December 2007 - Glasgow-haskell-users

Re: [Haskell-cafe] Dynamic typing of polymorphic functions
by Alfonso Acosta 19 Dec '07

19 Dec '07

On Dec 19, 2007 9:13 PM, Neil Mitchell <ndmitchell(a)gmail.com> wrote: > > OK, If you managed to read until this point, you might have noticed > > that, due to the monomorphism restriction implied by Data.Typeable, it > > is impossible to build polymorphic processes. > > Tom Shackell had similar issues with passing code around at runtime. > As a result Yhc contains the Yhc.Dynamic module which gives you all > polymorphic dynamic typing. It's integrated into the compiler and > runtime system. Thanks for your answer Neil. >From your description it _seems_ to be exactly what I need (I coudn't reallly figure it out form the sources since its implementation seems to rely on Yhc internals I totally ignore). Are there any examples of use available? Even if Yhc.Dynamic turns out to be what I need, my implementation unfortunately depends on Template Haskell. Could any GHC guru point out if this would be feasible (and how complicated) to implement in GHC? Thanks, Fons

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Foreign C with pointers
by Luis Cabellos 19 Dec '07

19 Dec '07

Hi, I am creating a binding to an existing library (lib*.a) from Haskell. How can i bind a function that get a pointer? e.g: I have in the c library. int GetData( Data * d ); The steps that i need are: 1 - create the Data in Haskell 2 - create the foreign import sentence in a lib*.hs 3 - use the Data after the call Where can i get a good tutorial about FFI? I try http://www.haskell.org/hdirect/ffi.html but it's too general (need i pair of examples, i think) -- Thanks a lot, Luis Cabellos

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Haddock won't build on 6.8.1
by Deborah Goldsmith 17 Dec '07

17 Dec '07

I see this: $ runhaskell ./Setup.lhs build Preprocessing executables for haddock-0.8... shift/reduce conflicts: 5 Building haddock-0.8... src/Main.hs:49:7: Could not find module `System.Directory': it is a member of package directory-1.0.0.0, which is hidden Any ideas? Thanks, Deborah

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ForeignPtr performance
by Scott Dillard 17 Dec '07

17 Dec '07

Hi, I do a lot of programming with small compound objects, things like 3d vectors or RGBA tuples for images. For performance reasons, I keep them in StorableArrays, since there is no way to use these things with UArrays. (But that's a topic for another thread.) Some how the notion got into my head that StorableArrays were slow, I think from old versions of GHC. So I've gotten into the habit of using withStorableArray and peek/poke to work on my arrays. This, as you can imagine, is cumbersome. Not only because of the extra typing, but also because I have to do everything in the IO monad, even if the array is read-only. So I'm thinking about writing my own IArray instance using ForeignPtr for storage and unsafePerformIO for access. I did a little experiment, comparing > forM_ [0..lots] $ \i -> withForeignPtr fp $ \p -> pokeElemOff p i i to > withForeignPtr fp $ \p -> forM_ [0..lots] $ \i -> pokeElemOff p i i And there seemed to be no difference in running time, which is great. So I'm thinking of making unsafeAt something like > unsafeAt (MyArray fp) i = unsafePerformIO $ withForeignPtr fp $ \p -> peekElemOff p i My question is: what exactly does GHC.Prim.touch# do? This appears to be the critical function of withForeignPtr, and if I write my IArray instance as above, I have no way to float this touch# out of my loops, as I currently do using withStorableArray. It appears from my simple experiment that touch# does nothing at runtime. But maybe this is only because no memory is allocated inside my simple loop? Maybe if I write non-trivial loops, then touch# will actually cost something? Intensely curious, Scott

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Type Relations
by Paulo J. Matos 17 Dec '07

17 Dec '07

Hello all, I've came across a problem which would be best implemented if I could define what I think to be a Type Relation. Let's consider classes as being type properties. When you say: class Num a where (+) :: a -> a -> a ... it defines a property of the type a. Basically all types a of class Num are addable, etc.. However, how can I (if I can) represent the fact that you can coerce a type a to a type b? A particular example of that is the fromInteger function from Num class where you say that if your type a is of class Num, you must be able to coerce an Int to a. I would like to define a relation between types and then say that two types are in the relation if they define the coerce function. Something I imagine would be: typerelation Coercible a b where coerce :: a -> b And then you would define instances of Coercible if you could define the function coerce between the types. You could then define function only over types which can be coerced from one to the other. Any suggestions on how to do something like this? -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Performance bug maybe
by Scott Dillard 17 Dec '07

17 Dec '07

Hi, I think I found a performance bug. Either that, or something is going on that I don't understand. I've attached an example program. Here's my session with it: =============== ghc --make -O2 MaybeBug.hs; time ./MaybeBug +RTS -tstderr [1 of 1] Compiling Main ( MaybeBug.hs, MaybeBug.o ) Linking MaybeBug ... ./MaybeBug +RTS -tstderr -1452071552 191905792 <<ghc: 800037360 bytes, 4 GCs, 40960/40960 avg/max bytes residency (1 samples), 383M in use, 0.00 INIT (0.00 elapsed), 3.96 MUT (4.36 elapsed), 0.00 GC (0.00 elapsed) :ghc>> real 0m4.408s user 0m3.960s sys 0m0.448s ghc --make -O2 MaybeBug.hs -DSLOW -no-recomp; time ./MaybeBug +RTS -tstderr [1 of 1] Compiling Main ( MaybeBug.hs, MaybeBug.o ) Linking MaybeBug ... ./MaybeBug +RTS -tstderr -1452071552 191905792 <<ghc: 4000037408 bytes, 6106 GCs, 400588800/400588800 avg/max bytes residency (2 samples), 765M in use, 0.00 INIT (0.00 elapsed), 9.71 MUT (10.53 elapsed), 0.05 GC (0.08 elapsed) :ghc>> real 0m10.717s user 0m9.761s sys 0m0.876s ================== There are two functions, initArray1 and initArray2, that have exactly the same implementation. I use them to initialize two arrays. If I use initArray1 on the first, and initArray2 on the second, then things get inlined and specialized wonderfully and no heap is used. If I use initArray1 to initialize both arrays, then less inlining happens. Since I'm doing possibly dangerous things with unsafePerformIO, I'm inclined to think this is not a bug, but I'm still wondering what's going on. On that note: is this use of unsafePerformIO in fact dangerous? I'm aware that allocating memory inside unsafePerformIO is not wise, especially when that function is to be inlined. If the function body is duplicated at the call site, I might get two identical arrays where I could have one that is shared. I can live with that possibility, but I'm more worried about the allocation being floated out of the unsafePerformIO, and having unintended sharing. Can this happen, the way I've written it? The inlining seems to be crucial for performance, so that constructor specialization can kick in. If there's a better way to do truly polymorphic unboxed immutable arrays, I'm all ears. (I'm aware of Bulat's ArrayRef library, but it doesn't seem to be maintained.. is it?) Scott

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mingw64
by Bulat Ziganshin 14 Dec '07

14 Dec '07

Hello Glasgow-haskell-users, are you know about http://sourceforge.net/projects/mingw-w64 ? this project was started just a few months ago, but it already provides downloads to test. the win64 support is very important for my project so please look at it -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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upgrading visual studio integration to use 6.8.2....
by Nicholls, Mark 14 Dec '07

14 Dec '07

I've just downloaded the vs integration package....which looks great....but it comes with 6.6.1, and I was hoping to move to 6.8.2....does anyone know how to do this? Lead Broadcast and Corporate Architect Technology London (44) 0207 284 7596

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Hugs faster than GHC
by Bernd Brassel 14 Dec '07

14 Dec '07

The following program reveals a performance bug in ghc 6.6 module HCBenchmark where type M1 a = () type M2 a = M1 a type M3 a = M2 a type M4 a = M3 a type M5 a = M4 a type M a = M4 a -- use M5 for 10 times the compile time and M3 for 1/10 f :: M (M (M (M (M (M (M (M (M (M x))))))))) f = () Each element you add in the chain of type synonyms adds a factor of ten to the compile time. Hugs is "a bit" faster :o) Trouble is, the module I wanted to compile originally, takes an estimated 6 thousand and 600 hours to compile and I am not sure that this bug is the only obstacle for the compiler ... Could anyone cope if that is still a problem with the newest instance of ghc? Thanks a lot Bernd

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where's the best place to post those really irratating noddy questions that people who are learning Haskell ask.....
by Nicholls, Mark 14 Dec '07

14 Dec '07

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