October 2005 - Haskell-Cafe

matrix computations based on the GSL
by Alberto Ruiz 21 Apr '06

21 Apr '06

Hello all! This is my first message to the list. In my work I often need linear algebra algorithms and other numeric computations. An option is using scientific computing systems like Matlab, Mathematica, Maple, etc. In Haskell there are several modules and bindings to matrix libraries; many of them are excellent. But I think that Haskell would be even more attractive if the basic matrix computations (what we find, for example, in the GNU Octave Quick Reference) were easily available in a standard library. We have all we need: an interactive environment, excellent graphics (HOpenGL), FFI and ForeignPtr with automatic garbage collection, QuickCheck, etc. Computations can be implemented by the GSL (GNU Scientific Library), a well designed numerical library written in C. Well... I know that I am trying to reinvent the wheel: I am working on a user friendly functional interface to matrix computations based on the GSL. I think that many numeric problems that can be solved using Octave or a similar system can be also solved more elegantly using Haskell. The library is very preliminary and incomplete but some simple problems can already be solved. The sources, haddock documentation and the draft of a tutorial can be found at: http://dis.um.es/~alberto/hmatrix/matrix.html I am not a Haskell expert, I only use the most basic programming techniques, so any suggestion will be greatly appreciated... Alberto

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RE: [Haskell-cafe] generics question, logical variables
by Ralf Lammel 03 Apr '06

03 Apr '06

Frederik, > As for your code example, it looks very interesting, but are you > saying that this could turn into an extension of the Data.Generics > library, or that this is something I could be implementing in terms of > what's already there? The posted code works with GHC 6.4 (SYB2) intentionally and actually. I have attached another attempt (again GHC 6.4, based on SYB2) which might be more useful for your purposes, and it may be useful in general, in fact. What I defined this time is a "certainty-improving" function: idify :: (Typeable1 f, Monad m, Data (a f), Data (a Id)) => (forall a. f a -> m a) -> a f -> m (a Id) That is, the function "idify get" takes a value whose type is parameterized in a type constructor f (such as Maybe or IORef), and the function attempts to establish Id instead of f on the basis of the function argument "get". > What is the 'a' parameter for in "force"? > > force :: ( Data (t Maybe a) > , Data (t Id a) > , Term t Maybe a > , Term t Id a > ) => t Maybe a -> t Id a The previous attempt was a more parameterized blow than required in your case. (I was guessing about what "typed logical variables" could mean. I was assuming that you would need some extra layer of embedding types on top of the Haskell term types. Looking at your code, this was not the case.) > For the part which I asked for help with, to get around my trouble > with generics, I defined a class GenFunctor and an example instance. > The intent is that generics should be able to provide this > functionality automatically later on, but you can see what the > functionality is. Let's look at the type of your GenFunctor: class GenFunctor f where gfmapM :: (Monad m, FunctorM b) => (forall x . a x -> m (b x)) -> f a -> m (f b) This type can be seen as a more relaxed version of the idify operation above. That is, idify fixes GenFunctor's b to Id. The particular encoding of idify (attached) takes advantage of this restriction. I wonder whether I should bother. (Exercise for reader :-)) > However, I am stuck on something else, the program doesn't typecheck > because of use of another function I defined, 'cast1'. Maybe you can > take a look. I had thought that I would be able to write a generic > 'unify' but I get the error: > > GenLogVar.hs:82:19: > Ambiguous type variable `a' in the constraint: > `Data a' arising from use of `cast1' at GenLogVar.hs:82:19-23 > Probable fix: add a type signature that fixes these type variable(s) > > This is because I need to do something special when I encounter a > "Var" variable in unification, but the compiler seems to not like the > fact that the type argument of the Var data type is not known. Please try to avoid new cast operations at all costs. :-) Your code can be arranged as follows: (i) Use extQ1 to dispatch to a special case for "Var x" for the first argument. (ii) In this special case, use again ext1Q to dispatch to a special case for "Var y" for the second argument. (iii) At this point, *cast* the variable value of *one* variable to the type of the other. So the problem with your code, as it stands, is that the target type of cast is ambiguous because you cast *both* arguments. The insight is to make the cast asymmetric. Then, not even polymorphism is in our way. Interesting stuff! Ralf

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Detecting Cycles in Datastructures
by Tom Hawkins 11 Dec '05

11 Dec '05

In a pure language, is it possible to detect cycles in recursive data structures? For example, is it possible to determine that "cyclic" has a loop? ... data Expr = Constant Int | Addition Expr Expr cyclic :: Expr cyclic = Addition (Constant 1) cyclic Or phased differently, is it possible to make "Expr" an instance of "Eq" such that cyclic == cyclic is smart enough to avoid a recursive decent? -Tom

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Threads talking back to parent
by Joel Reymont 04 Nov '05

04 Nov '05

Folks, I'm launching thousands of threads that send and receive commands. I would like the launching thread to keep a log of all the commands sent and received, in proper order. This would mean that each time a command is sent and received in the child thread it would need to be sent back to parent who would log the command on a first-come first-serve basis. How would I implement this in Haskell? Thanks, Joel -- http://wagerlabs.com/

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Computations that can time out + gathering feedback from threads
by Joel Reymont 31 Oct '05

31 Oct '05

Folks, I need to implement computations that can time out (connecting to a server, reading from socket, etc.), plus gather feedback from a few thousands of threads. What is the best way to implement this? Do I launch a thread to sleep and a thread to connect/read from socket and use something like Shared Transaction Memory to wait for either of the two to finish? Also, my "simple scripting engine" launches a few thousands of scripts that connect to a server, exchange commands and update their internal state. I need to wait for all these to finish and get the results of their execution. My scripts are a combination of monads and I run them with runXXX( ... ) from the IO monad so that I can inspect the resulting state. Is there an elegant solution for waiting for all the threads to complete and then figuring out how many of these are in good share according to their end state? Thanks, Joel -- http://wagerlabs.com/

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Value-weak hash tables for GHC?
by Juliusz Chroboczek 31 Oct '05

31 Oct '05

Hi, Does anyone have an implementation of value-weak hash tables for GHC he'd be willing to share? (Hash tables where an entry is removed when the value, not the key, is collected.) Since GHC has (usable) weak pointers, finalisers and hash tables, I could in principle roll my own, but I'd rather get an efficient implementation off someone competent. Oh, and it needs to be under a license that makes it suitable for inclusion in Darcs. Thanks a lot, Juliusz P.S. I am not subscribed to the list -- I'd be grateful if you could CC me with any replies.

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RE: [Haskell-cafe] Silly question
by Bayley, Alistair 31 Oct '05

31 Oct '05

> From: Joel Reymont [mailto:joelr1@gmail.com] > > I didn't know about "it" in ghci, thanks. Are there multiple levels > of it? > > I wanted z "a" :: Maybe Int which I understand now. > > Thanks, Joel What do you mean by multiple levels? "It" just stores the most recent expression/value. If you type "print it" then the result of the print becomes the most recent value e.g. *Foo> "boo" "boo" *Foo> :t it it :: [Char] *Foo> print it "boo" *Foo> :t it it :: () *Foo> print it () *Foo> The docs probably explain it better: http://www.haskell.org/ghc/docs/latest/html/users_guide/ch03s04.html#id31036 40 Alistair. ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] Silly question
by Bayley, Alistair 31 Oct '05

31 Oct '05

s> From: Joel Reymont [mailto:joelr1@gmail.com] > > How do I run z "a" from ghci or the IO monad? > > module Foo where > > import qualified Data.Map as M > import Control.Monad > import Control.Monad.List > import Prelude hiding ((!!)) > > (!!) :: (Ord k, MonadPlus m) => [M.Map k v] -> k -> m v > (!!) maps key = msum $ map (M.lookup key) maps > x :: M.Map String Int = M.empty > y :: M.Map String Int = M.fromList [("a", 1)] > z m = [x, y] !! m There are a couple of ways, depending on how you want to run it through MonadPlus, I think (although I don't understand your code much): main = do let a :: [Int]; a = z "a" print a b <- z "a" print b *Foo> main <<- running main from ghci [1] 1 Or directly the the ghci prompt: *Foo> z "a" *Foo> :t it it :: Int *Foo> print it 1 *Foo> z "a" :: [Int] [1] Did you know about the "it" variable in ghci? It holds the last expression typed at the prompt. Alistair. ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Silly question
by Joel Reymont 31 Oct '05

31 Oct '05

How do I run z "a" from ghci or the IO monad? module Foo where import qualified Data.Map as M import Control.Monad import Control.Monad.List import Prelude hiding ((!!)) (!!) :: (Ord k, MonadPlus m) => [M.Map k v] -> k -> m v (!!) maps key = msum $ map (M.lookup key) maps x :: M.Map String Int = M.empty y :: M.Map String Int = M.fromList [("a", 1)] z m = [x, y] !! m Thanks, Joel -- http://wagerlabs.com/

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RE: [Haskell-cafe] FFI and modifying haskell memory
by Simon Marlow 29 Oct '05

29 Oct '05

On 21 October 2005 14:40, Joel Reymont wrote: > I stumbled upon this when binding the Zlib library and want to make > sure I'm doing this right. > > foreign import ccall unsafe "uncompress" uncompress_ :: > Ptr Word8 -> Ptr CULong -> Ptr Word8 -> CULong -> IO CInt > > You give this function the address of your size as second argument > and it updates size for you. basic uncompress(char* buf, unsigned > long *size, ...) for you. > > Is it correct that I need to allocate a chunk of 4 bytes to be passed > into C and updated there and then need to peek my value from that > chunk of memory? yes, something like with $ \pculong -> r <- uncompress_ buf pculong ... size <- peek pculong > I thought StablePtr would give me the address of a Haskell value but > it gives me an offset into some table instead. That's right, the reason being that Haskell values move around when GC happens. Cheers, Simon

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