- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Is (==) commutative?
by Tristan Seligmann 24 Jul '12

24 Jul '12

Forgot to copy the list. Sorry for the duplicates :( On Jul 24, 2012 1:35 PM, "Tristan Seligmann" <mithrandi(a)mithrandi.net> wrote: > On Jul 24, 2012 12:32 PM, "Twan van Laarhoven" <twanvl(a)gmail.com> wrote: > > > > On 2012-07-24 10:10, Christian Sternagel wrote: > >> > >> Dear all, > >> > >> with respect to formal verification of Haskell code I was wondering > whether (==) > >> of the Eq class is intended to be commutative (for many classes such > >> requirements are informally stated in their description, since Eq does > not have > >> such a statement, I'm asking here). Or are there any known cases where > >> commutativity of (==) is violated (due to strictness issues)? > > > > > > Strictness plays no role for Eq, since to test for equality both sides > will have to be fully evaluated. > > I don't think this is necessarily true. For example: > > (==) :: (Eq a) => Maybe a -> Maybe a -> Bool > Nothing == Nothing = True > Nothing == Just _ = False > Just _ == Nothing = False > Just x == Just y = x == y > > This particular example is still commutative, however (at least I think it > is). >

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TLS 0.9.6, question about session resumption.
by C Gosch 23 Jul '12

23 Jul '12

Hi Cafe, I am trying to use the TLS package from hackage, and it works fine so far -- except when a client wants to do session resumption (note I am not an expert in TLS, so it might be something quite simple). In that case, I get an alert, "unexpected message", during handshake. The handshake goes like this: ClientHello (with a SessionID) ServerHello (with the same SessionID) ServerHelloDone and then the server says (AlertLevel_Fatal,UnexpectedMessage) I'm not sure whether the "ServerHelloDone" should happen when resuming. Does anyone have a hint what may be going wrong? I am using TLS10 and the tls package with version 0.9.6. Thanks, Christian

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Re: [Haskell-cafe] Cabal install fails due to recent HUnit
by Simon Hengel 23 Jul '12

23 Jul '12

On Mon, Jul 23, 2012 at 12:51:32PM -0500, Stephen Paul Weber wrote: > > Currently you would have to do the upgrade manually, as `cabal-install > > cabal-install` won't work (or alternatively edit your local > > ~/.cabl/packages/hackage.haskell.org/00-index.tar). > > Pending a fix on hackage (hopefully) I've been using `cabal copy && cabal > register` to grab specific packages today. > > How would I go about fixing my 00-index.tar? I tride de-tarring, deleting > the recent versions of HUnit, and re-tarring, and when I copy that file in > cabal starts telling me no packages exist...? E.g. with vim, you can edit files in the tar file (in-place). That is what I did, and it worked fine. Cheers, Simon

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Need help with learning Parsec
by C K Kashyap 23 Jul '12

23 Jul '12

Dear gentle Haskellers, I was trying to whet my Haskell by trying out Parsec today to try and parse out XML. Here's the code I cam up with - I wanted some help with the "gettext" parser that I've written. I had to do a dummy "char ' ') in there just to satisfy the "many" used in the xml parser. I'd appreciate it very much if someone could give me some feedback. data XML = Node String [XML] | Body String deriving Show gettext = do x <- many (letter <|> digit ) if (length x) > 0 then return (Body x) else (char ' ' >> (return $ Body "")) xml :: Parser XML xml = do { name <- openTag ; innerXML <- many innerXML ; endTag name ; return (Node name innerXML) } innerXML = do x <- (try xml <|> gettext) return x openTag :: Parser String openTag = do char '<' content <- many (noneOf ">") char '>' return content endTag :: String -> Parser String endTag str = do char '<' char '/' string str char '>' return str h1 = parse xml "" "<a>A</a>" h2 = parse xml "" "<a>A</a>" h3 = parse xml "" "<a><c></c></a>" h4 = parse xml "" "<a><c></c></a>" Regards, Kashyap

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Re: [Haskell-cafe] Haskell's type inference considered harmful
by oleg＠okmij.org 23 Jul '12

23 Jul '12

> 1. Haskell's type inference is NON-COMPOSITIONAL! Yes, it is -- and there are many examples of it. Here is an example which has nothing to do with MonomorphismRestriction or numeric literals {-# LANGUAGE ExtendedDefaultRules #-} class C a where m :: a -> Int instance C () where m _ = 1 instance C Bool where m _ = 2 main = do x <- return undefined let y = x print . fst $ (m x, show x) -- let dead = if False then not y else True return () The program prints 1. If you uncomment the (really) dead code, it will print 2. Why? The dead code doesn't even mention x, and it appears after the printing! What is the role of show x, which doesn't do anything? Exercises: what is the significance of the monadic bind to x? Why can't we replace it with "let x = undefined"? [Significant hint, don't look at it] Such a behavior always occurs when we have HM polymorphism restriction and some sort of non-parametricity -- coupled with default rules or overlapping instances or some other ways of resolving overloading. All these features are essential (type-classes are significant, defaulting is part of the standard and is being used more and more).

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System.Win32.Registry... Help?
by Anonymous Void 22 Jul '12

22 Jul '12

Hi, I'm working on a project that will require me to create and possibly set registry keys. I don't have much experience with programming on Windows either, but I'm having to learn as you don't get many *nix PCs at a computer repair shop, lol. I found a mailing list post showing how to read registry keys and was able to make a function based off of it, but I have no idea what to put into some of the arguments for regSetValueEx or regCreateKeyEx, so I'm stuck. Also, what's the best way to recursively traverse trees in the registry, are there any functions for it? Can someone please help me out with this? Thank you. {-# LANGUAGE ForeignFunctionInterface #-} import System.Win32.Types import System.Win32.Registry import Foreign.Ptr (castPtr) import Foreign.Marshal.Alloc (allocaBytes) import Foreign.C.String (peekCWString, withCWString) import Control.Exception (bracket, throwIO) -- // parse a string from a registry value of certain type parseRegString :: RegValueType -> LPBYTE -> IO String parseRegString ty mem | ty == rEG_SZ = peekCWString (castPtr mem) | ty == rEG_EXPAND_SZ = peekCWString (castPtr mem) >>= expandEnvironmentStrings | otherwise = ioError (userError "Invalid registry value type") -- // FFI import of the ExpandEnvironmentStrings function needed -- // to make use of the registry values expandEnvironmentStrings :: String -> IO String expandEnvironmentStrings toexpand = withCWString toexpand $ \input -> allocaBytes 512 $ \output -> do c_ExpandEnvironmentStrings input output 256 peekCWString output foreign import stdcall unsafe "windows.h ExpandEnvironmentStringsW" c_ExpandEnvironmentStrings :: LPCTSTR -> LPTSTR -> DWORD -> IO DWORD ---- get_key :: HKEY -> String -> String -> IO String get_key cat loc key = bracket op regCloseKey $ \x -> allocaBytes 512 $ \mem -> do ty <- regQueryValueEx x key mem 512 parseRegString ty mem where op = regOpenKeyEx cat loc kEY_QUERY_VALUE set_key :: HKEY -> String -> String -> IO () set_key cat loc key = regSetValueEx cat loc rEG_SZ??? "LPTSTR? What do I put here?" magic_win32_number_here? where op = regOpenKeyEx cat loc kEY_SET_VALUE main = get_key hKEY_CURRENT_USER loc key >>= print where loc = "Software\\7-Zip" key = "Test"

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How do I marshall a pointer over SendMessage LPARAM or WPARAM?
by Simon Peter Nicholls 22 Jul '12

22 Jul '12

Cross-posted from Haskell-beginners. Apologies for not posting in the right place (though I am a beginner and have probably made a simpleton error). I'm new to Haskell, and have had some good success with FFI so far, but using Win32's sendMessage to send a pointer in LPARAM or WPARAM is resulting in access violations at the other end. Is there some issue with my pointer conversions? Am I hitting some restriction, or missing some compiler options? It's driving me pretty crazy, after a very nice start to using Haskell. Some "sending" code: Foreign.C.String.withCWString "frustrator" $ \s -> do let wParam = System.Win32.Types.castPtrToUINT s :: System.Win32.Types.WPARAM Graphics.Win32.sendMessage wnd Graphics.Win32.wM_APP wParam 0 wndProc "receiving" code: | wmsg == Graphics.Win32.wM_APP = do s <- peekCWString $ System.Win32.Types.castUINTToPtr wParam putStrLn s return 0 The string will not be seen. Some extra notes: I can get wndProc messages and integral data generally. The pointer values match textually at both ends when "shown" to stdout. At the sending side I can pass the CWString to a regular FFI function call just fine, and castUINTToPtr will give me back a functioning Ptr for that call. I have also tried sending to & receiving from a working C++ program, without success. Access violations are reported when receiving, though again the address matches up. Silence from Haskell as before, when C++ is sending. I found someone else having an issue here: http://osdir.com/ml/haskell-cafe@haskell.org/2009-11/msg00731.html but no solution unfortunately. UPDATE since prior posting: Seems if I call out via FFI on the sending side, and set the WPARAM to L"hardcoding FTW" in a wrapper C function, the SendMessage I then invoke will work. The C code sees both the original string and the new one, but only the second works when used. I must be missing something obvious, but have toasted my grey cells at this point!

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