{v rqC} [lid] Specialisations: [([sc_s2wb{v} [lid], sc_s2wc{v} [lid], sc_s2wd{v} [lid], sc_s2we{v} [lid], sc_s2wf{v} [lid], sc_s2wg{v} [lid]], [main:Expr.Graph{v r51} [gid] sc_s2wg{v} [lid] sc_s2wf{v} [lid] sc_s2we{v} [lid] sc_s2wd{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Edge{v r4Q} [gid] sc_s2wc{v} [lid]) sc_s2wb{v} [lid]]), ([sc_s2wh{v} [lid], sc_s2wi{v} [lid], sc_s2wj{v} [lid], sc_s2wk{v} [lid], sc_s2wl{v} [lid], sc_s2wm{v} [lid]], [main:Expr.Graph{v r51} [gid] sc_s2wm{v} [lid] sc_s2wl{v} [lid] sc_s2wk{v} [lid] sc_s2wj{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Ghost{v r4P} [gid] sc_s2wi{v} [lid]) sc_s2wh{v} [lid]]), ([sc_s2wR{v} [lid], sc_s2wS{v} [lid], sc_s2wT{v} [lid]], [sc_s2wT{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Vertex{v r4R} [gid] sc_s2wS{v} [lid]) sc_s2wR{v} [lid]]), ([sc_s2wU{v} [lid], sc_s2wV{v} [lid], sc_s2wW{v} [lid]], [sc_s2wW{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Vertex{v r4R} [gid] (base:GHC.Base.I#{(w) v 6d} [gid] sc_s2wV{v} [lid])) sc_s2wU{v} [lid]])]

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"publishing" a Haskell package
by Galchin, Vasili 19 Apr '08

19 Apr '08

Hello, In the past someone sent me a URL on how to formally "publish" Haskell work for public review and consumption. Does anyone have his link? Regards, Vasili

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Re. intro to FDs - thanks!
by Alexis Hazell 19 Apr '08

19 Apr '08

Thanks to all who helped me with my query about functional dependencies - much appreciated. And special thanks to Bulat for reminding me that the GHC user guide is more than just a user guide. :-) Alexis.

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HTTP and file upload
by Adam Smyczek 19 Apr '08

19 Apr '08

Is form based file upload supported in HTTP module (HTTP-3001.0.4)? Adam

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RFC: A standardized interface between web servers and applications or frameworks (ala WSGI)
by Johan Tibell 18 Apr '08

18 Apr '08

Good day hackers, The Python community have been successful in standardizing an interface between web server and applications or frameworks resulting in users having more control over their web stack by being able to pick frameworks independently from web servers, and vice versa. I propose we try to do the same for Haskell. I've written half a draft for a Haskell version of Python's PEP 333 [1]. If you're interested in taking part in this effort please read through the Python spec first (as it is way more complete and you can understand this proposal better by reading it, I've skipped some important issues in my first draft) and then go read the Haskell spec [2]. I'm particularly interesting in feedback regarding: * Doing in this way won't work as it violates HTTP/CGI spec part X, Y and Z (the Python spec takes lots of things from the CGI spec including naming and semantics). * My server/framework could never provide/be run under this interface. * This interface has bad performance by design. * Using a different set of data types would work better. The spec needs to be extended to cover all the corners of HTTP. Some parts need to be motivated better. It is easier for me to motivate things if people would tell me what parts are badly motivated. Note: I'm open to a complete rewrite if needed. I'm not wedded to the current design and/or wording. In fact parts of the wording is borrowed from the Python spec. The parts with bad grammar are all mine. 1. http://www.python.org/dev/peps/pep-0333/ 2. http://www.haskell.org/haskellwiki/WebApplicationInterface -- Johan

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[ANN] cabal-rpm 0.4
by Bryan O'Sullivan 18 Apr '08

18 Apr '08

I've just uploaded version 0.4 of cabal-rpm to Hackage. This is a program that generates an RPM package from a Cabal package. RPM is the package format used by several major Linux distributions. New in this version are support for GHC 6.8.2 and the Cabal 1.2 release series. Download: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-rpm-0.4 Source: darcs get http://darcs.serpentine.com/cabal-rpm <b

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ANNOUNCE: cpuid 0.2 - Binding for the cpuid machine instruction
by Martin Grabmueller 18 Apr '08

18 Apr '08

Hello fellow Haskellers, I have just uploaded my new package cpuid to Hackage. Description: This module provides the function 'cpuid' for accessing information about the currently running IA-32 processor. Both a function for calling the 'cpuid' instruction directly, and some convenience functions for common uses are provided. This package is only portable to IA-32 machines. Home page: http://uebb.cs.tu-berlin.de/~magr/projects/cpuid/doc/ Hackage page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cpuid-0.2 I'm looking forward to comments, improvements and bug fixes. I hope it's useful to you! Happy Haskell Hacking, Martin

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GC'ing file handles and other resources
by Conal Elliott 18 Apr '08

18 Apr '08

Are Haskell folks satisfied with the practical necessity of imperatively & explicitly reclaiming resources such as file handles, fonts & brushes, video memory chunks, etc? Doesn't explicit freeing of these resources have the same modularity and correctness problems as explicit freeing of system memory (C/C++ programming)? I wrote a lovely purely functional graphics library that used video memory to lazily compute and cache infinite-resolution images, and I found that I don't know how to get my finalizers to run anytime soon after video memory chunks become inaccessible. Explicit freeing isn't an option, since the interface is functional, not imperative (IO). I guess I'm wondering a few things: * Are Haskell programmers generally content with imperative and bug-friendly interfaces involving explicit freeing/closing of resources? * Do people assume that these resources (or handling them frugally) aren't useful in purely functional interfaces? * Are there fundamental reasons why GC algorithms cannot usefully apply to resources like video memory, file descriptors, etc? * Are there resource management techniques that have the flexibility, efficiency, and accuracy of GC that I could be using for these other resources? Thanks, - Conal 2008/4/14 Abhay Parvate <abhay.parvate(a)gmail.com>: > Hello, > > In describing the Handle type, the GHC documentation says (in the > System.IO documentation): > > GHC note: a Handle will be automatically closed when the garbage collector > detects that it has become unreferenced by the program. However, relying on > this behaviour is not generally recommended: the garbage collector is > unpredictable. If possible, use explicit an explicit hClose to close Handles > when they are no longer required. GHC does not currently attempt to free up > file descriptors when they have run out, it is your responsibility to ensure > that this doesn't happen. > > But one cannot call hClose on Handles on which something like hGetContents > has been called; it just terminates the character list at the point till > which it has already read. Further the manual says that hGetContents puts > the handle in the semi-closed state, and further, > > A semi-closed handle becomes closed: > > - if hClose is applied to it; > - if an I/O error occurs when reading an item from the handle; > - or once the entire contents of the handle has been read. > > So do I safely assume here, according to the third point above, that it's > fine if I do not call hClose explicitly as far as I am consuming all the > contents returned by hGetContents? > > Thanks, > Abhay > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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