- Haskell-Cafe - Haskell.org

storing to a file
by Johan Steunenberg 18 Nov '02

18 Nov '02

Hello, I am new to haskell, and to functional programming, and wondering how to store a Double, or any non-char, to a file. Do I have to make a char array of the double and store that? Or is it preferred to use the show and read functions? Thanks in advance, johan steunenberg

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Formatting function types
by Lauri Alanko 17 Nov '02

17 Nov '02

Just a silly thing that's been nagging me. a very common way (at least in the base libraries) of formatting function types seems to be this: hPutBuf :: Handle -- handle to write to -> Ptr a -- address of buffer -> Int -- number of bytes of data in buffer -> IO () I remember when I first started learning Haskell, and these many-arrowed functions seemed very strange to me: "Okay, we give it a handle, and get a pointer, and, um, from this we get an int, and from this an action? Er?" The problem here is that the first parameter has a distinguished look while the other parameters and the return value all look the same. I think that a naive reader is inclined to assume that line breaks are situated at major structural boundaries. Consider two different interpretations of the structure of the type term: (((Handle (Handle -> Ptr a) ->(Ptr a -> Int) ->(Int -> IO ()) -> IO ()))) Which looks more natural? The point of this rant is just this: the aforementioned multi-line formatting style should only be used for left-associative infix operators. For right-associative ones (such as the function arrow), the One True Way is this: Handle -> Ptr a -> Int -> IO () Nuff said. Lauri Alanko la(a)iki.fi

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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

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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

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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

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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

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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

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Re: storing to a file
by C.Reinke 15 Nov '02

15 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)

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1 line simple cat in Haskell
by Ahn Ki-yung 15 Nov '02

15 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

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Binary read/write WAS [storing to a file]
by Shawn P. Garbett 15 Nov '02

15 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'

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Calling Haskell from Python / C++
by Jonathan Holt 15 Nov '02

15 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

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Re: Time library underspecified
by Tom Pledger 15 Nov '02

15 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

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RE: Calling Haskell from Python / C++
by Simon Peyton-Jones 14 Nov '02

14 Nov '02

I hope it's all described adequately in the GHC user manual http://haskell.org/ghc If not, let us know! Simon | -----Original Message----- | From: Peter Simons [mailto:simons@cryp.to] | Sent: 13 November 2002 10:07 | To: haskell-cafe(a)haskell.org | Subject: Re: Calling Haskell from Python / C++ | | Simon Peyton-Jones writes: | | > If Python uses C's calling convention, it might be easy; just use | > 'foreign import' and 'foreign export' (see the FFI spec at | > haskell.org) | | I remember reading that the main program, that wants to call Haskell | code, would have to be compiled with a special Haskell main() routine, | though? Is that true? And if, why is that necessary? | | -peter | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Library for symbolic integration/derivation/ODE solving?
by Marcus Lindblom 14 Nov '02

14 Nov '02

Hi! I would like a library that helps me with some symbolic integration/derivation/ODE solving. Sort of a Maple/Mathematica as a Haskell library, that operates on a expression tree. I've tried google and the haskell homepage library list but haven't found anything of the sort. A few pointers would be nice. ;) Regards /Marcus

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RE: ghc binary, fptools CVS jamming :-(
by Simon Marlow 14 Nov '02

14 Nov '02

> Do I have to compile every packages again if I upgrade ghc ? Yes. > It is a pleasure to make use of the utilities (greencard, > HOpenGL, etc). But what > > When I try to use CVS repository and stand-alone binary distribution > together (because I wanted to see hdirect is like and v 0.16 > does not work with ghc-5.xx) I felt very much discouraged. > > What should I do ? > Do I always have to compile everything from the fptools source again ? If you install a new version of GHC, then you need to recompile anything which was compiled with the old version. If all you want to do is compile HOpenGL, say, then you don't need to install a new version of GHC. Cheers, Simon

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HDirect
by Martin Huschenbett 14 Nov '02

14 Nov '02

Hi all together! Is there any newer version of HDirect than 0.17? Maybe an CVS-repository? THX, Huschi!

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ghc binary, fptools CVS jamming :-(
by Ahn Ki-yung 14 Nov '02

14 Nov '02

Do I have to compile every packages again if I upgrade ghc ? It is a pleasure to make use of the utilities (greencard, HOpenGL, etc). But what When I try to use CVS repository and stand-alone binary distribution together (because I wanted to see hdirect is like and v 0.16 does not work with ghc-5.xx) I felt very much discouraged. What should I do ? Do I always have to compile everything from the fptools source again ? I hope FFI related Haskell module gets stable and standardized soon. -- Ahn Ki-yung

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Re: Record of STRefs better than STRef to a Record?
by Jorge Adriano 14 Nov '02

14 Nov '02

> > I'm passing that 'record' around as an implicit value. The record as > > STRefs that I use to collect info, but it also has some pure fields with > > 'read-only' info. Something like, <opss let me change 'max' to 'm'> > > data E s = E{ > > refi :: STRef s Int, > > refc :: STRef s Char, > > m :: Int > > } > > > > In some functions I might need only some pure fields, and none of the > > STRefs, but since I pass something of type 'E s' around, everything ends > > up beeing monadic. > > there is no reason why that should be the case. you only need to be > within the ST monad when you read the references, so if in some function > you only look at the pure values, it can have a pure type. > hope this helped > iavor That's what I thought, untill I tried it. Well, actually you must be right since the pure field defines a pure (projection) function... Hmmm, ok, can someone explain this to me, data E s = E{ refi :: STRef s Int, refc :: STRef s Char, m :: Int } -- this is fine, obviously... pure :: E s -> Int pure e = m e -- but this is not... pure2 :: (?e :: E s) => Int pure2 = m (?e) Why exactly isn't this allowed? What is the workaround? Error msg: ------------------------------------------------------------------------ Ambiguous constraint `?e :: E s' At least one of the forall'd type variables mentioned by the constraint must be reachable from the type after the '=>' In the type: forall s. (?e :: E s) => Int While checking the type signature for `pure2' Failed, modules loaded: none. -------------------------------------------------------------------------- Thanks, J.A.

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Record of STRefs better than STRef to a Record?
by Jorge Adriano 14 Nov '02

14 Nov '02

Hi all, If I use an STRef to a record, will a new record be created each time I want to update a single field? Or can I expect GHC to optimize it and have the field of the record updated in place? Right now I'm using a record of STRefs, like: data E s = E{ refi :: STRef s Int, refc :: STRef s Char } but it can get a little messy, since thread s propagates to every datatype that uses the record type in it's definition. Thanks, J.A.

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monadic stack to register machine translator
by William Lee Irwin III 13 Nov '02

13 Nov '02

module GT where import Monad import Monoid import MonadState import MonadWriter import MonadRWS -- Just a quick exercise in using monads. -- Thought it'd be nice to share with the class. data GOp = PushVal Integer | Push Integer | Pop Integer | Slide Integer | Update Integer | GAdd | GSub | GMul | GDiv | GMod | GPow | GNeg | GAbs deriving (Eq, Ord, Read, Show) type Tmp = Integer data ROp = LoadImm Tmp Integer | RAdd Tmp Tmp Tmp | RSub Tmp Tmp Tmp | RMul Tmp Tmp Tmp | RDiv Tmp Tmp Tmp | RMod Tmp Tmp Tmp | RPow Tmp Tmp Tmp | RNeg Tmp Tmp | RAbs Tmp Tmp deriving (Eq, Ord, Read, Show) type CounterT m t = StateT Integer m t type StackT t = State [Integer] t type GST t = RWS () [ROp] (Integer, [Integer]) t class Stack f where pushVal, push, pop, update, slide :: Integral t => t -> f () popVal :: Integral t => f t instance Integral t => Stack (RWS () [ROp] (t, [t])) where pushVal n = do (ctr, stk) <- get put (ctr, fromIntegral n : stk) popVal = do (ctr, top:stk) <- get put (ctr, stk) return (fromIntegral top) push n = do (ctr, stk) <- get put (ctr, stk!!fromIntegral n : stk) pop n = do (ctr, stk) <- get put (ctr, drop (fromIntegral n) stk) slide n = do (ctr, top:stk) <- get put (ctr, top : drop (fromIntegral n) stk) update n = do (ctr, top:stk) <- get let (front, _:back) = splitAt (fromIntegral n) stk put (ctr, front ++ [top] ++ back) class Counter f where gen :: Enum t => f t instance Integral t => Counter (RWS () [ROp] (t, [t])) where gen = do (ctr, stk) <- get put (ctr + 1, stk) return . toEnum . fromIntegral $ ctr + 1 instance (Enum t, Monad m) => Counter (StateT t m) where gen = do ctr <- get put $ succ ctr ctr <- get return . toEnum $ fromEnum ctr translate gOps = snd $ evalRWS (mapM trans gOps) () (0,[]) trans :: GOp -> GST () trans i = case i of PushVal n -> do reg <- gen tell [LoadImm reg n] pushVal reg Push n -> push n Pop n -> pop n Slide n -> slide n Update n -> update n GAdd -> doBinOp RAdd GSub -> doBinOp RSub GMul -> doBinOp RMul GDiv -> doBinOp RDiv GMod -> doBinOp RMod GPow -> doBinOp RPow GNeg -> doUnOp RNeg GAbs -> doUnOp RAbs where doUnOp op = do x <- popVal y <- gen tell [op y x] pushVal y doBinOp op = do x <- popVal y <- popVal z <- gen tell [op z x y] pushVal z

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RE: Calling Haskell from Python / C++
by Simon Peyton-Jones 13 Nov '02

13 Nov '02

| 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). If Python uses C's calling convention, it might be easy; just use 'foreign import' and 'foreign export' (see the FFI spec at haskell.org) If it doesn't, someone would have to add the Python calling convention to the various implementations. Simon

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Re: 1 line simple cat in Haskell
by C.Reinke 13 Nov '02

13 Nov '02

> main = mapM (>>=putChar) getCharS where getCharS = getChar:getCharS > > How would you suggest to neatly insert the error handling code into ? \begin{code} -- some suggestions for a little zoo of cats module Main where import IO import Monad main0 = interact id main1 = getContents >>= putStr main2 = untilEOF (getChar>>=putChar) catchEOF io = catch io (\e->unless (IO.isEOFError e) (ioError e)) untilEOF io = catchEOF (sequence_ $ repeat io) main = main2 \end{code} Claus PS. I haven't kept up to date with buffering issues, and hugs/ghci may not like this kind of code..

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Haskell98 Report copyright
by Christopher Milton 13 Nov '02

13 Nov '02

I hope we don't have a repeat of the MathWorld website shutdown.* I also can't find a webpage with the definition of Standard ML... only avaible in print from MIT Press? Chris * http://mathworld.wolfram.com/erics_commentary.html ===== Christopher Milton cmiltonperl(a)yahoo.com __________________________________________________ Do you Yahoo!? U2 on LAUNCH - Exclusive greatest hits videos http://launch.yahoo.com/u2

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Re: showsPrec: cui bono?
by C.Reinke 12 Nov '02

12 Nov '02

> has anybody here used in a non-trivial way the showsPrec anti-parser? Isn't the idea to make things trivial while avoiding performance penalties? Perhaps: simple pretty-printing of abstract syntax trees? I often use it to get simple debugging output for complex internal data structures (first, use deriving; then, define showsPrec; if that's still not good enough, do some real thinking..). Anyway, this reminded me of a litte old hack of mine. Only trivial use of showsPrec, but perhaps you'll like it anyway?-) http://www.cs.ukc.ac.uk/people/staff/cr3/toolbox/haskell/R.hs As with anything else in my toolbox, no warranty for nothing.. Cheers, Claus ------------------- cut here {- Representative thingies.. A little hack to pair values with string representations of their expressions. Useful if you want to explain what map (+1) [1..4] or foldr1 (*) [1..5] do, or if you want to demonstrate the difference between foldr (+) 0 [1..4] and foldl (+) 0 [1..4] Load this module into Hugs (Hugs mode) and type in some of these examples to get an idea of what I mean. Also try map (+) [1..4] This could be extended in various directions, but I wanted to keep things simple. I'm not convinced that extra complications would be worth the effort. Claus Reinke -} default (R Integer) data R a = R {rep:: String ,val:: a } instance Show (R a) where showsPrec _ a = showString (rep a) instance Show (R a -> R b) where showsPrec _ f = showString ("\\x->"++(rep (f x))) where x = R{rep="x",val=error "variable"} instance Show (R a -> R b -> R c) where showsPrec _ f = showString ("\\x y->"++(rep (f x y))) where x = R{rep="x",val=error "variable"} y = R{rep="y",val=error "variable"} lift1 op a = R {rep="("++(rep op)++" "++(rep a)++")" ,val= ( (val op) (val a) ) } lift2 op a b = R {rep="("++(rep op)++" "++(rep a)++" "++(rep b)++")" ,val= ( (val op) (val a) (val b) ) } lift2infix op a b = R {rep="("++(rep a)++" "++(rep op)++" "++(rep b)++")" ,val= ( (val a) `iop` (val b) ) } where iop = val op instance (Num a,Show a) => Num (R a) where (+) = lift2infix R{rep="+",val=(+)} (-) = lift2infix R{rep="-",val=(-)} (*) = lift2infix R{rep="*",val=(*)} negate = lift1 R{rep="-",val=negate} fromInteger a = (\fIa->R{rep=show fIa,val=fIa}) (fromInteger a) instance (Eq a,Num a) => Eq (R a) where a == b = (val a) == (val b) instance (Ord a,Num a) => Ord (R a) where a <= b = (val a) <= (val b) instance (Enum a,Num a,Show a) => Enum (R a) where fromEnum = fromEnum.val toEnum a = R{rep=show a,val=toEnum a} enumFrom x = map toEnum [fromEnum x..] -- missing in Hugs Prelude..

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Rational sequence
by Ferenc Wagner 12 Nov '02

12 Nov '02

With GHC-5.02.2, I do $ ghci Prelude> :m Ratio Ratio> [1%2..10%2] [1 % 2,3 % 2,5 % 2,7 % 2,9 % 2,11 % 2] The question is, why is there 11%2 at the end of the list? It's inconsistent with the (good) rules for Integer, since Ratio> [1,3..10] [1,3,5,7,9] Is this intentional? Feri.

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H98 copyright notice
by Ross Paterson 08 Nov '02

08 Nov '02

The new copyright notice is inconsistent. If it is not otherwise changed, I suggest that the first clause, "The publisher intends this Report to belong to the entire Haskell community, ..." be deleted. Maybe the word "belong" could be replaced with "be available", but that doesn't seem completely accurate.

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producing and consuming lists
by Jorge Adriano 06 Nov '02

06 Nov '02

I might have been not very clear in my last mail. I decided to post again, and go straight to the point, with some small examples. Consider the following function streams. streams :: (Int->Bool, Int->Bool)->(Int, Int)->([Int],[Int]) streams (p,q) (x,y) = (xs',ys') where (xs,ys) = streams (p,q) ((x+y),(y-x)) xs' = if p x then x:xs else xs ys' = if q y then y:xs else ys - produces a pair of ('infinite') lists - produced lists are not indepentent (you need to calculate elements of one list to calculate elements of the other) - in each recursive call an element is added to the 1st/2nd list iff it satisfies a given (Int->Bool) function p/q How should one consume (part of) both lists, avoiding space leaks? A particular example of consuming both lists might be writing them to files: main :: IO() main = do let (s1,s2)=stream ... -- stream applied to some arguments (p,q) (x,y) p' = ... q' = ... writeFile "f1.txt" (show$ takeWhile p' s1) writeFile "f2.txt" (show$ takeWhile q' s2) In this example all elements of s2 required to evaluate (takeWhile p' s1) are kept in memory, until the first file is writen. Notice that writing one element from s1 and one from s2 successively might still cause space leaks to arise. Fusing the consuming functions with the producer is a possible, but IMO dirty, way out. If my question doesn't seem to make sense for any reason, please tell me, maybe I am missing something obvious here. Thanks, J.A.

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GUI Library for Visualizing Data
by Peter Simons 06 Nov '02

06 Nov '02

Hi, the Haskell language is fairly new to me, so please pardon me if this is a frequently asked question, but I wasn't able to find an answer myself. I'm looking for a library that allows me to visualize data under X11 screen, for instance as a pie chart or as a bar chart. I seem to recall that something like that was available for the GTK toolkit, but has this library been ported to Haskell? Or any other library? I'd appreciate any pointers! -peter

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Job
by Mark Carroll 05 Nov '02

05 Nov '02

The company I'm involved with - Aetion, a tiny defense contractor in Columbus, Ohio - will likely be wanting to hire an affordable Haskell programmer before the end of the year. So, on the offchance that any of you guys are be interested, or know of someone who might be, feel free to e-mail me for more information or to supply your resume. Unfortunately, the nature of the work is such that US citizenship or a security clearance would help, and telecommuting isn't really an option. On the upside, the work should be varied and interesting - more challenging than mundane. (I wonder if I should mention this in comp.lang.functional, too.) For the next Haskell communities report thing in a few months, I'll try to say a bit about what we're up to. (-: At this stage, we're just starting, so there's little to say. -- Mark

2 1

streaming, state, etc...
by Jorge Adriano 03 Nov '02

03 Nov '02

Hi, OK so I had completed my (search) algorithm implementation. And the 'last' function was something like: "until p f x0". But now I wanted all iterations, I wanted to analyze the behavior of the algorithm. What could I do? I thought about using Trace... but it didn't seem adequate, I wanted to manipulate and use that info... maybe monads... that would cluter my algorithm so bad... This feeling lasted for a few seconds, then I thought, Lazy Evaluation! "iterate p f x0" There, done :) I had an infinite list, and I'd just take as much as I wanted. *Analysis* and the *algorithm* itself were *completely separated*, yet it was using the information as soon as it was calculated - or maybe "it was being calculated only when it was needed". I could not do this kind of stuff in imperative languages. Very nice :) ------------ Problem 1: ------------ I've used that many times since then, but lately it doesn't really seem to serve my needs. What if I need to get two different kinds of data, say (list1, list2), and now I want to write them: writeFile file1 (analyze list1) writeFile file2 (analyze list2) What if you need to evaluate list2 to evaluate list1? Not good! I'm evaluating list2 and not consuming it, therefore space leak. I could alternate between one list and another, but what if at a certain point, evaluating the next elem in "list1" caused the evaluation of a few thousand elements of "list2"? So how do you usually deal with this kind of stuff? Concurrency?? ------------ Problem 2: ------------ Yes "iterate p f x0" looks good. What if function "f" is not a pure one? If my monad is a Lazy one, streaming can be used(*), if it is not, that won't work. So streaming could work with monads like State, (lazy) ST. Not with IO, or (lazy) ST. (*) do list<-myalgorithm writeFile file1 (analyze list) So, how do you usually do this kind of thing - extracting lists of info from an algorithm? J.A.

1 0

dozen
by Nuno Silva 31 Oct '02

31 Oct '02

can anyone help me how to get the dozen number? using Int example dozen 1020 the expected result is: 2 --------------------------------- Do you Yahoo!? Y! Web Hosting - Let the expert host your web site

7 14

RE: Concurrent Haskell, asynchronous exceptions and interruptibility (again)
by Simon Marlow 30 Oct '02

30 Oct '02

> For Concurrent Haskell, with respect to asynchronous > exceptions, can a > thread receive an asynchronous exception if it's evaluating > an expression > that could "potentially block on IO" when evaluated within > the "block" > function? > > Specifically, is interruptibility determined statically in > the language definition or dynamically at run time? "dynamically at runtime", according to the semantics given in our paper. GHC does actually implement this, although it implements a slightly different version of throwTo from the one described in the paper (ours blocks until the exception has been delivered). > For example, assume a thread is evaluating the following function. > > transfer :: MVar a -> IO (MVar a) > transfer from = > block $ do > to <- newEmptyMVar > x <- takeMVar from > putMVar to x > return to > > In general, since putMVar is an IO function, it can > potentially block (if > its MVar argument is already occupied with a value). However, > since "MVar > to" will obviously be vacant in this instance, the putMVar > can not block on > IO. > > Would ghc allow an asynchronous exception to be raised at the > point that "putMVar to x" is evaluated? Not unless the putMVar blocks. The proof that it never blocks depends on the behaviour of other threads though: if you have another thread doing putMVar on the same MVar, then it may block. Other threads must be "well-behaved" for it to work. > If not, then I guess transfer is exception safe. > > Would that answer scale? Could I replace the "putMVar to x" with an > arbitrarily large/complex IO function that I might write that > is guaranteed > (by me at least) to be non-blocking and still be sure that an > exception > would not be delivered during evaluation of that function? Yes. Cheers, Simon

2 1

Last generator in do {...}
by George Kosmidis 30 Oct '02

30 Oct '02

I am sure there are a billion errors in this. This is the first one: Line:17 - Last generator in do {...} must be an expression Can Anyone point the errors and maybe tell me if there is a possibility for this to work? import IO import System import List import Maybe import Char import Numeric type Name=String type Room=Int type Dr=String type PatientTup=(Name,(Room,Dr)) type PatientList=[PatientTup] main=do userText<-getText ----------------------------------------------------------------------------------- ________getText::IO String ________getText=do nc<-getText ___________________return (:nc) ----------------------------------------------------------------------------------- ________PatientList::[(String,(Int,String))] ________PatientList=[("Robson, Brian",(2,"MJH")), _____________________("Hitchin, Linda",(1,"ILR")), _____________________("Reeve, Paul", (2,"ILR"))] ----------------------------------------------------------------------------------- ________getWards::Int->Int ________getWards PatientList=[Room | (Name,(Room,Dr)) <- PatientList, Room==getText] _________________do putSpc (length Room) _________________return(RoomLength) ----------------------------------------------------------------------------------- ________getPatient::String->String ________getPatient PatientList=[Name,Room,Dr | (Name,(Room,Dr)) <- PatientList, Name==getText] ___________________do putChar (Name) ______________________putChar 'is in ward' ______________________putChar (Room) ______________________putChar 'and is treated by' ______________________putChar (Dr) ______________________return (wl) ----------------------------------------------------------------------------------- ________printFreq::WordList->IO() ________printFreq wl ----------------------------------------------------------------------------------- ________NumberInWard::printFreq(getWards(getText)) ----------------------------------------------------------------------------------- ________PatientInfo::printFreq(getPatient(getText)) ----------------------------------------------------------------------------------- ________setABC::ReceptionReport -> ReceptionReport ________setABC [ ]___=[ ] ________setABC(wn:ws)=setABC[(w,wc) | (w,wc)<-ws, not(isBigger(w,wc)wn)] _________++[wn]++setABC[(w,wc) | (w,wc) <- ws, isBigger (w,wc) wn] __________________where ___________________isBigger (w1,wc1) (w2,wc2) ____________________| ord(head(w1))-ord(head(w2))>0=True ____________________| ord(head(w1))-ord(head(w2))<0=False ____________________|length w1>1&&length w2>1 _____________________=isBigger(tail(w1),wc1) (tail(w2),wc2) ____________________|length w1>1&&length w2==1=True ____________________|otherwise=false ----------------------------------------------------------------------------------- ________ReseptionList::String->String ________ReseptionList PatientList=[Name,Room | (Name,(Room,Dr)) <- PatientList] ______________________do sequence (map ReceptionReport wl) __________________________________where ______________________________________ReceptionReport::setABC((Name,Room))->IO() ______________________________________ReceptionReport(w,wc)=do putStr w _______________________________________________________________putStr (show w) _______________________________________________________________putChar 'in' _______________________________________________________________putStr (show wc) _______________________________________________________________putChar '\n' _______________________________________________________________return wl ----------------------------------------------------------------------------------- ________setNum::DoctorsReport -> DoctorsReport ________setNum [ ] = [ ] ________setNum((wn,wcn):DoctorsReport[(w,wc)|(w,wc)<-ws, _________________________ws<=wcn++[(wn,wcn)]DoctorsReport[(w,wc)|(w,wc)<-ws,wc>wcn] ----------------------------------------------------------------------------------- ________DoctorsList::String->String ________DoctorsList PatientList=[Name,Room | (Name,(Room,Dr)) <- PatientList, Dr==getText] ____________________do sequence (map DoctorsReport wl) ________________________________where ____________________________________DoctorsReport::setNum((Name,Room))->IO() ____________________________________DoctorsReport(w,wc)=do putStr w ___________________________________________________________putStr (show w) ___________________________________________________________putChar 'in' ___________________________________________________________putStr (show wc) ___________________________________________________________putChar '\n' _______________________________________________________________return wl -----------------------------------------------------------------------------------

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Concurrent Haskell, asynchronous exceptions and interruptibility (again)
by Patrick Bosworth 30 Oct '02

30 Oct '02

For Concurrent Haskell, with respect to asynchronous exceptions, can a thread receive an asynchronous exception if it's evaluating an expression that could "potentially block on IO" when evaluated within the "block" function? Specifically, is interruptibility determined statically in the language definition or dynamically at run time? For example, assume a thread is evaluating the following function. transfer :: MVar a -> IO (MVar a) transfer from = block $ do to <- newEmptyMVar x <- takeMVar from putMVar to x return to In general, since putMVar is an IO function, it can potentially block (if its MVar argument is already occupied with a value). However, since "MVar to" will obviously be vacant in this instance, the putMVar can not block on IO. Would ghc allow an asynchronous exception to be raised at the point that "putMVar to x" is evaluated? If not, then I guess transfer is exception safe. Would that answer scale? Could I replace the "putMVar to x" with an arbitrarily large/complex IO function that I might write that is guaranteed (by me at least) to be non-blocking and still be sure that an exception would not be delivered during evaluation of that function? Cheers, Patrick. _________________________________________________________________ Internet access plans that fit your lifestyle -- join MSN. http://resourcecenter.msn.com/access/plans/default.asp

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RE: Odd Performance
by Garner, Robin 30 Oct '02

30 Oct '02

You may also like to compare this definition. > import List > > primes = unfoldr sieve [2..] > sieve (x:xs) = Just (x,filter (x `notDivisor`) xs) > x `notDivisor` y = y `mod` x /= 0 Slower, but (IMO) cuter. Cheers, -----Original Message----- From: Tim Otten [mailto:tim@cap.american.edu] Sent: Wednesday, 23 October 2002 15:10 To: haskell-cafe(a)haskell.org Cc: scasey(a)cap.american.edu Subject: Re: Odd Performance Tom Pledger writes: > Probably. Try replacing this > (\z -> z <= (intsqrt x)) > with this > (\z -> z^2 <= x) Yes! This is significantly nicer. Taking 4000 primes, this is about twice as fast as the original (loose) algorithm, and it appears that it gets better as n grows. (Call this version #3). > or moving the O(sqrt(n)) step[1] into a let expression This does help -- the implementation with this adjustment seems to hover around 1.1 times the speed of the loose algorithm. (Call this version #4) > Only if the predicate function (the p in takeWhile p xs) is > significantly more expensive than a constant-cost piece of arithmetic > and pattern-matching. This makes sense and explains the results above: squaring z is easier than square-rooting x. In fact, the way intsqrt is written, #4's takeWhile indirectly tests (z*z > x) once for each [z | 1<=z<=sqrt(x)]. #3's takeWhile expression will only evaluate (z^2 < x) once for each [ z | z <- primes, z<=sqrt(x) ]. Moreover, #4's takeWhile will do additional tests (z <= sqrt) for each [z | z <- primes, z <= sqrt(x)]. In retrospect, the intsqrt-approach seems rather silly. :) Thank you, Tim _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Active Haskell development and libraries
by Mark Carroll 29 Oct '02

29 Oct '02

The Libraries and Tools For Haskell page has quite a list of things, but a few broken links, and links to very many projects that seem to have started off as interesting research projects that, as the page itself says, were more proofs of concept and are no longer maintained. We can't help but be slightly worried by this when we're thinking about what language(s) try out for production code, given that when we look at things like Objective Caml there are nice things like the Caml Humps at http://caml.inria.fr/humps/ that shows a lot of recent work, and even a new O'Caml O'Reilly book coming out (well, mostly a translation of the French book, I think). The Haskell Communities and Activities Report is a great start for showing some recent Haskell activity, and the Libraries and Tools page at least separates out the unsupported GUI-libraries, and there is certainly some good stuff for Haskell - for instance, we like HUnit and Haddock, and I'm thinking that Parsec might come in handy. Still, at our end we're wanting to stop and think: I'm looking forward to reading the next communities and activities report. In particular, I want to reassure myself that there is a lot of useful actively-maintained stuff we can use, and more coming. Although I'm very happy that the libraries and tools page exists at all, I wonder if there's any possibility of the Haskell stuff being listed in a nice table too, including information as when the latest release was and with what versions of which Haskell compilers each thing was last found to be working. Maybe there is some existing free software that can make such a site easily maintained? Obviously I'm not demanding that all you guys write lots of extra libraries and tools for Haskell for free for me. (-: (Of course, stuff is already being written and released by posters here.) Partly I'm suggesting that, to avoid putting people off using Haskell, maybe the active and currently-working things should be highlighted and easy to find relative to the other things, right on the libraries and tools page, given that currently there are a few broken links and projects that are practically stalled, and partly I'm wanting to see what other people think about where Haskell is now relative to things like O'Caml from the point of view of commercial instead of research users. At least, am I right in thinking that more industrial Haskell users are wanted, and that it's not meant to be solely an interesting piece of research? To take an example, right now we're not exactly sure what the recommended way is to have Java code call our Haskell functions, for instance. (Background: I have reasonable influence in a tiny company and would like us to use Haskell to create some of our core products - I'm a big believer that use of rich, strong, static typing and a functional style will bring us benefits - at least, I'd like to exploit the fruits of programming language research in recent decades. However, although I'm fairly optimistic about Haskell's future, I've jumped the wrong way before, and I'm wanting to make sure that the momentum in the Haskell community is in the right direction. We would be very happy to contribute stuff ourselves if we needed to to bring things the final few inches toward what we need, as we're generally opensource fans, but we're too small to fund a whole lot of general stuff right now.) Apologies if this seems to be a reiteration of previous vocal worrying I've been doing. (-: Really, I'm just hoping that some changes on the libraries and tools page can help users of Haskell and encourage more people to join the community - especially given that much of the legwork is already being done by Claus anyway - and to help us be sure that Haskell and its associated things is, strategically, a good technology for us to rely on, from the point of view of there being a vibrant and growing repository of useful libraries. -- Mark

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RE: Interruptibility and thread safeness with readMVar
by Simon Marlow 28 Oct '02

28 Oct '02

> How does the ghc run-time system determine if an operation is > interruptible > when within "block" function scope? Explanations I've read in various > papers/web pages state that an operation is interruptible, > even within a the > scope of the "block" function, if that operation can potentially block > waiting for input/output. That's correct. > Take the readMVar function. In my implementation > of ghc, readMVar is defined as follows. > > readMVar :: MVar a -> IO a > readMVar m = > block $ do > a <- takeMVar m > putMVar m a > return a > > Since, by the above definition of interruptible, both > takeMVar and putMVar > are both interruptible operations, isn't there a possibility that, on > receipt of an exception while blocked on the "putMVar m a" > operation, "MVar > m" would be left in an inconsistent state (i.e. empty)? Am I right in > thinking that readMVar should only ever be used when no other > thread can > potentially access "MVar m"? Or have I completely missed the point? Yes, that's right. The safetey of readMVar depends on there being no malicious threads doing putMVar without first doing takeMVar (in fact this condition extends to many MVar operations). > My second question concerns readMVar directly. Is this > function intended to > be an atomic operation in the context of multiple threads > accessing the > "MVar m" parameter (i.e. thread safe)? Presumably, if a > context switch takes > place after takeMVar but before putMVar, there is a > possibility that another > thread will successfully evaluate putMVar on "MVar m", > inserting a different > value. Is this intended? I suppose it's necessary to resort > to some kind of > cunning ploy of using more MVars/threads to control access to > MVar m if we > wan't to guarantee readMVar's thread safeness? If so, does > anyone have such an implementation? The assumption is that there are no threads attempting to do putMVar without first doing takeMVar (the same condition as above). > Both of these questions are a result of trying to implement a > function that > will atomically (in the presence of both other threads and > asynchronous > exceptions) transfer the contents of one MVar to another (or > leave both > MVars in their original states of the operation fails). In order to do this, you'll need to be clear about what another thread in the system is allowed to do with the MVars involved. This will affect whether you need extra locking (more MVars) or not. Also, doing atomic operations on two MVars simultaneously is tricky, because there is a risk of deadlock with another thread doing the same operation on the same MVars but in the opposite order. Usually some ordering is applied to the MVars to avoid this problem. Cheers, Simon

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RE: autocompletion mode for ghci?
by Simon Marlow 28 Oct '02

28 Oct '02

> is there any mode for the ghci, which provides autocompletion > like e.g. the bash? GHCi on Unix provides autocomplete through the readline library (the same library used by bash) albeit only for filenames and directories. We don't compile with readline support on Windows (I'm not sure why). If you want completion of function names, type names, etc. then the answer is that we haven't implemented it, but it would be an interesting project. Cheers, Simon

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Interruptibility and thread safeness with readMVar
by Patrick Bosworth 28 Oct '02

28 Oct '02

How does the ghc run-time system determine if an operation is interruptible when within "block" function scope? Explanations I've read in various papers/web pages state that an operation is interruptible, even within a the scope of the "block" function, if that operation can potentially block waiting for input/output. Take the readMVar function. In my implementation of ghc, readMVar is defined as follows. readMVar :: MVar a -> IO a readMVar m = block $ do a <- takeMVar m putMVar m a return a Since, by the above definition of interruptible, both takeMVar and putMVar are both interruptible operations, isn't there a possibility that, on receipt of an exception while blocked on the "putMVar m a" operation, "MVar m" would be left in an inconsistent state (i.e. empty)? Am I right in thinking that readMVar should only ever be used when no other thread can potentially access "MVar m"? Or have I completely missed the point? My second question concerns readMVar directly. Is this function intended to be an atomic operation in the context of multiple threads accessing the "MVar m" parameter (i.e. thread safe)? Presumably, if a context switch takes place after takeMVar but before putMVar, there is a possibility that another thread will successfully evaluate putMVar on "MVar m", inserting a different value. Is this intended? I suppose it's necessary to resort to some kind of cunning ploy of using more MVars/threads to control access to MVar m if we wan't to guarantee readMVar's thread safeness? If so, does anyone have such an implementation? Both of these questions are a result of trying to implement a function that will atomically (in the presence of both other threads and asynchronous exceptions) transfer the contents of one MVar to another (or leave both MVars in their original states of the operation fails). ********************************************************************** This message is intended only for the stated addressee(s) and may be confidential. Access to this email by anyone else is unauthorised. Any opinions expressed in this email do not necessarily reflect the opinions of royalblue. Any unauthorised disclosure, use of dissemination, either whole or part is prohibited. If you are not the intended recipient of this message, please notify the sender immediately. **********************************************************************

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autocompletion mode for ghci?
by Steffen Mazanek 26 Oct '02

26 Oct '02

Hallo, is there any mode for the ghci, which provides autocompletion like e.g. the bash? Thanks, Steffen Mazanek

1 0

New to Haskell
by Andy Schmidt 24 Oct '02

24 Oct '02

Hi, My name is Andy Schmidt and I am new to Haskell. My prof just introduced me to it yesterday actually. I want to learn more. It sounds like a really interesting thing. So let me know what I need to do in order to get started. Thanks Andy _________________________________________________________________ Surf the Web without missing calls! Get MSN Broadband. http://resourcecenter.msn.com/access/plans/freeactivation.asp

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RE: Rational sequence
by Simon Peyton-Jones 24 Oct '02

24 Oct '02

| > The Report says that the Enum instance for Ratio uses the | > same rule as for Float/Double, | | Now I can see that the revised Report contains more about | this than the one on haskell.org. But I still can't see the | statement you cited above. Where should I look? You look in the instance declaration for Ratio, which is given in the Ratio chapter of the Library report. | As I told above, I can't see anything to change, except | implementation. As I understand it, GHC conforms to the Report, but Hugs perhaps does not, yet. Simon

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Re: Odd Performance
by Tim Otten 23 Oct '02

23 Oct '02

Tom Pledger writes: > Probably. Try replacing this > (\z -> z <= (intsqrt x)) > with this > (\z -> z^2 <= x) Yes! This is significantly nicer. Taking 4000 primes, this is about twice as fast as the original (loose) algorithm, and it appears that it gets better as n grows. (Call this version #3). > or moving the O(sqrt(n)) step[1] into a let expression This does help -- the implementation with this adjustment seems to hover around 1.1 times the speed of the loose algorithm. (Call this version #4) > Only if the predicate function (the p in takeWhile p xs) is > significantly more expensive than a constant-cost piece of arithmetic > and pattern-matching. This makes sense and explains the results above: squaring z is easier than square-rooting x. In fact, the way intsqrt is written, #4's takeWhile indirectly tests (z*z > x) once for each [z | 1<=z<=sqrt(x)]. #3's takeWhile expression will only evaluate (z^2 < x) once for each [ z | z <- primes, z<=sqrt(x) ]. Moreover, #4's takeWhile will do additional tests (z <= sqrt) for each [z | z <- primes, z <= sqrt(x)]. In retrospect, the intsqrt-approach seems rather silly. :) Thank you, Tim

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using IOExts...
by Jason Smith 23 Oct '02

23 Oct '02

Hi All I don't know what I'm doing wrong here but for some reason no matter what esoteric command line option I seem to be able to dream up I can get ghc to include IOExts..I want to use the side-affect IO commands but cannot. I am using ghc-5.02.2 for Win32. Can someone just give me a command line example that alows me to compile a file that includes IOExts? Thanks J.

2 1

representation getting verbose...
by None 23 Oct '02

23 Oct '02

Greetings Haskellers, I'm running into a problem representing some fairly complicated types, and I'll try to put together a simpler example to get your suggestions. In Paul Hudak's SOE, I find a definition of expression: data Expr = C Float | V String | Expr :+ Expr | Expr :- Expr | Expr :* Expr | Expr :/ Expr Now this is compelling, but sometimes, I might want to have a function that takes a variable only, not just any kind of expression. I could write something like: typeOfVariable :: Expression -> Type typeOfVariable (V s) = ... _ = error... But thats not very satisfying from a type-checking perspective. So it makes sense to create a constructor: data Variable = VVariable String data Expr = C Float | V Variable | Expr :+ Expr | Expr :- Expr | Expr :* Expr | Expr :/ Expr and make typeOfVariable :: Variable -> Type. But then when I want to match or create a variable expression, things are starting to get verbose: case expr of C f -> ... V (Variable (VVariable s)) -> ... ... And if I want a still more accurate hierarchy, the construction and destruction of Variables can really become cumbersome. For an interpreter I'm writing, I found myself writing a function "constructVarExpr :: String -> Expr" just to make it easier. This all seems very inelegant, and I get the feeling that there's a better way to do this. Any suggestions on how I could better represent Expressions or Variables to keep the type-checking but decrease the verbosity? peace, Isaac Jones

4 4

Odd Performance
by Tim Otten 23 Oct '02

23 Oct '02

As a student in an undergraduate 'Intro to Discrete Structures' course, I recently did a project which required generating the first n primes. We discussed the sieve of Eratosthenes in class. Although the professor is not familiar with Haskell, he allowed me to use it, and I mistakenly wrote the following code[1]: -- A list of all prime numbers primes = [2] ++ [ x | x <- [3..], isPrime x ] -- x is prime if the set of primes [2 <= p <= sqrt(x)] is empty isPrime x = ( [] == [ y | y <- (take (intsqrt x) primes), x `mod` y == 0 ] ) This works, and I used it as I developed my project. However, it tests more primes than necessary. A better version would seem to be: -- x is prime if the set of prime factors [2 <= p <= sqrt(x)] is empty isPrime x = ( [] == [ y | y <- (takeWhile (\z -> z <= (intsqrt x)) primes), x `mod` y == 0 ]) Surprisingly, the second (tight) version grows significantly more costly than the first (loose) version as we take more primes. Taking 20 primes, > take 20 primes both algorithms require under a second. Taking 200 primes, the tight algorithm requires twice as much time as the loose one. Taking 2000 primes, the tight algorithm requires about 4.5 times as much time as the loose one.[2] Can anyone suggest why the tighter algorithm exhibits significantly worse performance? Is takeWhile significicantly more expensive than take? Is the \z lambda expression expensive? The intsqrt isn't recalculated each time takeWhile evalutes a prime, is it? Slightly confused, Tim Otten [1] Note that intsqrt calculates the floor of the square root of x. intsqrt x = intsqrt' 1 x intsqrt' n x | (n*n > x) = n-1 | otherwise = intsqrt' (n+1) x [2] A few comments on the test apparatus: * Debian GNU/Linux 3.0 * Hugs98 circa Sept 2001 * Tested on a G3 400mhz and a PII 233mhz. The G3 was consistently faster, but the relationship (Time(Alg2)/Time(Alg1)) remains comparable across chips * Times were established using the bash shell's 'time' command * Each test was performed in a new instance of 'runhugs' * Tests were repeated and staggered (Test alg1 @ n primes; test alg2 @ n primes; test alg1 @ n primes; test alg2 @ n primes) Codes, scripts, etc. available upon request.

2 2

SOE exercise
by Isaac Jones 22 Oct '02

22 Oct '02

(I'm not sure why my postings seem somewhat anonymous, I'll mess with the headers in this post to see if that fixes it. I post to other mailman lists and haven't noticed this problem.) I'm working through Paul Hudak's SOE, and have a question about problem 9.4, which is to define a function applyEach. I've come up with several versions, but not one which usefully uses currying (the exercise doesn't explicitly say to use currying, but that's the only thing in this section). My real question is whether I should be trying to apply currying here. Solutions are welcome, but I can think of several good reasons not to post solutions to this forum. The behavior of applyEach should be obvious from my attempts below. Output: applyEach [(+1), (+3), (+2)] 1 => [2,4,3] :: [Integer] Recursive version: > applyEach :: [a->b] -> a -> [b] > applyEach [] _ = [] > applyEach (h:t) x = (h x) : (applyEach t x) Now with higher order functions: > applyEach' :: [a->b] -> a -> [b] > applyEach' funs x = map applyx funs where applyx (fun) = fun x With Lambda: > applyEach'' :: [a->b] -> a -> [b] > applyEach'' funs x = map (\fun-> fun x) funs With Currying: ? peace, isaac P.S. I'm enjoying this book a great deal :-)

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RE: Rational sequence
by Simon Peyton-Jones 22 Oct '02

22 Oct '02

The Report says that the Enum instance for Ratio uses the same rule as for Float/Double, namely that [a..b] means takeWhile (<= (b+1/2)) [a, a+1, a+2, ...] You may say that the "<=" should be "<" but that's what the Report says. Certainly if you do [1%3..10%3] you'll get more values than your rule suggests. I'm not sure what your rule should be, though. (What about [2%4, ... 20%4]?) Anyway, it's a bit late to change the Report Simon | -----Original Message----- | From: Ferenc Wagner [mailto:wferi@bolyai1.elte.hu] | Sent: 22 October 2002 11:12 | To: haskell-cafe(a)haskell.org | Subject: Rational sequence | | With GHC-5.02.2, I do | | $ ghci | Prelude> :m Ratio | Ratio> [1%2..10%2] | [1 % 2,3 % 2,5 % 2,7 % 2,9 % 2,11 % 2] | | The question is, why is there 11%2 at the end of the list? | It's inconsistent with the (good) rules for Integer, since | | Ratio> [1,3..10] | [1,3,5,7,9] | | Is this intentional? | Feri. | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Re: Haskell-Cafe digest, Vol 1 #518 - 4 msgs
by Yoann Padioleau 21 Oct '02

21 Oct '02

"Berron Lara Virginia" <berron(a)nuyoo.utm.mx> writes: > Hi everybody! > Is there any tool for draWING the graph of a function in one or two variables what do you mean ? you mean drawing the trace of execution of a function ? cos there is the Hat tools. > in Haskell? > Thanks in advance > Virgin > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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FFI and GHC
by João Ferreira 21 Oct '02

21 Oct '02

Hello all, I am trying to learn how to use FFI with GHC. I've tried the example in GHC's user's guide but it didn't worked. Here it is: 1) The haskell code =================== [bracaman@tucano hib]$ cat foo.hs foreign export ccall foo :: Int -> IO Int foo :: Int -> IO Int foo n = return (length (f n)) f :: Int -> [Int] f 0 = [] f n = n:(f (n-1)) 2) The error ============ [bracaman@tucano hib]$ ghc -c foo.hs foo.hs:3: Type signature given for an expression [bracaman@tucano hib]$ I just can't understand why this is happening... can someone, please, help? Thank you in advance, João Ferreira

3 2