- Glasgow-haskell-users

RE: unsafePerformIO around FFI calls
by Simon Marlow 09 Jul '02

09 Jul '02

> Hal Daume <hdaume(a)ISI.EDU> writes: > > I'm curious exactly what is "safe" and what is "unsafe" to wrap > > unsafePerformIO around when it comes to FFI calls. > > Here's a simple test: > > Could you imagine an alternative implementation of the same API in > pure Haskell? (Don't consider efficiency or effort required to write > the implementation, just whether it can be done.) > > If so, then it is ok to use unsafePerformIO and the ffi to implement > the API instead. > > If it fails that test, it is incredibly unlikely that it is ok and a > proof that it is ok is likely to be pretty complex - maybe worth a > PLDI paper or some such. That's a nice succinct way to describe it. Another way, which boils down to the same thing but which is a little more concrete, is to ask: - Does the function's result depend only on the values of its arguments? (obviously only makes sense for a top-level IO function which you want to wrap in unsafePerformIO - for a non-top-level function or expression just replace 'arguments' with 'arguments and free variables'). Cheers, Simon

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unsafePerformIO around FFI calls
by Hal Daume III 08 Jul '02

08 Jul '02

I'm curious exactly what is "safe" and what is "unsafe" to wrap unsafePerformIO around when it comes to FFI calls. If there's a general discussion of this somewhere and someone could send me a pointer that would be another acceptable solution. I googled for "unsafePerformIO FFI" but nothing relevant turned up. Anyway, here's the issue at hand. I have a function which takes a value and an IO action, allocates some memory, put the value in there, executes the IO action (which is an FFI function), gets the result, deallocates the memory and then returns the rest. For instance, we have an FFI function from LAPack which computes, say, eigenvalues. Because this is from fortran and in fortran everything is by reference, the function is an IO action of (simplified) type: > maxEigenvalue :: HMatrix -> IO Double implemented using FFI. We also have the CPS function to make the HMatrix: > withMatrix :: [[Double]] -> (HMatrix -> IO Double) -> IO Double which looks basically like (I'm on vacation so I don't have the real code right now): > withHMatrix m action = > do memory <- allocate memory > hm <- write m into memory > result <- action hm > deallocate memory > return result (it's slightly more complex because i need to make it safe in the case that action throws an exception and the memory is still deallocated before the exception is re-raised.) i can then wrap these together and say: > compEig :: [[Double]] -> IO Double > compEig m = withHMatrix m maxEigenvalue so the question is: Under what circumstances is it safe to make instead: > compEig' :: [[Double]] -> Double > compEig' m = unsafePerformIO $ withHMatrix m maxEigenvalue ??? Thanks! - Hal p.s., Please continue to CC Carl as this issue came up in conversations with him -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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RE: functional dependencies - bug?
by Simon Peyton-Jones 08 Jul '02

08 Jul '02

Christian This is clearly a bug in 5.03. However, it works fine in the (about to be released) 5.04. My plan, therefore, is to add it to our regression test suite and delcare victory. If I was being totally thorough I'd find out what went wrong in 5.03, in case it's still wrong in 5.04, but I'm not going to be totally thorough! Please yell if any similar mysterious things happen. I appreciate the report. Simon | -----Original Message----- | From: Christian Maeder [mailto:maeder@Informatik.Uni-Bremen.DE] | Sent: 28 June 2002 18:09 | To: glasgow-haskell-users(a)haskell.org | Subject: functional dependencies - bug? | | | Hi, | | does anybody have experiences with "Type Classes with | Functional Dependencies" (LNCS 1782, ESOP 2000, March 200, by | Mark P. Jones)? | | I've tried the example in the above paper: | | class Collects e ce | ce -> e where | empty :: ce | insert :: e -> ce -> ce | member :: e -> ce -> Bool | | This works correctly for: | | instance Eq a => Collects a [a] where | instance Collects Char Int where | | and complains correctly for: | | instance Collects Char Int where | instance Collects Bool Int where | | with "Functional dependencies conflict between instance declarations" | | Now to my problem with a context in the class declaration: | | class Eq ce => Collects e ce | ce -> e where | empty :: ce | ... | | This works as above, but correctly complains for: | | data Stupid = Stupid -- without equality | instance Collects Bool Stupid where | | with "No instance for (Eq Stupid)". | | However, if I supply an (admittingly stupid) default | definition for "empty" within the class declaration: | | class Eq ce => Collects e ce | ce -> e where | empty :: ce | empty = error("empty") | ... | | I no longer get the above "No instance for (Eq | Stupid)"-Message, and I guess that is a bug (or a feature | that I don't understand). | | When I change the context to "Eq e" everything is fine again: | | instance Collects Stupid Int where | | complains as above. | | I don't know when (even meaningful) default definitions cause | that context conditions are not checked. In fact I noticed | this error only after I've removed a default definition just | to find out that my instance declaration were wrong that | seemed to be correct before (but instances for contexts were missing). | | Are there any other (known) pitfalls with functional dependencies? | | Regards Christian _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users |

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Type of newForeignPtr & addForeignPtrFinalizer
by Adrian Hey 07 Jul '02

07 Jul '02

Hello, I have a feeling this may be a stupid question, but why are the types of these.. newForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a) addForeignPtrFinalizer :: ForeignPtr a -> IO () -> IO () (second arg being the finalizer) Won't a finaliser almost always take a pointer to the thing being finalised as an argument? If so it would be more convienient to have newForeignPtr.. newForeignPtr :: Ptr a -> (Ptr a -> IO ()) -> IO (ForeignPtr a) or maybe.. newForeignPtr :: Ptr a -> (ForeignPtr a -> IO ()) -> IO (ForeignPtr a) and.. addForeignPtrFinalizer :: ForeignPtr a -> (ForeignPtr a -> IO ()) -> IO () The first of these is easy to implement yourself I suppose.. myNewForeignPtr :: Ptr a -> (Ptr a -> IO ()) -> IO (ForeignPtr a) myNewForeignPtr p fin = newForeignPtr p (fin p) The second seems to require this bit of weirdness.. myNewForeignPtr :: Ptr a -> (ForeignPtr a -> IO ()) -> IO (ForeignPtr a) myNewForeignPtr p fin = do newfp <- newForeignPtr p (return ()) addForeignPtrFinalizer newfp (fin newfp) return newfp Unless I'm missing something, you have to use a pointless dummy finaliser (return ()) to get a ForeignPtr to use as the argument of the real finaliser. The reason I ask is I've been trying to use C2HS recently to produce a Haskell binding to GNU plot library, and have had to do something very similar. The relevant bits of .chs file being.. -- Haskell: newtype Plotter = Plotter (ForeignPtr Plotter) {#pointer *plPlotter as Plotter foreign newtype#} -- Haskell: newtype PlotterParams = PlotterParams (ForeignPtr PlotterParams) {#pointer *plPlotterParams as PlotterParams foreign newtype#} -- Destructor for the plPlotter type. -- C proto: int pl_deletepl_r (plPlotter *plotter); -- Haskell: deletePlotter :: Plotter -> IO () {#fun unsafe pl_deletepl_r as deletePlotter {id `Plotter'} -> `()' -- id as marshaller?? #} -- Create a new X Plotter -- C proto: plPlotter* new_x_plotter (plPlotterParams* plotter_params); -- Haskell: newXPlotter :: PlotterParams -> IO Plotter -- newXPLotter'_ :: PlotterParams -> IO (Ptr Plotter) {#fun unsafe new_x_plotter as newXPlotter {id `PlotterParams'} -> `Plotter' marshalPlotter * #} marshalPlotter :: Ptr Plotter -> IO Plotter marshalPlotter p = do foreignPlotter <- newForeignPtr p (return ()) let plotter = Plotter foreignPlotter addForeignPtrFinalizer foreignPlotter (deletePlotter plotter) return plotter I suppose the thing that's worrying me most is will this work? (I don't have enough done yet to find out). It seems overly complicated to me, but I can't see any alternative to this complexity. Am I using C2HS right? Have I missed something obvious? Thanks -- Adrian Hey

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Timeout
by D.J.Steinitz 04 Jul '02

04 Jul '02

There seem to be a few variants of timeout around. I've copied the one from asynchronous exceptions. Is this the right one to use? There didn't seem to be a library version which I thought there should have been. I'd welcome any advice. Dominic. module ConcurrUtil(eitherIO, timeout) where import Prelude hiding (catch) import Exception import Concurrent data EitherRet a b = A a | B b | X Exception eitherIO :: IO a -> IO b -> IO (Either a b) eitherIO a b = do m <- newEmptyMVar block (do a_id <- forkIO (catch (do r <- unblock a putMVar m (A r)) (\e -> putMVar m (X e))) b_id <- forkIO (catch (do r <- unblock b putMVar m (B r)) (\e -> putMVar m (X e))) let loop = catch (takeMVar m) (\e -> do throwTo a_id e throwTo b_id e loop) r <- loop killThread a_id killThread b_id case r of A r -> return (Left r) B r -> return (Right r) X e -> throw e) timeout :: Int -> IO a -> IO (Maybe a) timeout t a = do r <- eitherIO (threadDelay t) a case r of Left _ -> return Nothing Right a -> return (Just a)

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RE: where is dynApply?
by Simon Marlow 03 Jul '02

03 Jul '02

> my documentation (sec-dynamic.html#SEC-DYNAMIC-UTIL, below) lists > functions that I cannot find. I would like to use dynApply unless you > give me a reason that I should not. (Could I implement this function > myself without knowing internals?) > > Our installation GHC Interactive, version 5.02.3 has the ":info" for > Dynamic but not for dynApply. These functions exist, but aren't exported. It looks like an oversight, I've fixed it for the forthcoming release. Cheers, Simon

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where is dynApply?
by Christian Maeder 03 Jul '02

03 Jul '02

Hello, my documentation (sec-dynamic.html#SEC-DYNAMIC-UTIL, below) lists functions that I cannot find. I would like to use dynApply unless you give me a reason that I should not. (Could I implement this function myself without knowing internals?) Our installation GHC Interactive, version 5.02.3 has the ":info" for Dynamic but not for dynApply. Thanks in advance, Christian 5.11.3. Utility functions Operations for applying a dynamic function type to a dynamically typed argument are commonly useful, and also provided: dynApply :: Dynamic -> Dynamic -> Dynamic -- unsafe. dynApplyMb :: Dynamic -> Dynamic -> Maybe Dynamic

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Can anyone get at c2hs in cvs?
by Adrian Hey 03 Jul '02

03 Jul '02

Hello, Sorry if this is slightly off topic, but I don't seem to be able to get at c2hs and I wonder if anyone here has had the same problem or knows of a fix. The instructions here.. http://www.cse.unsw.edu.au/~chak/haskell/c2hs/ say to login using.. cvs -d :pserver:anonymous@ceres.cse.unsw.edu.au:/home/chakcvs/cvs login then enter password (anonymous). When I do this all I get back is.. cvs [login aborted]: connect to ceres.cse.unsw.edu.au:2401 failed: Connection refused Thanks -- Adrian Hey

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Re: trying to compile a cgi library from hugs
by Chris Moline 02 Jul '02

02 Jul '02

On Wed, 03 Jul 2002 00:31:25 +0200, Wolfgang Thaller <wolfgang.thaller(a)gmx.net> wrote: > Your invocation of the ghc command is telling GHC to just compile > Counter.lhs to Counter.o and then create an executable (link) with just > Counter.o, not CGI.o. The linker then complains about undefined > references to the CGI module. > Solution 1: GHC works just like an ordinary UNIX-style C compiler. > Specify CGI.o on the command line. > Solution 2: use ghc --make Counter.lhs > GHC will automatically look for all required modules and compile them if > necessary (for GHC versions >= 5). ah, thanks. if anyone wants to know the cgi library works without any changes except youll have to change the .cgi's to .lhs so ghc will handle them right. sincerly, chris moline

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trying to compile a cgi library from hugs
by Chris Moline 02 Jul '02

02 Jul '02

hi. since my webhoster doesnt have haskell installed i have decided to use compiled cgi programs instead. since there doesnt seem to be a cgi library for ghc i was hoping to port the library i found at http://www.pms.informatik.uni-muenchen.de/mitarbeiter/panne/haskell_libs/CG… the library (CGI.lhs) seems to compile fine. but when i go to compile the programs i get the following [04:13pm 0.13 0.15 0.16 ~/CGI]$ mv Counter.cgi Counter.lhs Counter.cgi -> Counter.lhs [04:14pm 0.11 0.15 0.16 ~/CGI]$ ghc Counter.lhs compilation IS NOT required Counter.o: In function `__stginit_Main': Counter.o(.text+0x16): undefined reference to `__stginit_CGI' Counter.o: In function `Main_main_srt': Counter.o(.text+0x288): undefined reference to `CGI_h1_closure' Counter.o(.text+0x2a8): undefined reference to `CGI_submit_closure' Counter.o(.text+0x2ac): undefined reference to `CGI_gui_closure' Counter.o(.text+0x2b8): undefined reference to `CGI_page_closure' Counter.o(.text+0x2c0): undefined reference to `CGI_zdfShowCgiOut_closure' Counter.o(.text+0x2c4): undefined reference to `CGI_zdfMimeHTML_closure' Counter.o(.text+0x2c8): undefined reference to `CGI_wrapper_closure' Counter.o: In function `s13m_entry': Counter.o(.text+0x35e): undefined reference to `CGI_h1_closure' Counter.o: In function `s13Z_entry': Counter.o(.text+0x689): undefined reference to `CGI_submit_closure' Counter.o: In function `s144_entry': Counter.o(.text+0x6fe): undefined reference to `CGI_gui_closure' Counter.o: In function `s14f_entry': Counter.o(.text+0x7e2): undefined reference to `CGI_page_closure' Counter.o: In function `s14v_ret': Counter.o(.text+0x81a): undefined reference to `CGI_Content_con_info' Counter.o: In function `s13d_entry': Counter.o(.text+0x898): undefined reference to `CGI_zdfMimeHTML_closure' Counter.o(.text+0x89d): undefined reference to `CGI_zdfShowCgiOut_closure' Counter.o: In function `Main_main_entry': Counter.o(.text+0x91e): undefined reference to `CGI_wrapper_closure' [04:14pm 0.15 0.15 0.16 ~/CGI]$ i do not know what is happening here. any hints on how to proceed? sincerly, chris moline

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