- Haskell-Cafe - Haskell.org

The (!) operation
by Christopher Done 08 Mar '12

08 Mar '12

‘Ello. Is there a generalization of this operator? It's all over the place, it's basically (!) :: (Monad m, Indexed collection index value) => index -> container -> m value We have `(!!)` on lists, `(!)` on maps, vectors, json objects, … (doesn't seem there's one for bytestring) (Though I seem to recall the monadic return value being frowned upon but I don't recall why.) Thoughts? Ciao!

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GHCi fails to load C++ object files (missing symbol)
by Yves Parès 08 Mar '12

08 Mar '12

Hi, I'm trying to have GHCi load a haskell file that depends on a C++ object file, which causes GHCi to fail because of an unknown symbol (*and I did link with **libstdc++*), whereas the link into an executable with ghc works and runs perfectly. I've reduced my code to the smallest one that produces the problem: The C++ part defines a mere class and C externals that enable communication with Haskell: // Header Stuff.hpp class Base { public: Base(); ~Base(); } extern "C" { Base* mkthing(); void delthing(Base*); } ----------- // Source Stuff.cpp #include <iostream> #include "Stuff.hpp" using namespace std; Base::Base() { cout << "Base created" << endl; } Base::~Base() { cout << "Base deleted" << endl; } extern "C" { Base* mkthing() { return new Base(); } void delthing(Base* b) { delete b; } } Haskell code (just for reference, but I'm not sure it's relevant), Main.hs: {-# LANGUAGE ForeignFunctionInterface #-} import Foreign import Foreign.C foreign import ccall unsafe "mkthing" mkthing :: IO (Ptr ()) foreign import ccall unsafe "delthing" delthing :: Ptr () -> IO () main = do p <- mkthing delthing p I then compile it with: g++ -c Stuff.cpp ghci Main.hs Stuff.o -lstdc++ And then the error is: Loading object (static) Stuff.o ... done Loading object (dynamic) /usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5.2/libstdc++.so ... done final link ... ghc: Stuff.o: unknown symbol `__dso_handle' linking extra libraries/objects failed Whereas 'ghc Main.hs Stuff.o -lstdc++' works just fine. Does GHCi lacks a link directive that regular GHC has?

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Type classes for converting to Text and String
by Simon Hengel 08 Mar '12

08 Mar '12

Hi! When writing library code that should work with both String and Text I find my self repeatedly introducing classes like: class ToString a where toString :: a -> String class ToText a where toText :: a -> Text (I use this with newtype wrapped value types backed by Text or ByteString.) So I wonder whether it would be a good idea to have a package that provides those classes. Or maybe just ToText, and provide default implementations of toString and toText, like: class ToText a where toText :: a -> Text toText = Text.pack . toString toString :: a -> String toString = Text.unpack . toText How do you guys deal with that? Any thoughts? Cheers, Simon

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Monad.Reader #20 call for copy
by Edward Z. Yang 08 Mar '12

08 Mar '12

I'm pleased to announce that I, Edward Z. Yang, will be taking over Brent Yorgey's role as lead editor of the Monad Reader! Call for Copy: The Monad.Reader - Issue 20 -------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 20 will be: **Monday, March 5** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (ezyang(a)mit.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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Re: [Haskell-cafe] Summer of Code idea: Haskell Web Toolkit
by Greg Weber 07 Mar '12

07 Mar '12

This is obviously a great concept. However, it may not be appropriate for a GSoC. The design space is far too open, and it is not clear if anything in that space will end up beating plain old javascript. I think my proposal for an awesome websockets library [1] shows that this is putting the cart before the horse. For some of the potential javascript solutions, websockets is a dependency anyways. Without being able to hold open a connection to the server, required server interaction will be too slow. Yesod has been successful by explicitly avoiding any new radical paradigms, but by instead just trying to get the basics of web development established correctly. Without websockets, we don't have the basics down for fluid client-side interaction. The groundwork for this ticket has been laid: it should be easy to accomplish with time left over. So we are guaranteed a clear win for the community. With the time left over, a solution to the javascript problem can be attempted. But there is no burden for it to be solved within the summer. Next year the landscape will be more mature and we can try to come up with a solid proposal for the javscript problem if the community hasn't already created a solution. [1] http://hackage.haskell.org/trac/summer-of-code/ticket/1607

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Summer of Code idea: Haskell Web Toolkit
by Dimitry Golubovsky 07 Mar '12

07 Mar '12

Hi, Alejandro Serrano Mena wrote: > My idea is to make a client-side Haskell Web Toolkit, in the spirit of > Google Web Toolkit, which would allow to program in Haskell the client part > of a web application, and would complement the web frameworks already > existing for Haskell (such as Yesod and Snap). http://www.haskell.org/haskellwiki/Haskell_web_toolkit - In memoriam ;) Feel free to use these ideas. It would be nice if you could pick it up and complete - I never did... -- Dimitry Golubovsky Anywhere on the Web

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Re: [Haskell-cafe] [Haskell] Higher types in contexts
by Henning Thielemann 06 Mar '12

06 Mar '12

moved to haskell-cafe On Mon, 5 Mar 2012, Barney Hilken wrote: > Is there any deep reason why I can't write a polymorphic type in a context? I think the record update problem can be (sort of) solved if you could write: > > class Has r Rev (forall a. [a] -> [a]) => HRClass r where > setHRClass :: (forall a.[a] -> [a]) -> r -> r > > but polymorphic types are not allowed in contexts. Is this one of the problems SPJ considers "Hard" or is it a feasible extension? I don't know what you want to do, but you may wrap the (forall a. [a] -> [a]) in an existential type: data ListFunc = forall a. ListFunc ([a] -> [a]) class Has r Rev ListFunc => HRClass r where setHRClass :: ListFunc -> r -> r

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Re: [Haskell-cafe] Adding type annotations to an AST?
by oleg＠okmij.org 06 Mar '12

06 Mar '12

> How do I add type annotations to interior locations in an abstract > syntax tree?.... > i.e. the type it [algorithm] infers is the type of the whole > program, I would also like the types of any internal let-rec > definitions so I can label my AST. I had exactly the same problem: type reconstruction and the annotation of all sub-terms with their inferred types. Even if the overall type inference fails, the user can still see what the type checker was able to infer before the error. Here is the solution http://okmij.org/ftp/Computation/FLOLAC/TEvalNR.hs There is a bit of the explanation here: http://okmij.org/ftp/Computation/FLOLAC/lecture.pdf

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Double-dispatch
by Clark Gaebel 06 Mar '12

06 Mar '12

Is there any way in Haskell to have the correct function selected based on the types of two different types? For example, let's say I'm writing intersection tests: aABBandAABB :: AABB -> AABB -> Bool oBBandOBB :: OBB -> OBB -> Bool oBBandPoint :: OBB -> Point -> Bool Is there some way (such as with Type Families) that I can write some sort of generic method for this: intersects :: Intersectable -> Intersectable -> Bool which automatically selects the right function to call based on the types of the two intersectables? Regards, - clark

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Conduit Sink fork
by t helfferich 05 Mar '12

05 Mar '12

Hi! So, it turns out I have a need for a sink that forks input into two other sinks using the Conduit package. Here is what I came up with: https://gist.github.com/1975383 Is this the right way to write it? Also, what left over value(s) should I return in the Done constructor? I appreciate any help you can give me. Thanks so much,Grant

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