- Haskell-Cafe - Haskell.org

RE: Bug? [was: Implicit params]
by Simon Peyton-Jones 15 Nov '02

15 Nov '02

| > this seems like a bug in GHC. Hugs is happy with it. The "s" in the | > pure2 signature is not ambiguous because it is determined when you give | > the value of the implicit parameter. in fact the way i think about | > implicit parameters is simply as a nice notation for computations in the | > environment monad, so in my mind the above two definitions are pretty | > much the same. | | Thanks Iavor! | GHC people, can you confirm this, Is it a bug? I'm using Ghc 5.0.4 SuSE rpms. | Is there a workaround? Interesting. I hadn't thought of that. Yes, I think you can consider it a bug, or at least the lack of a feature. I'll fix it next week S

1 0

RE: deriving (was Re: storing to a file)
by Simon Peyton-Jones 15 Nov '02

15 Nov '02

| I was thinking that it might be nice to be able to write Haskell to add, | say, a "deriving XML" or "deriving ASN1" feature whose instances provide | methods to convert between Haskell data structures and those formats, | instead of having to hack the compiler to achieve such automated method | writing. When Template Haskell is out, you'll be able to do just that, I think. Simon

1 0

RE: Time library underspecified
by Simon Peyton-Jones 15 Nov '02

15 Nov '02

This is a community thing! They aren't our libraries, they're yours. Haddockising them is something anyone can do -- it doesn't have to be GHC head office (we are always way behind on our to-do list). Please help! Simon | -----Original Message----- | From: Mark Carroll [mailto:mark@chaos.x-philes.com] | Sent: 15 November 2002 13:54 | To: haskell-cafe(a)haskell.org | Subject: Re: Time library underspecified | | On Fri, 15 Nov 2002, George Russell wrote: | | > The TimeExts library probably does most of what you want. I really ought to | Haddockify | > the comments some time . . . | | Heh - this reminds me that one problem I often have with the GHC online | library Haddock docs is that many library function definitions are | uncommented and I have to try them out in ghci to figure out exactly what | they do. (-: | | --- Mark | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Re: Time library underspecified
by Mark Carroll 15 Nov '02

15 Nov '02

On Fri, 15 Nov 2002, George Russell wrote: > The TimeExts library probably does most of what you want. I really ought to Haddockify > the comments some time . . . Heh - this reminds me that one problem I often have with the GHC online library Haddock docs is that many library function definitions are uncommented and I have to try them out in ghci to figure out exactly what they do. (-: --- Mark

1 0

RE: Calling Haskell from Python / C++
by Simon Marlow 15 Nov '02

15 Nov '02

> Simon Peyton-Jones writes: > > > I hope it's all described adequately in the GHC user manual > > http://haskell.org/ghc > > I'll take a look at it, thanks! > > Unfortunately, before I can worry about this I have to get GHC built > at all. It appears that the available binary versions don't work an a > pure gcc 3.x system. There aren't by any chance _statically_ linked > GHC versions available for Linux somewhere, are they? I'm not sure what you mean by a pure gcc 3.x system. GHC 5.04.1 works fine with gcc 3.x, but previous versions didn't. Cheers, Simon

2 1

Re: storing to a file
by C.Reinke 14 Nov '02

14 Nov '02

> There's been mention of a Binary module; .. That said, there was > also a post about using plain text. I tend to agree, except for > certain cases. However, that is *not* to say that you should > necessarily use Show/Read. | Actually, "deriving binary" would be a nice thing to have in general | - even more, a way to add your own "deriving" things from within | Haskell, although I have no idea how such a thing would work. In this context, DrIFT should be mentioned: http://www.haskell.org/communities/11-2002/html/report.html#sect5.2.1.1 http://repetae.net/john/computer/haskell/DrIFT/ Binary is already supported, I think (what about the new Binary variants?), but adding new instance derivation rules for your own external formats shouldn't be too difficult. So nobody has to stick to Show/Read for convenience reasons if there are other arguments against (such as those listed by Hal). Claus PS The Haskell ATerm Library might also be relevant? http://www.haskell.org/pipermail/haskell/2002-May/009589.html (the jDrift mentioned there is subsumed by the upcoming Drift2.0, merging various previous versions)

2 1

1 line simple cat in Haskell
by Ahn Ki-yung 14 Nov '02

14 Nov '02

If you are steaming with compicated codes, then how about taking a break. Let's play with a simple cat. \begin{code} main = mapM (>>=putChar) getCharS where getCharS = getChar:getCharS \end{code} Tested with ghc. Works good except that you get some messages on stderror because eof is not handled. How would you suggest to neatly insert the error handling code into ? P.S. Instead of coding with C++, I want to write my server main code like this. server_main = mapM (>>=process.reply) where getReqS = getReq:getReqS Only if I had enough time ... :-p Using HDirect and so on ... -- Ahn Ki-yung

4 5

Binary read/write WAS [storing to a file]
by Shawn P. Garbett 14 Nov '02

14 Nov '02

I've been fiddling with binary read/write in Haskell. I put together a little example demonstrating my lack of understanding. It creates a connection requestion XAtom and spits it out over a socket. My real hangup occurs when I get a String back from the Socket and would like it nicely marshalled into the ConnectSuccess type. These techniques I would assume apply to binary read/write for files as well. Any criticism/suggestions are appreciated. import IO import Monad import GHC.IO import GHC.Storable import Network import Network.Socket import Data.Char import Data.Word import System.Environment import Parsec -- A Parser that looks for everything before the colon beforeColon :: Parser String beforeColon = many1 (satisfy $ \c -> c /= ':') -- Parse out the display name from the environment Variable DISPLAY parseDisplay :: String -> String parseDisplay s = if (name == "") then "localhost" else name where name = case (parse beforeColon "" s) of Left err -> "" Right x -> x -- repeat an IO action multiple times repeat' :: Int -> IO a -> IO () repeat' n f = foldr (>>) (return ()) (take n (repeat f)) -- Send the X11 Connection Request to a handle sendConnectReq :: Handle -> IO () sendConnectReq h = do -- O'Reilly claims this should be '\x66' -- Try it with this and it crashes like so -- *** Exception: user error -- Reason: Pattern match failure in do expression, open.hs:xx hPutChar h '\x6c' --platform dependent byte-ordering, MSB hPutChar h '\x00' --unused hPutChar h '\x0b' --protocol major 11 hPutChar h '\x00' --protocol minor 0 repeat' 8 (hPutChar h '\x00') -- pad it out to boundary hFlush h -- Totally clueless on this one -- What's the best for this? marshallSuccess :: String -> ConnectSuccess marshallSuccess s = let v = drop 39 s in ConnectSuccess 0 0 0 0 0 0 0 0 0 0 0 0 v -- Get the reply getConnectReply :: Handle -> IO (Either ConnectFail ConnectSuccess) getConnectReply h = do (r:rs) <- hGetContents h -- This is the line that crashes with an invalid request (reply ""?) if (r == '\x00') then return (Left (drop 7 rs)) else return (Right (marshallSuccess rs)) -- Error string, this probably should be an ioexception type type ConnectFail = String -- Connection success type to fill up with wonderful stuff data ConnectSuccess = ConnectSuccess { release :: Word32, id_base :: Word32, id_mask :: Word32, motion_buf :: Word32, max_req :: Word16, screens :: Word8, img_order :: Word8, bit_order :: Word8, bit_unit :: Word8, bit_pad :: Word8, min_keycode :: Word8, max_keycode :: Word8, vendor :: String } -- Main program to make an X11 connection request. main :: IO () main = withSocketsDo $ do hostname <- liftM parseDisplay (getEnv "DISPLAY") h <- connectTo hostname (Service "x11") sendConnectReq h reply <- getConnectReply h case reply of Left s -> putStr "Failure: " >> putStr s >> putChar '\n' Right s -> putStr "Success: " >> putStr (vendor s) >> putChar '\n'

2 2

Calling Haskell from Python / C++
by Jonathan Holt 14 Nov '02

14 Nov '02

Hi, I've just recently learned about Haskell, and I'm impressed by the abstractions and expressiveness that it affords. I'm particularly interested in it for a small parser project that I'm planning. However, my main programming languages are Python and C++, and for various reasons switching entirely to Haskell is completely out of the question (no flame-bait intended!). So here is what I envision: I write the main application in Python. I write a (hopefully) small Haskell module that: a) Calls back to the main Python app for reading the text to be parsed, preferably using laziness. b) Parses the text, and maybe processes a bit. c) Returns the parsed data-structure. (This may be tricky, but I think I know how to do it). This all seems quite cool. And it can also be very useful in a wide veriety of other circumstances, i.e. whenever a complex computation can be "outsourced" from Python/C++ to Haskell. But I can't find any way to interface Python and Haskell in this way. The FFI seems to allow for this, but there arn't any tools to actually do it... So my question is: How would one do such a thing? Thanks in advance, -JH __________________________________________________ Do you Yahoo!? U2 on LAUNCH - Exclusive greatest hits videos http://launch.yahoo.com/u2

3 2

Re: Time library underspecified
by Tom Pledger 14 Nov '02

14 Nov '02

John Meacham writes: : | another useful thing would be | endOfTime and beginningOfTime constants, representing the minimum and | maximum values representable by ClockTime. : | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell If you plan to use endOfTime and beginningOfTime mainly as sentinels, and are happy to treat them as +infinity and -infinity for arithmetic, a separate wrapper type may help. I've found the following (abridged) useful for 'closing off' data types which were otherwise open-ended: > data Close a > = Lo > | Mid a > | Hi > deriving (Eq, Ord, Show) > > instance Bounded (Close a) where > minBound = Lo > maxBound = Hi > > instance Num a => Num (Close a) where ... > instance Real a => Real (Close a) where ... > instance Enum a => Enum (Close a) where ... > instance Integral a => Integral (Close a) where ... Regards, Tom

1 0