- Haskell-Cafe - Haskell.org

Can't figure out source of race condition when using System.Process
by Rafal Kolanski 19 Nov '08

19 Nov '08

Greetings Gentlemen (and Ladies), As part of my small and simple framework for making presentations in Haskell I have a module which, given some text, makes a .tex file and then converts it to .svg for loading into a cairo canvas. It features simple caching (performs md5 on the contents of the .tex file and uses that as the file name... if an .svg by that name exists, it'll get loaded rather than regenerated). Unfortunately, it also segfaults once in a while, probably indicating I have some kind of race condition ... but I can't figure out why. This is the only point in my code that I think I'm using any concurrency, although I'm compiling with ghc --make -threaded. Compiling without -threaded results in a deadlock. Ubuntu 8.10, 64-bit, GHC 6.8.2. Any advice? I'm attaching the module in question (79 lines). Sincerely, Rafal Kolanski.

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Re: [Haskell-cafe] FFI and returning structs
by Donn Cave 19 Nov '08

19 Nov '08

Quoth Maurcio <briqueabraque(a)yahoo.com>: | I have not found examples of this in documentation | or hackage packages: how can I deal with funcions | returning not pointers to structs but structs | thenselves? | | struct example {... | | example function_name (... | | My intuition said I could try a data declaration | that is a properly done instance of Storable, and | then use that. | | data Example = ... | | instance Storable Example where ... | | But it seems I was wrong. Eh! probably not wrong - the problem is not how the data looks, but where it is. If it were my problem, I would try a wrapper function that returns the same value as an argument - e.g., if you have struct X f(), then void f_(struct X *x) { *x = f(); } ... so your wrapper will look like ccall "f_" f :: Ptr X -> IO () instead of ccall "f" f :: IO X Ironically, that's actually just what the original function is doing - cf. "struct return convention", where the caller allocates space and passes a pointer as a hidden parameter. But you can't just declare it that way, because the original f bumps the stack pointer on return (you may see "ret $4" in the assembly code, on Intel hardware) Try to compile the wrapper with the same compiler that compiled the original function. I'm really just guessing that your Haskell compiler doesn't understand or perfectly handle this particular C wart. If my suggestion doesn't work, I may have guessed wrong. Donn

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Pattern matching on numbers?
by Ryan Ingram 19 Nov '08

19 Nov '08

How does this work? > fac n = case n of > 0 -> 1 > _ -> n * fac (n-1) ghci> :t fac fac :: (Num t) => t -> t The first line of "fac" pattern matches on 0. So how does this work over any value of the Num typeclass? I know that the "1" on the rhs of fac are replaced with (fromInteger 1), but what about numeric literals in patterns? Does it turn into a call to (==)? Should whatever technique is used be extended to other typeclasses too? -- ryan

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Monadic bind with associated types + PHOAS?
by oleg＠okmij.org 19 Nov '08

19 Nov '08

Ryan Ingram wrote: > One thing that often comes up is a desire to do a pass on the > resultant code to optimize it, but it's pretty difficult with the > standard monadic formulation because of embedded functions. You can't > do introspection on functions in Haskell; they aren't elements of Eq > or Show. This has caused, for example, some implementations of FRP to > switch to using arrows instead. However, I find the arrow syntax > cumbersome and I feel it tends to obfuscate what is actually going on > in the code. > > An earlier talk at ICFP mentioned using PHOAS instead of HOAS to > encode expressions. In HOAS, a lambda expression in the target > language is represented by a function in the source language: If one uses a tagless final representation, one benefits from the conveniences of the higher-order abstract syntax and yet gains the ability to show terms, count the number of constructors and even partially evaluate them (i.e., optimize). For the example, please see http://okmij.org/ftp/tagless-final/Incope.hs (in particular, see Semantics2 for the partial evaluator that does not use GADT). We can easily add an interpreter that does a CPS transform, thus letting us embed a CBV object language in a Haskell without any use of monad or the change of syntax of the object language. We can easily extend the language to add state (as we have done in the OCaml part of the project). http://okmij.org/ftp/tagless-final/README.txt

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Haskell library support
by Galchin, Vasili 19 Nov '08

19 Nov '08

Hello, I am looking for something to work on. Where are there perceived holes in the Haskell library support? Regards, Vasili

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FFI and returning structs
by Maurício 19 Nov '08

19 Nov '08

Hi, I have not found examples of this in documentation or hackage packages: how can I deal with funcions returning not pointers to structs but structs thenselves? struct example {... example function_name (... My intuition said I could try a data declaration that is a properly done instance of Storable, and then use that. data Example = ... instance Storable Example where ... But it seems I was wrong. Do you have any tips on how to proceed? Thanks, Maurício

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How to make interface IO() to IO a in a command-driven tool
by Kyongho Min 18 Nov '08

18 Nov '08

Dear Haskell users, I am working on a command-driven tool (::IO ()). If the system invokes itself again, and the system's returned data type would be 'IO a', say (IO Exp) (Expression). tool:: a -> IO() The nested tool invocation returns (IO Exp) to the previous tool. I am using dynamic error exception handling like (throwDyn exp). If I use 'forkIO', then it works for this situation (e.g. IO Exp) but it is not working for 'quit' command (quit the tool:: thread blocked indefinitely) - quit the subtool and return to the previous tool. Any suggestion would be helpful? tool = {tool1, tool2, tool3, ...} tool1:: (IO ()) invoked tool2:: (IO a) may invoked tool3:: (IO a) tool2 is working basically on IO() and if I got the Exp (proved), then return it to the previous tool (say tool1). Thanks in advance, min

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clojure's data structures (was: Re: [Haskell-cafe] implementing python-style dictionary in Haskell)
by Evan Laforge 18 Nov '08

18 Nov '08

This actually brings up something I was wondering about lately. I recently stumbled across a language called clojure, which is a lisp-like that runs on the JVM. The interesting thing is that mutations go through a transactional mutable reference system, and the other datastructures are all immutable. The author implements an immutable hash map with a trie on each 5 bits of the hash, so each node potentially has 32 children. This means that lookups are effectively O(1) and alterations have to copy a maximum of 7 chunks of 32 pointers. Extendable "vectors" are implemented similarly. The hash tables sound basically like an IntMap on the hash code, except as far as I know, IntMap's patricia tree splits up the bit space adaptively instead of being hardcoded on 5 bit segments. I'm not sure what the performance difference would be. I haven't run any tests, but the clojure author claims his vector and hash map are quite fast. I suppose the extendable vector corresponds to the current crop of ByteString-like variants, so we sort of already have that, though in a kind of chaotic and inchoate way. I'm also curious about how far the generalized trie SoC project got... it should be more accessible now that we have a mainline release with ATs, right?

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Deploying a Binary Haskell Package
by John Van Enk 18 Nov '08

18 Nov '08

This question isn't directly related to Haskell, but I figure some one might know here. I want to deploy an application. I could either: 1) Tell people how to download GHC, have them check out the repository, have them install all the needed hackage packages, ... or 2) Give them a setup.msi/exe I'm curious if any one has done this with a Haskell package. It seems that it's something that might make sense being integrated into Cabal (runhaskell Setup msi). Either way, has some one deployed a Haskell binary as a MSI package? /jve

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Proof of a multi-threaded application
by Silviu ANDRICA 18 Nov '08

18 Nov '08

Hello, I am very new to Haskell, this is my first day, and I wanted to know if it is possible to prove correctness of a multi-threaded application written in Haskell. Basically, I want to check that a multi-threaded implementation of an algorithm that detects cycles in a dynamically changing graph, actually detects the cycles. Also, the algorithm prevents previously detected cycles from happening by not allowing certain edges to be added. And I want to check that this property also holds. Thank you and I hope you can help me, Silviu ANDRICA.

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