January 2009 - Haskell-Cafe

errno handling in concurrent haskell
by Manlio Perillo 13 Jan '09

13 Jan '09

Hi. I have some doubts about errno handling in a Concurrent Haskell program. Let's suppose that GHC non threaded runtime is used, so that each Haskell thread is bound to an OS thread. Let's suppose there are two threads running (`A` and `B`). Thread `A` calls a function `f`, that, in turn, calls via FFI a C function `c_f`. Function `c_f` fails, settings errno; however the GHC scheduler suspends execution of thread `A` and switch to thread `B`, before the current value of errno is read. Now, let's suppose thread `B` calls a function `g`, that, in turn, calls via FFI a C function `c_g`. Function `c_g`, too, fails, setting errno. Is this possible? P.S.: I have found this is C.Foreign.Error.hs, in base package (not the latest version): throwErrnoIfRetry :: (a -> Bool) -> String -> IO a -> IO a throwErrnoIfRetry pred loc f = do res <- f if pred res then do err <- getErrno if err == eINTR then throwErrnoIfRetry pred loc f else throwErrno loc else return res This function calls getErrno two times. Is this safe? Why the throwErrno function does not accept errno as parameter? Thanks Manlio Perillo

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[Haskell-cafe] type error using ghc 6.10.1, not in previous versions
by Tim Bauer 13 Jan '09

13 Jan '09

Hi all. Under I have some old code that broke under ghc 6.10.1. Under (6.6.1), 6.8.1 (and I think 6.8.2), this compiles. import Prelude hiding(catch) import Control.Concurrent import Control.Exception(catch,throw,evaluate) async :: IO a -> IO (MVar a) async ioa = do mVar <- newEmptyMVar forkIO $ catch (ioa >>= putMVar mVar) (putMVar mVar . throw) return mVar Under 6.10. I now get a type error. TypeError.hs:13:55: Ambiguous type variable `e' in the constraint: `GHC.Exception.Exception e' arising from a use of `throw' at TypeError.hs:13:55-59 Probable fix: add a type signature that fixes these type variable(s) Prelude> :t catch catch :: IO a -> (IOError -> IO a) -> IO a Prelude> :t Control.Exception.catch Control.Exception.catch :: (GHC.Exception.Exception e) => IO a -> (e -> IO a) -> IO a What changed that causes this to break between 6.8 and 6.10? In fact, I don't even see the type error (the message is of little help). The function throw returns an `a', so it should unify with the required type for the handler. I tried simplifying the problematic line to: forkIO $ catch (ioa >>= putMVar mVar) (error "boom") (error "boom") should also unify with anything. Can anyone suggest other things I can try and perhaps what is going on? Thanks, - Tim

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ANN: bytestring-trie 0.1.4
by wren ng thornton 13 Jan '09

13 Jan '09

-------------------------------------------- -- bytestring-trie 0.1.4 -------------------------------------------- Another release for efficient finite maps from (byte)strings to values. Equal parts bugfix release and real release. This upgrade is suggested for all users and is what 0.1.0 should have been. -------------------------------------------- -- Changes (since 0.1.2) -------------------------------------------- * Fixed a number of obscure bugs when "" is used as a key. * Added keys and toListBy (which generalizes toList, keys,...). Also did some optimization of toListBy so it shouldn't be as egregiously inefficient as it was before. * Added fromListL, fromListR, and fromListS to Data.Trie.Convenience along with notes about when each is more efficient than the others. * Separated Data.Trie (the main module for users) from Data.Trie.Internal (gritty details, and core implementation). Data.Trie re-exports most of Data.Trie.Internal so this is primarily a cosmetic change. The showTrie and lookupBy_ functions are no longer exported from Data.Trie however. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie Darcs: http://code.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://code.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring-trie/ -- Live well, ~wren

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Functions with generic return types
by Stephen Hicks 12 Jan '09

12 Jan '09

Hi, I'm trying to write a small module for conveniently writing functions that can return any of a finite number of types. That is, I'd like to be able to write something like foo :: StringOrInt t => String -> IO t This is pretty easy to do if I hard-code the classes as above, but I run into difficulties making it more general. In particular, it's convenient to use incoherent instances so that I can insert the polymorphism at whatever level I choose (see the third instance declaration below, as well as the function foo). But when I try to make this work for monads, it fails - that is, my function bar (below) always prints both "str" and "int" because I can't make it lazy enough - in order to get even a thunk in the non-monadic type, it seems like the side effect has to happen (unsafeInterleaveIO + seq would sort of fix this here, but not generally), whereas the type alone should be sufficient in selecting which monad needs to be evaluated. I'm having a hard time explaining this abstractly, but I think the following concrete code explains pretty clearly what I'm trying to do, and I would appreciate any comments or suggestions for either a better way to go about this, or something already written that achieves something similar? Thanks, steve ==== {-# LANGUAGE TypeSynonymInstances, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, IncoherentInstances #-} class Pick c a where pick :: c -> a instance Pick (a,b) a where pick = fst instance Pick (a,b) b where pick = snd instance Pick (a,b) c => Pick (d -> a, d -> b) (d -> c) where pick (a,b) d = pick (a d,b d) -- This instance definition is broken... instance (Monad m,Pick (a,b) c) => Pick (m a,m b) (m c) where pick (ma,mb) = do { a <- ma; b <- mb; return $ pick (a,b) } foo :: Pick (String,Int) t => String -> t foo = pick (id :: String -> String, length :: String -> Int) toStr :: String -> IO String toStr s = putStrLn "str" >> return s toInt :: String -> IO Int toInt s = putStrLn "int" >> return (length s) bar :: Pick (String,Int) t => String -> IO t bar = pick (toStr,toInt)

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Re: [Haskell-cafe] Looking for Haskellers on Windows
by Günther Schmidt 12 Jan '09

12 Jan '09

Hi Bulat, I do :), but I was amazed that there was no response to a post with, what I thought, would be a rather common problem for an application developer. That post was about writing to an MS-Access database via HDBC-ODBC, which fails. When I then asked the HDBC maintainer himself it turned out that he doesn't use Windows either and thus can't help. Thus my cry for help explicitly to Haskellers on Windows. Günther Am 10.01.2009, 15:18 Uhr, schrieb Bulat Ziganshin <bulat.ziganshin(a)gmail.com>: > Hello Günther, > > Saturday, January 10, 2009, 5:03:18 PM, you wrote: > > there are lot of windows haskellers so you can just write your questions > in > haskell-cafe > >> Hi all, > >> I'm looking for Haskell programmer that use Windows as their >> OS-Plattform. >> I developing an application in Haskell on Windows and run into problems >> that seem to have a lower priority with the greater Haskell community as >> most of them are using Linux where the problems do not occur or are >> already solved. > >> So it'd be nice to get in touch with other Haskellers that face the same >> problems. > >> Günther > >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

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System.CPUTime and picoseconds
by Manlio Perillo 12 Jan '09

12 Jan '09

Hi. Just out of curiosity, but why Haskell 98 System.CPUTime library module uses picoseconds instead of, say, nanoseconds? At least on POSIX systems, picoseconds precision is *never* specified. Thanks Manlio Perillo

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Gtk2Hs on Windows for GHC 6.10.1
by Peter Verswyvelen 12 Jan '09

12 Jan '09

Gtk2Hs 0.9.13 has an annoying bug on Windows that makes it impossible to run via GHCi. So to see if the latest development version works better, I tried to build it on Windows using GHC 6.10.1, but I failed to do so with both MSYS and Cygwin. Does anybody know how to build it on Windows? Thanks, Peter

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Data analysis with Haskell
by Daniel Kraft 12 Jan '09

12 Jan '09

Hi all, I'm going to start a project where I'll have to do some data analysis (statistics about product orders) based on database entries; it will mostly be some very basic stuff like grouping by certain rules and finding averages as well as summing up and such. It will however be more than what can be done directly in the database using SQL, so there will be some processing in my program. I'm thinking about trying to do this in Haskell (because I like this language a lot); however, it is surely not my most proficient language and I tried to do some number crunching (real one that time) before in Haskell where I had to deal with some 4 million integer lists, and this failed; the program took a lot more memory than would have been necessary and ran for several minutes (kept swapping all the time, too). A rewrite in Fortran did give the result in 6s and didn't run out of space. This was probably my fault at that time, because I surely did something completely wrong for the Haskell style. However, I fear I could run into problems like that in the new project, too. So I want to ask for your opinions, do you think Haskell is the right language to do data analysis of this kind? And do you think it is hard for still beginner Haskell programmer to get this right so Haskell does not use up a lot of memory for thunks or list-overhead or things like that? And finally, are there database bindings for Haskell I could use for the queries? Thanks a lot! Daniel

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A pattern type signature cannot bind scoped type variables `t'
by rodrigo.bonifacio 12 Jan '09

12 Jan '09

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Fw: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses
by Rob Hoelz 12 Jan '09

12 Jan '09

Forwarding to Haskell Cafe per Eric's suggestion. Begin forwarded message: Date: Sun, 11 Jan 2009 23:01:31 -0600 From: Rob Hoelz <rob(a)hoelzro.net> To: darcs-devel(a)darcs.net Subject: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses Hello again, Darcs users and developers, As I mentioned in my last e-mail, I'm working on http://bugs.darcs.net/issue291. It's actually gone pretty well, and I feel I'm just about finished (I've done all but sorting out the changes after leaving the editor), only I've encountered the compiler error you see in the subject of this message. This error only appears when compiling with witnesses. Here's the source for the function that it's complaining about: compare_changes_with_old (x :>: xs) (y :>: ys) = case compare_changes_with_old xs ys of nx :> ny -> if unsafeCompare x y then ((x :>: nx) :> ny) else (NilFL :> (y :>: ys)) compare_changes_with_old NilFL NilFL = (NilFL :> NilFL) compare_changes_with_old NilFL ys@(_ :>: _) = (NilFL :> ys) compare_changes_with_old x@(_ :>: _) NilFL = (NilFL :> NilFL) Now, I have two questions: 1) What exactly does this error mean, and how do I get around it? 2) What are witness types, and what are they used for? I will gladly accept links to fine manuals as answers to either question, but simple explanations would be nice. =D I thought I understood Haskell pretty well, but existentially qualified types have thrown me for a loop. Thanks much, Rob Hoelz _______________________________________________ darcs-devel mailing list (AUTOMATIC POSTINGS ONLY PLEASE!) darcs-devel(a)darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-devel

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