- Haskell-Cafe - Haskell.org

Haskell Cheat Sheet?
by Evan Klitzke 27 Sep '07

27 Sep '07

Has anybody made (or have a link to) a Haskell reference cheat sheet? I'm thinking of a nice LaTeXed PDF in the 1-10 page range (e.g. something like this http://www.tug.org/texshowcase/cheat.pdf) with the basics of the language syntax, the type declarations for the common type classes, the type signatures of the most commonly used functions in the Prelude and other common modules, and so forth? The Haskell standard library is very large for a newcomer (even just the Prelude!), and as a learner of the language I find myself spending a lot of time looking up Prelude functions and syntax details -- having all of this in a short PDF document that I could have offline would be very useful. -- Evan Klitzke <evan(a)yelp.com>

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Troubles understanding memoization in SOE
by Peter Verswyvelen 27 Sep '07

27 Sep '07

Hi, in SOE, the following memoization function is implemented: memo1 :: (a->b) -> (a->b) memo1 f = unsafePerformIO $ do cache <- newIORef [] return $ \x -> unsafePerformIO $ do vals <- readIORef cache case x `inCache` vals of Nothing -> do let y = f x writeIORef cache [(x,y)] -- ((x,y) : -- if null vals then [] else [head vals]) return y Just y -> do return y inCache :: a -> [(a,b)] -> Maybe b x `inCache` [] = Nothing x `inCache` ((x',y'):xys) = if unsafePtrEq x x' then Just y' else x `inCache` xys This is then used in type Time = Float type UserAction = G.Event data G.Event = Key Char Bool | Button Point Bool Bool | MouseMove Point | Resize | Closed deriving Show newtype Behavior a = Behavior (([Maybe UserAction],[Time]) -> [a]) newtype Event a = Event (([Maybe UserAction],[Time]) -> [Maybe a]) Behavior fb `untilB` Event fe = memoB $ Behavior (\uts@(us,ts) -> loop us ts (fe uts) (fb uts)) where loop (_:us) (_:ts) ~(e:es) (b:bs) = b : case e of Nothing -> loop us ts es bs Just (Behavior fb') -> fb' (us,ts) memoB :: Behavior a -> Behavior a memoB (Behavior fb) = Behavior (memo1 fb) If I understand it correctly, the memoization is required because otherwise recursive "streams" wouldn't work. For example, in the Pong game example, a ballPositionX stream is generated by integrating a ballVelocityX stream, but the ballVelocityX stream changes sign when the ball hits the left or right walls, and to determine that event, the ballPositionX stream is required. So both streams are mutually recursive, and without memoization, the program would be stuck (at least my own FRP experiments, which don't use memoization yet, gets stuck :-)). Another trick to prevent this, is the "b : case e of" code in untilB, which causes the event to be handled a bit too late, to avoid cyclic interdependencies. I hope I got that right. Now my questions. So, the keys (x) and values (y) in (memo1 fb) are streams (aka infinite lists)? More correctly, memo1 uses a pointer to the head of the list as a key, for fast comparing (as you can't compare infinite lists)? But since both key and value are infinite streams, won't this approach cause a serious space leak because the whole list cannot be reclaimed by the garbage collector? So the full ballPositionX and ballVelocityX streams would remain in memory, until the program exits? Since this doesn't happen when I run the SOE examples (I guess!), I clearly misunderstand this whole thing. I could explain it when the pointer to the list is actually a pointer to the delayed computation (a "thunk"?) of the tail, but the code doesn't seem to do that. Thanks for any help, I hope I explained the problem well enough. Peter Verswyvelen

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YI on Windows?
by Peter Verswyvelen 26 Sep '07

26 Sep '07

I retrieved the latest version of YI, but I failed to compile it for Windows (GHC 6.6.1, and I got alex-2.1.0, haddock-0.8, HsColour, GTK, but not VTY) Before even pasting the error log, is it supported in Windows? I see that "make" gave the error "ghc.exe: unknown package: unix" and some more errors. "make emacs" worked better, but failed starting "dist/build/yi/yi -B. -fgtk --as=emacs" with the error "\driver\package.conf.inplaceonf as c:\app\ghc" Thanks, Peter BTW: I was using MinGW/Msys

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Does Haddock respect hiding?
by Adrian Hey 26 Sep '07

26 Sep '07

Hello folks, Using Haddock 0.8, if I use haddock to document a module like this.. module MyModule ( module MyOtherModule ) where import MyOtherModule hiding (foo) I still get foo documented in the API for MyModule, despite the fact that foo is not really made available by importing MyModule. This looks like a bug to me, or maybe there's something else I should be doing. Anyone care to comment? Thanks -- Adrian Hey

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GHC 6.7 on Windows / containers-0.1 package?
by Peter Verswyvelen 26 Sep '07

26 Sep '07

Since I'm hopelessly stuck with an weird infinite loop (that consumes 0% CPU time!), and since I can't find the problem using "trace", I wanted to try GHC 6.7, which seems to container a debugger (woohoo!!!). Currently I'm using GHC-6.6.1 on Windows. So I grabbed ghc-6.7.20070824 (=the latest one for Windows I could find) and the "extra-libs", compiled and installed the GLUT package (which I needed), but when I compile my library, I get Could not find module `Data.Map': it is a member of package containers-0.1, which is hidden So I tried to expose it using GHC-PKG expose containers-0.1 which worked find, because GHC-PKG list prints d:/app/ghc/ghc-6.7.20070824\package.conf: Cabal-1.1.7, GLUT-2.1.1, HUnit-1.1.1, OpenGL-2.2.1, QuickCheck-1.0.1, Win32-2.1, array-0.1, arrows-0.2.1, base-2.1, bytestring-0.1, cgi-3001.1.5, containers-0.1, directory-1.0, fgl-5.4.1, filepath-1.0, (ghc-6.7.20070824), haskell-src-1.0.1, haskell98-1.0, hpc-0.5, html-1.0.1, mtl-1.0.1, network-2.0.1, old-locale-1.0, old-time-1.0, packedstring-0.1, parallel-1.0, parsec-2.0, pretty-1.0, process-1.0, random-1.0, regex-base-0.72, rts-1.0, stm-2.1, template-haskell-0.1, xhtml-3000.0.2 Anybody has an idea how I can continue? Thanks (again!), Peter

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C's fmod in Haskell
by Peter Verswyvelen 26 Sep '07

26 Sep '07

Maybe this is a stupid question, but I don't find something similar in Haskell. I find mod and rem, which work on integers. But I'm looking for a function similar to C's fmod. Of course I can write it myself, but I guess it must already exist under a different name? Thanks, Peter

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Re: Math.Statistics
by ok 26 Sep '07

26 Sep '07

There are a number of interesting issues raised by mbeddoe's Math.Statistics. 0. Coding issues. Why use foldr1 (*) instead of product? covm xs = split' (length xs) cs where cs = [ cov a b | a <- xs, b <- xs] split' n = unfoldr (\y -> if null y then Nothing else Just $ splitAt n y) seems a rather odd way to write covm xs = [[cov a b | b <- xs] | a <- xs] which in turn is a bit wasteful because the matrix must be symmetric. I believe the author may have misunderstood "numerically stable". The obvious (sum xs)/(fromIntegral $ length $ xs) is fine for the mean, and the obvious two-pass algorithm for the variance is fine. The tricky algorithms are needed for incremental one-pass calculation. 1. How do you decide how general to make things? mean, hmean: Floating a But only + and / are needed, so Fractional a is enough. gmean: Floating a You need + and /, so fair enough. median: Floating a, Ord a High median and low median (not provided) need only Ord a. Standard median needs + and /, so fair enough. modes: Ord a Right. range: Num a, Ord a But range needs only Ord a. avgdev: Floating a Right. pvar, var, stddev, skew, kurtosis, cov, covm: Floating a How should one define the variance of a collection of complex numbers? I'm not at all sure that it makes sense. If I were dealing with complex numbers, I might treat them as 2-vectors, in which case the "standard deviation" would be a 2x2 matrix, not a complex number. I suspect that these should require Floating a, Ord a. For skew this is quite certain: one looks for "positive" or "negative" skew, and this makes no sense for complex numbers. iqr: type not specified but appears to be :: [a] -> [a] This is certainly the wrong type as the inter-quartile range is supposed to be a NUMBER, not a sequence. You have to use comparisons and subtraction, so iqr :: Num a, Ord a => [a] -> a 2. Statistics This is an odd bunch of things; mostly things that are dangerous to use. A collection of the techniques from "Applications, Basics, and Computing" by Hoaglin and Velleman (and was there someone else? memory fails me) would be nice. A few basic robust measures like - median absolute deviation - trimmed mean - Winsorized standard deviation - percentage bend correlation might be nice. 3. How laziness could help. Suppose you ask for the mean, standard deviation, and skewness of a bunch of numbers using this library. Calculating the standard deviation will recalculate the mean. Calculating the skewness will recalculate the standard deviation, which will recalculate the mean. Robust statistics, like the trimmed mean, often want to sort the data. And you don't want to keep on sorting the data over and over again. This is a case where Haskell's laziness really shines: it translates into direct practical gains for simple code. data (Floating a, Ord a) => Simple_Continuous_Variate a = SCV [a] Int a a (Array Int a) list_to_variate xs = SCV xs n m s o where n = length xs m = sum xs / fromIntegral n s = sum [(x - m)^2 | x <- xs] / fromIntegral (n - 1) o = listArray (1,n) (sort xs) vLength (SCV _ n _ _ _) = n vMean (SCV _ _ m _ _) = m vSd (SCV _ _ _ s _) = s vMin (SCV _ _ _ _ a) = a ! 1 vMax (SCV _ n _ _ a) = a ! n vRange scv = vMax scv - vMin scv vMedian (SCV _ n _ _ a) | odd n = a ! ((n+1)`div`2) | even n = ((a ! l) + (a ! u))/2 where l = n `div` 2 u = n - l ..... The wonderful thing about this is that there are no bangs in SCV, so the various pieces of information don't get evaluated until they are needed, and then don't get evaluated *again*. For example, vRange will cause the sorted array to be built at most once. Trimmed mean is very little harder to write than vMedian. Esoteric uses of lazy evaluation abound, but this one makes sense even to an old number-crunching programmer. I once built something like this in C, but boy, it was a pain.

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Pierce on type theory and category theory
by Seth Gordon 26 Sep '07

26 Sep '07

Are Benjamin C. Pierce's _Types and Programming Languages_ and/or _Basic Category Theory for Computer Scientists_ suitable for self-study? (Do they have problem sets that can be checked by either looking up answers in The Back of the Book, or by trying to compile/run some code that the student writes?) FWIW, my formal math education--well, all the formal math education that I can remember at this point--stopped at freshman calculus plus one semester of statistics.

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Re: [Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
by Andrzej Jaworski 26 Sep '07

26 Sep '07

Flooding haskell-cafe with extra traffic from haskel@ will lead to comp.soft-sys.matlab syndrom where few people read anything but their own postings and discussion is virtually unknown. I can see only two options available to us right now to preserve readability in the fast growing Haskell community: divide haskell@ into more specific lists (haskell-cafe should preserve its right to long threads !!!) or ascribe volunteers (on monthly bases) to moderate and process haskell@ so as to receive tagged tree of links. -A.J.

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Re: [Haskell-cafe] Composition Operator
by PR Stanley 26 Sep '07

26 Sep '07

Ah, I understand now. Let me get this right: The resulting (a -> c) is a kind of abstract function formed by the pairing of <a -> b) and (b -> c), hence the lambda \x -> g (f x), correct? Thanks, Paul At 05:11 22/09/2007, you wrote: >Hello, > >It's probably easiest to think of composition as a function which >takes two arguments (both functions), (g :: b -> c) and (f :: a -> b), >and returns a new function of type a -> c. We could write this >explicitly as > > composition :: (b -> c, a -> b) -> a -> c > composition (g,f) = \x -> g (f x) > >then (.) is the currying of composition: > > (.) = curry composition > >or > > (.) g f = \x -> g (f x) > >-Jeff > > >On 9/21/07, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > (.) :: (b -> c) -> (a -> b) -> (a -> c) > > While I understand the purpose and the semantics of the (.) operator > > I'm not sure about the above definition. > > Is the definition interpreted sequentially - (.) is a fun taht takes > > a fun of type (b -> c) and returns another fun of type (a -> b) etc? > > Any ideas? > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] Re: Very crazy
by Andrew Coppin 26 Sep '07

26 Sep '07

Chaddaï Fouché wrote: > 2007/9/25, Andrew Coppin <andrewcoppin(a)btinternet.com>: > > >> I just found it rather surprising. Every time *I* try to compose with >> functions of more than 1 argument, the type checker complains. >> > > There is no function that takes more than one argument in Haskell. ;-) > map _could_ be seen as a function with 2 arguments, but in this case > it's more useful to think of it as a function that take one argument > f, a function that turn 'a into 'b and turn it into a new function > that turn a list of 'a into a list of 'b. > This is why I found it so surprising - and annoying - that you can't use a 2-argument function in a point-free expression. For example, "zipWith (*)" expects two arguments, and yet sum . zipWith (*) fails to type-check. You just instead write \xs ys -> sum $ zipWith(*) xs ys which works as expected. I can't figure out why map . map works, but sum . zipWith (*) doesn't work. As I say, the only reason I can see is that the type checker hates me and wants to force me to write everything the long way round...

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Very crazy
by Andrew Coppin 26 Sep '07

26 Sep '07

While using Haskell, I often find myself writing convoluted constructions such as this: show_system = unlines . zipWith (\l ms -> "Eq" ++ show l ++ ": " ++ (concat $ intersperse " + " $ zipWith (\n x -> x ++ " x" ++ show n) [1..] (init ms)) ++ " = " ++ last ms ) [1..] . map (map (take 8 . show)) And people complain that *Perl* is bad? This function is quite obviously absurd. I mean, it works, but can *you* figure out what it does without running it? The question is, can anybody think of a better way to write this function? (And more generally, functions like it.)

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Re: [Haskell-cafe] Very crazy
by Andrew Coppin 26 Sep '07

26 Sep '07

Chaddaï Fouché wrote: > 2007/9/25, Andrew Coppin <andrewcoppin(a)btinternet.com>: > >>> printf don't always perform IO : if you ask it for a String it will >>> happily turn into sprintf for you, if you use it in the IO Monad, it >>> will indeed perform IO, but there's nothing fundamentally IO bound in >>> printf logic. >>> >>> >> That's even *more* impossible... o_O >> > > Why ? > How can one function have more than one type signature?

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Re: [Haskell-cafe] Haskell Cheat Sheet?
by Ben 26 Sep '07

26 Sep '07

+1 > Message: 9 > Date: Tue, 25 Sep 2007 13:04:56 -0700 > From: Evan Klitzke <evan(a)yelp.com> > Subject: [Haskell-cafe] Haskell Cheat Sheet? > To: haskell-cafe(a)haskell.org > Message-ID: <1190750696.5459.7.camel@thinkpad> > Content-Type: text/plain > > Has anybody made (or have a link to) a Haskell reference cheat sheet? > I'm thinking of a nice LaTeXed PDF in the 1-10 page range (e.g. > something like this http://www.tug.org/texshowcase/cheat.pdf) with the > basics of the language syntax, the type declarations for the common type > classes, the type signatures of the most commonly used functions in the > Prelude and other common modules, and so forth? The Haskell standard > library is very large for a newcomer (even just the Prelude!), and as a > learner of the language I find myself spending a lot of time looking up > Prelude functions and syntax details -- having all of this in a short > PDF document that I could have offline would be very useful. > > -- > Evan Klitzke <evan(a)yelp.com> > > > > ------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > End of Haskell-Cafe Digest, Vol 49, Issue 118 > ********************************************* >

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Re: [Haskell-cafe] Troubles understanding memoization in SOE
by Peter Verswyvelen 26 Sep '07

26 Sep '07

Thanks Paul & Paul for the answers. I'll certainly read the paper Paul Liu reported. I just deleted 100 lines of text which explained my problem in more detail, and while I was explaining it, I answered it myself. Typical. I thought the lambda function that memo1 returns would be called over and over again, and instead of reevaluating the stream from the beginning, it would just return the stream since it is in the cache, but actually it just gets called twice in recursive situations: the first time it evaluates y = f x, stores the thunk in the cache, and returns the thunk, the second time it finds the same thunk in the cache, and then computation of the rest of the stream continues without consulting the cache anymore right? From my clumsy explanation you can see that I'm still thinking too imperative, too eager. Haskell is more lazy than I am, which is an incredible achievement :-) It would really help if I could see the lazy computation; do you think this kind of memo code is traceable using HAT? I'll guess I'll have to check out arrows / yampa again. A year ago I did not understand a single thing in those papers, but I should try it again now I read the SOE book :-) Thanks a lot, Peter

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Shouldnt this be lazy too?
by Vimal 25 Sep '07

25 Sep '07

Hi all, I was surprised to find out that the following piece of code: > length [1..] > 10 isnt lazily evaluated! I wouldnt expect this to be a bug, but in this case, shouldnt the computation end when the length function evaluation goes something like: > 10 + length [11..] ? -- -- Vimal

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representing differencial equations in haskell
by Thomas Girod 25 Sep '07

25 Sep '07

Hi there. Let's say I have mathematical model composed of several differential equations, such as : di/dt = cos(i) dc/dt = alpha * (i(t) - c(t)) (sorry my maths are really bad, but I hope you get the point) I would like to approximate the evolution of such a system iteratively. How would you do that in haskell ? cheers, Thomas

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Template Haskell newbie questions
by Hugo Pacheco 25 Sep '07

25 Sep '07

Hi all, I'm try to write some function in TH that I don't even know if it is possible. Consider the example from the tutorials sel 1 2 = [| \(x,_) -> x |] sel 2 2 = [| \(_,x) -> x |] If I want to write some function that will dynamically create a selection function according to its arguments and do something with it. fsel :: Int -> Int -> x fsel x y = $(sel x y) ... Imagine a stupid case for fsel :: Int -> Int -> Int fsel x y = $(sel x y) (x,y) The compiler complains that GHC stage restriction: `x' is used in a top-level splice, and must be imported, not defined locally In the first argument of `sel', namely `x' In the expression: $[splice](sel x y) (x, y) In the definition of `fsel': fsel x y = $[splice](sel x y) (x, y) what, indeed, makes sense. However, is there a possible way to generate Haskell functions from TH without having to instanciate its arguments? Thanks, hugo

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Re: [Haskell] Re: [Haskell-cafe] PROPOSAL: Rename haskell@to haskell-announce@
by Andrzej Jaworski 25 Sep '07

25 Sep '07

> (...) I subscribe to haskell so as not to miss > anything important, and when something I'm interested in moves to > haskell-cafe, (...) Wrong assumption. Even if you know the author it makes more sense to wait and see how others respond to his newest input. This means that you should check up Cafe first to see if it is worth moving to Cafe;-) Another false believe is that best things appear first in Haskell@. On the contrary, long threads in Cafe indicate that participants know what to expect of each other and pick up a thread on the same principle some pick up new Stephen King novel. Before reviews on the amazon.com publishers had to result to tricks like labelling a collection of science fiction stories by various lesser known authors with Isaac Asimov name even when his only cotribution was a preface. As I said ealier - really big traffic will need intelligent screening and lebbeling. And to what pathology it leads when unattended see comp.soft-sys.matlab. Cheers, -Andrzej Jaworski

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Re: [Haskell-cafe] Very crazy
by Miguel Mitrofanov 25 Sep '07

25 Sep '07

> How can one function have more than one type signature? It's called "polymorphism".

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Re: [Haskell] simple function: stack overflow in hugs vs none in ghc
by Isaac Dupree 25 Sep '07

25 Sep '07

(redirecting to haskell-cafe) Tom Pledger wrote: > | > sqnc p ts = let ( r, ts' ) = p ts in case r of > | > Nothing -> ([],ts') > | > Just x -> let (r',ts'') = (sqnc p ts') in > (x:r', ts'' ) > : > > > I don't know how ghc is avoiding the stack overflow, but it looks like > it's caused by strict pattern matching. The first call to sqnc wants > reassurance that the second call is returning a pair (as opposed to _|_) > before deciding that it's OK to return a pair. > > Try putting a tilde in the recursive call, to make the pattern lazy. > > let ~(r',ts'') = ... "let" already is lazy -- putting a tilde there has no effect. In fact that tuple is produced from (x:r', ts'' ) even if (sqnc p ts') is _|_. (The "let" at the beginning of the function can be discovered to be strict by strictness analysis (which GHC does), because "r" is analyzed by the "case", and the pair needs to be deconstructed to find the value of "r".) Isaac

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Re: [Haskell-cafe] Very crazy
by Andrew Coppin 25 Sep '07

25 Sep '07

Chaddaï Fouché wrote: > 2007/9/25, Andrew Coppin <andrewcoppin(a)btinternet.com>: > >> Forget PrintfType - I can't even understand the haddoc page yet! >> >> (printf performs I/O, yet it is outside the I/O monad. It seems to >> accept an arbitrary number of arguments, which is obviously impossible. >> It's *almost* as scary as the original. Although the original will crash >> your program is you use it wr... oh, so does this one. Cool.) >> > > printf don't always perform IO : if you ask it for a String it will > happily turn into sprintf for you, if you use it in the IO Monad, it > will indeed perform IO, but there's nothing fundamentally IO bound in > printf logic. > That's even *more* impossible... o_O > Well printf crash your program because it can't check if you're > arguments correspond to the chain at compile time (quite obviously > since the chain itself can be dynamic), but it won't corrupt memory or > anything from this kind, so it's arguably much better than the C one. > All I know is that in C, any attempt to use printf() has an 80% probability of causing the machine to dump the entire contents of RAM to stdout until it happens upon a zero byte. Probably the most annoying part was all the 0x07 bytes in there. Do you know what a room full of beeping PCs is like? I do... Sometimes when we got bored, we would "race" our PCs to see which one would stop beeping first. (But they stopped that when I worked out how to make mine "win" every race...)

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bindings to the xmms_remote API, GList and something more
by Andrea Rossato 25 Sep '07

25 Sep '07

Hi, I think there's a huge flaw in the Haskell design: if you conceive such a powerful and expressive programming language, one that empowers even the most ignorant computer user to write complex applications, instead of sticking to basic shell scripting - the only domain graspable by such stupid pseudo-coders -, well you end up with messages like this one flooding the mailing list you are providing for your users. This is what I call inevitability... To make a short story long, I needed some client for the Audacious media player, something I could use to remote control it and, since I'm addicted to Haskell, instead of surfing the web to find a suitable client I surfed the web to find the API documentation. With the FFI, in a matter of half an hour I had the client with all the commands I needed. (The long introduction was written just to let you know that you'd better not waste too much time with what follows...;-) So far so good, but, since everything was so easy, I thought: "Why not packaging a library (and the client) for Hackage? Maybe others may be willing to use it, maybe..." And then the problems start, problems due to my basic ignorance of the C language ("How comes you dare to import in Haskell functions written in a language you don't even understand?" you should replay!). Basically there are two C types I'm having problem with: 1. GList: even though I read the gtk2hs code I do not exactly understand how to create a GList to feed to this function: void xmms_remote_playlist_add(gint session, GList * list); As far as I understand this function takes a session number and a list of files' names. Still I seem not to be able to create a wrapper function around the imported one. 2. what a "gchar ** list" is? As far as my C goes, it should be an array of strings, right? What should I use on the Haskell side, newArray? void xmms_remote_playlist(gint session, gchar ** list, gint num, gboolean enqueue); 3. c2hs v. hsc2hs? Which should I prefer? In c2hs I write {#pointer * GList#}. What is the equivalent in hsc2hs? 4. As I said, I solved my personal problem: I imported enough functions to create the client I needed. Releasing the library is just something I would like to do, to give something back to the Haskell community. So this is something useful if I can provide robust and well designed code. This is a name space question: what is the name of the exported module? Sound.XMMS, Sound.XmmsRemote? Is the some kind of convention I should be referring to? Sorry if I was too long. Moreover I'm asking help I need just to publish a piece of software that is probably useless. Do not waste your time. But if you do, well... thank you, I really appreciate. Andrea

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PROPOSAL: Rename haskell@ to haskell-announce@
by Neil Mitchell 25 Sep '07

25 Sep '07

Hi I've just replied to another first poster with "wrong list". Its entirely not their fault, but its also probably a bit off-putting that your very first post gets a (very polite) "you got it wrong" message. To steal the reasons and explanations from Ian: --------------------- pretty much what would be haskell-announce@ anywhere else, and haskell-cafe@ what would be haskell@ elsewhere. A haskell-cafe@ list elsewhere would probably be where the haskell@ people discuss things which aren't actually related to Haskell (like e.g. the demon.local newsgroup). I think the number of posts in the wrong place would be lower if these were more conventionally named (although there aren't a lot of them anyway). I think it would make sense to: * Rename haskell@ to haskell-announce@, and redirect mails from haskell@ to haskell-announce@ for some period. (more, but can be done step by step in the future) --------------------- I suggest that following somewhat the http://www.haskell.org/haskellwiki/Library_submissions we allow 4 weeks for discussion, and depending on the result we then move the mailing list. Bug to track: http://hackage.haskell.org/trac/ghc/ticket/1732 Deadline: 23 October 2007 Thanks Neil

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non POSIX/Win32 API support
by Galchin Vasili 25 Sep '07

25 Sep '07

Hello, I want to provide support for another OS. How are the POISIX modules implemented? E.g. in terms of Green Card? Kind regards, Bill Halchin

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Re:[Haskell] Math behind Haskell
by Tomas Caithaml 25 Sep '07

25 Sep '07

Hi and thanks for that many replies! > The haskell-cafe@ mailing list is more appropriate for messages such > as this. haskell@ is just for announcements (it should be called > haskell-annouce@ !) Sorry for misposting. > I'd add: > * Discrete Maths - booleans, relations, functions etc. > * Type theory > * Logic programming (Prolog) > * Semantics > Most of these are computer science courses, rather than maths ones. My formulation was somewhat unfortunate on this point. I wasn't strictly looking for math courses, so Type Theory a Semantics seems to be a very good suggestions. Alas, I do not recall any course named "Type Theory" being taught here. But thanks for these hints. I should have probably also noted that I am not new to university (4th year), so I have already taken some courses. Notably: several semesters of real analysis Linear algebra, Algebra Discrete mathematics and graph theory few algorithm and datastructure courses, Logic Programming introduction courses into: Propositional and Predicate logic Computability theory Computational complexity theory Set Theory And I think I have enough of basic mathematical fluency to be able to follow (at least to some degree) any math course if it is not extra hard or if it does not heavily depend on other courses. >Just take the computer science courses that the fewest of your fellow > students see any point to. Well, there are too many of those :-) Thanks, Tom.

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Re: [Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
by Simon Michael 25 Sep '07

25 Sep '07

The gmane newsgroups corresponding to haskell mail lists can be found eg at: http://gmane.org/find.php?list=haskell

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Complexity question
by Andrew Coppin 25 Sep '07

25 Sep '07

Greetings. Anybody happen to know what the time complexity of "transpose" is?

5 5

Re: [Haskell] Math behind Haskell
by Neil Mitchell 24 Sep '07

24 Sep '07

Hi The haskell-cafe@ mailing list is more appropriate for messages such as this. haskell@ is just for announcements (it should be called haskell-annouce@ !) > * Lambda calculus - the basis of functional languages > > * Category theory - where all these mysterious things like monads, > arrows, and functors come from. I'd add: * Discrete Maths - booleans, relations, functions etc. * Type theory * Logic programming (Prolog) * Semantics Most of these are computer science courses, rather than maths ones. Thanks Neil

4 3

PROPOSAL: Rename haskell@ to haskell-announce@
by Andrzej Jaworski 24 Sep '07

24 Sep '07

Sorry guys for joining the thread of sinners on haskell@ today. Forgot to check if guests had not moved to cafe;-) It is a good example though how insufficient this verbal principles are, since I have myself raised the need to revise names of the lists here. On second thought, with rising traffic the problem should be approached by introducing AI automation - sort of scunning, learning, summerising and sending tagged abstracts onto haskel@. In other words the only possible input onto haskell list should be robotic:-) Be brave guys, you are ment to make history:-) Cheers, -Andrzej Jaworski

1 0

Noob question and sequence of operations (print then do something else)
by John Wicket 24 Sep '07

24 Sep '07

I am still in an imperative way of thinking. In this example here; how would I call "putStrLn" and then set the function with a value. Eg: aa :: String -> IO () aa instr = do putStrLn "abc" putStrLn "abc" return "123" --- The error I am getting. Couldn't match expected type `()' against inferred type `[Char]' In the first argument of `return', namely `"123"' In the expression: return "123" In the expression: do putStrLn "abc" putStrLn "abc" return "123"

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Re: [Haskell] Re: [Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
by Neil Mitchell 24 Sep '07

24 Sep '07

Hi > Forgive me, but I would much prefer a newsgroup to a mailing list. > > True, I could unsubscribe now and just browse the mailman archives - > but for posting, I'd have to temporarily re-subscribe, which is awkward. > (Indeed that's the only reason I'm not doing it.) I believe you can post from the gmane archives, and that they also offer a newsgroup feed of this list. I think the general consensus (and my personal view) is that a mailing list is the preferred mechanism for most people. Thanks Neil > > We already have comp.lang.haskell. - Best regards, > -- > -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- > ---- http://www.imn.htwk-leipzig.de/~waldmann/ ------- > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0

Re: [Haskell] Math behind Haskell
by Andrzej Jaworski 24 Sep '07

24 Sep '07

Tomas Caithaml wrote: > Hi all. > > When I was playing with Haskell and reading stuff about it, I noticed > that there is a lot of information in a form of research papers and a > lot of mathematics involved. I must confess that I quite like this > academic approach. > > Well, I thought that while I am still at university I could take some > math classes to help me better understand these things and > broaden my horizon. > > Any suggestion what could be relevant? > > I came up with: > > * Lambda calculus - the basis of functional languages > > * Category theory - where all these mysterious things like monads, > arrows, and functors come from. > > * Topology (?) - don't know; I thought it is important rather in > mathematical analysis (which I don't like much) but I have seen > somewhere some references to it in context of CS. > > Any other suggestions? All the advice you got was good but I will add: never go after one thing in mathematics before you really know what you are doing. Topology is probably best intellectually but must be combined with functional analysis or algebra to have something to experiment with. Similarly category theory alone will never teach you to think mathematically - to build models. For CS inclined entering into logic study is today a very rewording experiance but you must feel confident with CT for reading modern proof theory and additionally operator algebra entering into linear logic. The reward: you get closer to functional programming or quantum mechanics without even knowing that:-) Digging in Haskell can make you fluent in writing some computational stuff to experiment with, but it will most certainly follow thought experiment not vice versa. Relying on such hybrid thinking too much can be harmful though. A mathematician follows intuition, ideally he should not even plan what he will do before he spots something that will drive his attention. Programming is the opposite - planning lies at the heart of it. Even languages that fancy mathematical concepts belong still to technology and thechnology is to eliminate problems and not to investigate them. Mathematics and logic will definitelly influence CS more with each decade untill it becomes a science like phisics. However how shellow this science still is in CS today demonstrate the havoc with multicore, where linguistics, model theory, tamplet programming, object programming, Haskell(?) compete head in head to cover the bare ass... Cheers, -Andrzej Jaworski Moving to Haskell Cafe <haskell-cafe(a)haskell.org>

1 0

Library Process (was Building "production stable" software in Haskell)
by Dominic Steinitz 24 Sep '07

24 Sep '07

This discussion has sparked a question in my mind: What is the process for the inclusion of modules / packages in ghc, hugs and other compilers & interpreters? I thought the master plan was that less would come with the compiler / interpreter and the user would install packages using cabal. I was thus mildly surprised earlier this year to find e.g. data.bytestring was included with ghc. Now that I look at the haddock, it says this is a base package but doesn't say anything about its status. On the other hand, there are lots of modules that are base packages that are experimental. Should something experimental be a base package? And shouldn't all modules that are base packages declare their status? Perhaps these are questions for the libraries mailing list but I thought I'd start here. Yours confused of Kingston.

17 20

Re: [Haskell-cafe] Win32 Open GL / Glut Applications
by Ronald Guida 24 Sep '07

24 Sep '07

John Wicket wrote: > yea, that is probably what I need. Can you post in a step-by-step way. Here is a set of instructions for what I had to do to get FreeGLUT working with GHCi. <RANT> Just a little warning: my instructions could be wrong in some places. Normally, in order to verify my instructions, I would uninstall everything, start from scratch, follow my instructions step by step, and verify that they work. Unfortunately: * I have low confidence in the accuracy of the following instructions. * I have only one machine. * I went through lots of frustration to get freeglut to work with GHCi. * I am not willing to risk breaking my current installation. * I do not know of any easy way to preserve my current installation and still start over from scratch to test my instuctions. </RANT> If my instructions are wrong, then please let me know, and I might attempt to fix it. Although I just don't understand why freeglut, the Haskell GLUT library, and GHCi won't work together in the first place. Here's what I would have to do if I were starting from scratch: 7-Zip http://www.7-zip.org/ This is an open source file archiver for Windows that can handle *.gz and *.bz2 files. Download the Windows version and install it. GHC 6.6.1 http://haskell.org/ghc/download_ghc_661.html#windows This is the version of GHC that I'm using. Download and install it. MinGW/MSYS Download all three files, then install them in the order I have listed them. Note: MinGW and MSYS need to "live" in different directories. http://sourceforge.net/project/showfiles.php?group_id=2435 Automated MinGW installer MinGW-5.1.3.exe ** When you run the installer, it will download several more files for you. MSYS: Minimal System MSYS-1.0.10.exe http://sourceforge.net/project/showfiles.php?group_id=2435&package_id=67879 MSYS Supplementary Tools msysDTK-1.0.1.exe ** Scroll down to find "Current Release: msysDTK-1.0.1" and expand this tab. Select "msysDTK-1.0.1.exe" zlib-1.2.3 http://www.zlib.net/ The links for "zlib source code" are about halfway down the page. After downloading, the process (IIRC) is 1. Unzip the source code. 2. Start MSYS and cd to the source-code directory. 3. Execute "./configure" 4. Execute "make" 5. Execute "make install" I don't remember having any problems with zlib. cURL-7.16.4 http://curl.haxx.se/download.html Note: It appears that cURL just had a minor revision on Sept 13. I guess you can just try the latest version and see of things work. IIRC, the process is exactly the same as the zlib install process, and I didn't have any problems here either. darcs-1.0.9 http://darcs.net/darcs-1.0.9.tar.gz Dependencies that you'll need: cURL and zlib :-) Your darcs installation will have some holes, for example it won't support SSL because we didn't satisfy the SSL dependency. IIRC, the process here is 1. Unzip the source code. 2. Start MSYS and cd to the source-code directory. 3. Execute "autoconf" 4. Execute "./configure" 5. Execute "make" 6. Execute "make install" Freeglut-2.4.0 http://freeglut.sourceforge.net/index.php#download Here's where the hackery starts. 1. Download the Freeglut source code and unzip it. 2. Start MSYS and cd to the source-code directory. 3. Execute "./configure" 4. Download this custom makefile and put in in the "./src" directory. http://hpaste.org/2841 5. Download this custom def file and put in in the "./src" directory. http://hpaste.org/2842 6. Cd to the "./src" directory and then execute "make". 7. Copy the "freeglut.dll" to GHC's bin directory (ghc-6.6.1/bin). 8. Copy the *.h files from "freeglut-2.4.0/include/GL" to "ghc-6.6.1/include/mingw/GL". Note that "glut.h" will be overwritten. GLUT-2.1.1 You need to use darcs to download GLUT-2.1.1. 1. Start MSYS, create a directory for the GLUT source code, and cd to it. 2. Execute "darcs get http://darcs.haskell.org/libraries/GLUT/" and wait a while. 3. Cd to your new source directory. 4. Execute "autoreconf" and wait a while. 5. Modify "GLUT.cabal" as follows: Locate the line start starts with "build-depends:" and remove the dependencies "array" and "containers" 5. Execute "runghc Setup.hs configure" and wait. 6. Modify "GLUT-2.1.1/Graphics/UI/GLUT/Extensions.hs" as follows: Look at the last two lines: foreign import ccall unsafe "hs_GLUT_getProcAddress" hs_GLUT_getProcAddress :: CString -> IO (FunPtr a) Change "hs_GLUT_getProcAddress" to "glutGetProcAddress" 7. Modify "GLUT-2.1.1/cbits/HsGLUT.c" as follows: Look for "void* hs_GLUT_getProcAddress(char *procName)" and remove the whole function. 8. Execute "runghc Setup.hs build" and wait. 9. Execute "ghc-pkc unregister GLUT". This unregisters the existing GLUT haskell library. Also, search through GHC's directories, locate any glut library files (*.a, *.o, *.hi) that are in there and remove them. You are deleting the existing GLUT that came with GHC 6.6.1 in order to replace it with Freeglut. 10. Execute "runghc Setup.hs install". 11. In GHC's directory, there is a file named "package.conf". This file contains one extremely long line. You need to find an editor that will let you insert some text into this line without introducing any line breaks. Emacs can do this. You need to locate the text << pkgName = "GLUT" >> and then you need to locate << hsLibraries = ["HSGLUT-2.1.1"] >> to the right of there. The very next thing to the right of "hsLibraries" should be << extraLibraries = [] >>. You need to change it to << extraLibraries = ["freeglut"] >>. 12. At this point, you should be able to start GHCi, load an example that depends on freeglut, and run it from GHCi, without getting any crashes or weird errors. 13. If you want to /compile/ with GHCi, then you'll need to copy the freeglut.dll file into the same directory as your newly-compiled program.exe file. I haven't tried static linking yet; that would require recompiling freeglut as a static library instead of a DLL. Good Luck! -- Ron

3 4

Haskell Weekly News: September 23, 2007
by Don Stewart 24 Sep '07

24 Sep '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20070923 Issue 65 - September 23, 2007 --------------------------------------------------------------------------- Welcome to issue 65 of HWN, a newsletter covering developments in the [1]Haskell community. This episode comes to you live from Portland, Oregon, where the HWN automaton now lives, running on a virtual host at Galois, Inc. 1. http://haskell.org/ Announcements Cabal 1.2.0 released. Thomas Schilling [2]announced that [3]Cabal version 1.2.0 is available. The major new feature in this release is support for Cabal configurations. This allows package authors to more easily adopt their package descriptions to different system parameters such as operating system, architecture, or compiler. In addition, some optional features may be enabled or disabled explicitly by the package user. 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7491 3. http://www.haskell.org/cabal SparseCheck. Matthew Naylor [4]announced SparseCheck, a library for typed, depth-bounded logic programming in Haskell allowing convenient expression of test-data generators for properties with sparse domains. More information on the [5]home page. 4. http://article.gmane.org/gmane.comp.lang.haskell.general/15593 5. http://www.cs.york.ac.uk/~mfn/sparsecheck/ monadLib 3.3.0. Iavor Diatchki [6]announced a new version of monadLib, adding the identiy transformer, and a family of deriving functions. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/15586 ListLlke. John Goerzen [7]announced ListLike, a generic interface to the various list-like structures in Haskell. 7. http://article.gmane.org/gmane.comp.lang.haskell.general/15582 HaL2: Meeting Haskell in Leipzig 2: videos. Klaus Meier [8]announced that the videos and presentations of the talks given at HaL2 are [9]now online. 8. http://article.gmane.org/gmane.comp.lang.haskell.general/15565 9. http://iba-cg.de/haskell.html The Monad.Reader: Issue 8. Wouter Swierstra [10]announced that the latest issue of The Monad.Reader is [11]now available. Issue 8 consists of the following two articles: Brent Yorgey on 'Generating Multiset Partitions' and Conrad Parker's 'Type-Level Instant Insanity' 10. http://article.gmane.org/gmane.comp.lang.haskell.general/15554 11. http://www.haskell.org/haskellwiki/The_Monad.Reader Haskell mode plugins for Vim. Claus Reinke [12]announced improved [13]haskell mode plugins for vim. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/15553 13. http://article.gmane.org/gmane.comp.lang.haskell.general/15553 hstats-0.1. Marshall Beddoe [14]announced a statistical computing module for Haskell. 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/28973 HIntegerByInt. Isaac Dupree [15]announced a [16]reimplementation, in Haskell, of the Integer type that Haskell provides in its Prelude. It is designed in mind of being actually usable as the implementation of that type, for compilers. It is also a module that exports a working Integer type. It is in terms of only basic Prelude functions, lists, and Int. It is NOT a purely inductive definition, because Int is much faster than a purely inductive definition would allow, and nevertheless often easier to come by (more portable, license-wise, size-wise, nuisance-wise...) than GMP or other C bignum libraries. 15. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/7285 16. http://isaac.cedarswampstudios.org/2007/HIntegerByInt OzHaskell: Australian Haskell Programmers Group. Manuel Chakravarty [17]started organising OzHaskell, and Australian Haskell user's group. 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/28883 Israeli Haskell Programmers Group. B K [18]also seeks to form an Israeli Haskell user's group 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/28877 xmonad 0.3. Don Stewart [19]announced the 0.3 release of [20]xmonad. xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/28425 20. http://xmonad.org/ HPDF 1.0. alpheccar [21]announced version 1.0 of the [22]HPDF library. 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/28371 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF-1.0 pcap: user-level network packet capture. Bryan O'Sullivan [23]announced the release of pcap 0.3.1 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/28139 Gtk2Hs Tutorial. Hans van Thiel [24]announced a port of the GTK2 [25]tutorial by Tony Gail and Ian Main to Haskell's gtk2hs. 24. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27954 25. http://tinyurl.com/yrbtsu An efficient lazy suffix tree library. Bryan O'Sullivan [26]posted a [27]suffix tree library to hackage. It implements Giegerich and Kurtz's lazy construction algorithm, with a few tweaks for better performance and resource usage. 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27891 27. http://www.serpentine.com/software/suffixtree/ Bay Area Functional Programmers. Keith Fahlgren [28]announced the formation of the [29]Bay Area Functional Programmers group. This group is for anyone using or interested in functional programming and functional programming languages, particularly strongly typed languages such as Haskell, OCaml and SML. 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27860 29. http://bayfp.org/ Haskell irc channel reaches 400 users. Don Stewart [30]noticed that, five and a half years after its inception, under the guiding hand of Shae Erisson (aka shapr), the [31]Haskell IRC channel on freenode has reached 400 users! 30. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27826 31. http://www.haskell.org/haskellwiki/IRC_channel Guihaskell and PropLang 0.1. Asumu Takikawa [32]announced the results of his Google Summer of Code project: Guihaskell, a graphical REPL using PropLang and work on PropLang, a GUI library built on Gtk2hs that allows for high level design. 32. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27598 HAppS-Data 0.9: XML, Pairs, HList, deriveAll. Alex Jacobson [33]announced that the components of [34]HAppS are being released as individual useful packages. HAppS-Data is the first in a series, and provides useful operations on XML data. 33. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27597 34. http://happs.org/ Introduction to proving Haskell code. Tim Newsham [35]put together [36]a small intro lesson on proving Haskell code using quickcheck, equational reasoning and Isabelle/HOL. 35. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27523 36. http://www.thenewsh.com/%7Enewsham/formal/reverse/ Very Fast Searching of ByteStrings. Chris Kuklewicz [37]announced a Boyer-Moore algorithm implemented for strict and lazy bytestrings (and combinations thereof). It finds all the overlapping instances of the pattern inside the target. 37. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7363 Infinity 0.1. Austin Seipp [38]announced Infinity v0.1. an IRC bot in the essence of lambdabot; that is, it should be extendable through plugins and plugins should be easy to write, modify and contribute. 38. http://article.gmane.org/gmane.comp.lang.haskell.cafe/27445 Haskell' This section covers the [39]Haskell' standardisation process. * [40]Instances of Read on bounded integral types should detect overflow 39. http://hackage.haskell.org/trac/haskell-prime 40. http://article.gmane.org/gmane.comp.lang.haskell.prime/2302 Libraries This week's proposals and extensions to the [41]standard libraries. * [42]Add &&& and *** to Data.Tuple * [43]Add System.Info.isWindows * [44]Make arrays safer * [45]GenT monad transformer variant of Gen * [46]Test.HUnit documentation * [47]Data.List.groupBy with non-transitive equality predicate * [48]Add dropped STM invariants functions * [49]Add Data.Eq.equating to match Data.Ord.comparing * [50]Fix abstract unix sockets 41. http://haskell.org/haskellwiki/Library_submissions 42. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/7550 43. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/7261 44. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/7283 45. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7294 46. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7394 47. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7400 48. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7598 49. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7594 50. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7544 Hackage This week's new libraries in [51]the Hackage library database. 51. http://hackage.haskell.org/ * numbers-2007.9.23. Lennart Augustsson. [52]numbers: instances of the numerical classes for a variety of different numbers. 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/numbers-2007.9.23 * hmp3 1.3. Don Stewart. [53]hmp3: An mp3 player with a curses frontend. 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmp3-1.3 * utf-string 0.2. Eric Mertens. [54]utf8-string: A UTF8 layer for IO and Strings. 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/utf8-string-0.2 * hstats 0.1. Marshall Beddoe. [55]hstats: A library of commonly used statistical functions. 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hstats-0.1 * sparsecheck 0.2. Matthew Naylor. [56]SparseCheck: a library for logic programming in Haskell that allows convenient description of test-data generators. 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sparsecheck-0.1 * monadLib 3.3.0. Iavor Diatchki. [57]monadLib: A collection of monad transformers. 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monadLib-3.3.0 * ListLike 1.0.0. John Goerzen. [58]ListLike: Generic support for list-like structures in Haskell. 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ListLike-1.0.0 * cabal-test 0.1. David Himmelstrup. [59]cabal-test: Cabal-test is a tool for testing cabal projects. 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-test-0.1 * HPDF 1.1. alpheccar. [60]HPDF: a PDF library. 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF-1.1 * hxt 7.3. Uwe Schmidt. [61]hxt: The Haskell XML Toolbox. 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hxt-7.3 * irc 0.1. Trevor Elliott. [62]irc: A small library for parsing IRC messages 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/irc-0.1 * hsdns 1.0. Peter Simons. [63]hsdns: an asynchronous DNS resolver based on GNU ADNS. 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsdns-1.0 * streamproc 1.0. Peter Simons. [64]streamproc: Stream Processer Arrow 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/streamproc-1.0 * hsemail 1.0. Peter Simons. [65]hsemail: Parsers for the syntax defined in RFC2821 and 2822 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsemail-1.0 * funcmp 1.0. Peter Simons. [66]funcmp: Functional MetaPost is a Haskell frontend to the MetaPost language 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/funcmp-1.0 * cabal-rpm 0.3.1. Bryan OSullivan. [67]cabal-rpm: This package turns Haskell Cabal source packages into source and binary RPM packages. 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-rpm-0.3.1 * Finance-Quote-Yahoo 0.3. Brad Clawsie. [68]Finance-Quote-Yahoo: Obtain quote data from finance.yahoo.com 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… * xmonad 0.3. Spencer Janssen. [69]xmonad: a minimalist tiling window manager for X 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-0.3 * hint 0.1. Daniel Gorin. [70]hint: an interpreter monad for Haskell expressions based on ghc-api 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hint-0.1 * ipprint 0.2. Gleb Alexeyev. [71]ipprint: Tiny helper for pretty-printing values in ghci console 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ipprint-0.2 * pandoc 0.44. John MacFarlane. [72]pandoc: Conversion between markup formats 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pandoc-0.44 * X11-extras 0.3. Spencer Janssen. [73]X11-extras: Missing bindings to the X11 graphics library 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-extras-0.3 * dsp 0.2. Matthew Donadio. [74]dsp: Digital Signal Processing, Fourier Transform, Filter design, Frequency estimation, Interpolation, Linear Algebra, Polynomials 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dsp-0.2 * c2hs 0.15.0. Manuel Chakravarty. [75]c2hs: C->Haskell assists in the development of Haskell bindings to C libraries. 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/c2hs-0.15.0 * regex-base 0.92. Chris Kuklewicz. [76]regex-base: Interface API for regex-posix,pcre,parsec,tdfa,dfa 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-base-0.92 * soegtk 0.9.12.2. Duncan Coutts. [77]soegtk: SOE api for gtk2hs 77. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/soegtk-0.9.12.2 * hsns 0.5.3. Austin Seipp. [78]hsns: a miniature network sniffer 78. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsns-0.5.3 * anydbm 1.0.4. John Goerzen. [79]anydbm: Interface for DBM-like database systems 79. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/anydbm-1.0.4 * suffixtree 0.2.1. Bryan O'Sullivan [80]suffixtree: An efficient, lazy suffix tree implementation. 80. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/suffixtree-0.2.1 Discussion A regressive view of support for imperative programming in Haskell. Paul Hudak [81]sparked an interesting thread about the effect on the language of rich support for imperative programming. 81. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/27214 Conference roundup * [82]Sydney Area Programming Language INterest Group: Call for Abstracts 82. http://article.gmane.org/gmane.comp.lang.haskell.cafe/29046 Jobs A top tier Investment bank is looking for a Haskell developer. Kyle McBeath [83]announced that there is a available a permanent position in London, joining a cross asset team. You will be able to use Haskell commercially everyday, be generously compensated and be on the forefront of technology in banking. This is a great opportunity for PhD students or above with proven experience of Haskell programming. 83. http://article.gmane.org/gmane.comp.lang.haskell.general/15552 Postdoctoral Fellowship in Functional Programming. Graham Hutton [84]announced that applications are invited for a 3-year postdoctoral research fellowship in functional programming, to work on the EPSRC-funded project 'Reasoning About Exceptions and Interrupts'. 84. http://article.gmane.org/gmane.comp.lang.haskell.general/15600 Blog noise [85]Haskell news from the [86]blogosphere. * [87]Peano induction for binary numbers * [88]Simple proof of stack correctness for a tiny language (in Agda) * [89]Peano, episode 3 * [90]Proving Haskell programs correct with QuickCheck and Isabelle/HOL * [91]Coq and simple group theory * [92]Coq and The Monad Laws: Introduction * [93]Coq and The Monad Laws: The First and Second * [94]Coq and The Monad Laws: The Third * [95]Revisiting board-building in chess * [96]Monads * [97]A Monad Tutorial for Ocaml * [98]Learning about (Computational) Monads * [99]I Wasn't Joking about One-Argument Haskell Functions * [100]Mapping Programming Language IRC Channels 85. http://planet.haskell.org/ 86. http://haskell.org/haskellwiki/Blog_articles 87. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=70 88. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=71 89. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=72 90. http://www.thenewsh.com/%7Enewsham/formal/reverse/ 91. http://blog.mikael.johanssons.org/archive/2007/08/coq-and-simple-group-theo… 92. http://sequence.complete.org/node/358 93. http://sequence.complete.org/node/359 94. http://sequence.complete.org/node/360 95. http://sequence.complete.org/node/361 96. http://www.bofh.org.uk/articles/2007/08/07/monads 97. http://enfranchisedmind.com/blog/archive/2007/08/06/307 98. http://dailykibitz.blogspot.com/2007/08/learning-about-computational-monads… 99. http://www.oreillynet.com/onlamp/blog/2007/08/i_wasnt_joking_about_oneargum… 100. http://chneukirchen.org/blog/archive/2007/08/mapping-programming-language-i… * [101]ASCII Rave in Haskell * [102]Today is similar to the programming languages situation of twenty years ago * [103]Haskell - the videos * [104]QuickCheck : Why Testing code should be Laissez-faire * [105]Rationals! * [106]Grass-Roots Functional Languages? * [107]Programming in C, ummm, Haskell * [108]And what about C arrays? * [109]Real Quicksort in Haskell * [110]Pointer tagging * [111]n00b Thoughts on Haskell * [112]'The way forward involves functional programming' * [113]Does Syntax Matter? * [114]A bright future: security and modern type systems * [115]Making the transition from sequential to implicit parallel programming: How sequential languages obscure parallelism * [116]Software Transactional Memory - Making multithreading easier * [117]Roman Numerals in Haskell * [118]A history of monad tutorials * [119]Java Becoming Solution for Safety-Critical Applications * [120]Dunno (about STM) 101. http://doc.gold.ac.uk/~ma503am/alex/asciirave 102. http://pinderkent.blogsavy.com/archives/97 103. http://www.elbeno.com/blog/?p=353 104. http://disparatemathematician.blogspot.com/2007/08/why-testing-code-should-… 105. http://byorgey.wordpress.com/2007/08/13/rationals/ 106. http://www.falvotech.com/blog/index.php?/archives/68-Grass-Roots-Functional… 107. http://augustss.blogspot.com/2007/08/programming-in-c-ummm-haskell-heres.ht… 108. http://augustss.blogspot.com/2007/08/what-about-arrays-after-doing-my-littl… 109. http://augustss.blogspot.com/2007/08/quicksort-in-haskell-quicksort-is.html 110. http://community.livejournal.com/evan_tech/229988.html 111. http://dumpendebat.net/2007/08/14/n00b-thoughts-on-haskell/ 112. http://notes-on-haskell.blogspot.com/2007/08/haskell-more-than-just-hype.ht… 113. http://notes-on-haskell.blogspot.com/2007/08/does-syntax-matter.html 114. http://blog.moertel.com/articles/2007/08/15/a-bright-future-security-and-mo… 115. http://www.embedded.com/design/multicore/201500267 116. http://weblogs.asp.net/ralfw/archive/2007/07/03/software-transactional-memo… 117. http://fawcett.blogspot.com/2007/08/roman-numerals-in-haskell.html 118. http://koweycode.blogspot.com/2007/08/history-of-monad-tutorials.html 119. http://www.sdtimes.com/article/latestnews-20070815-04.html 120. http://patricklogan.blogspot.com/2007/08/dunno.html * [121]A thesis about language niches * [122]Thoughts on Scheme from a Haskeller * [123]Arrow Transformers for sample rate conversion * [124]Playing with Propositional Logic in Haskell * [125]Programmer productivity, feature set implementation, and runtime performance * [126]Anatomy of a new monad * [127]Welcome to Haskell for Maths Blog * [128]Monadic Parser Combinators using C# 3.0 * [129]Blog Rewrite - I'm giving up * [130]Implementing The Kelly Criterion * [131]Universal Floating Point Errors * [132]Polyglot Programming - is it all Greek to you? * [133]Factor with a dash of curry * [134]How to learn to program * [135]I dated Haskell Curry's daughter * [136]Importance and Prominence and the wave: FP * [137]A Beautiful Regex Matcher... In Haskell * [138]San Francisco Bay Area FP Group * [139]AngloHaskell: The Aftermath * [140]Leibniz Equality, Decomposition, and Definability 121. http://utcc.utoronto.ca/~cks/space/blog/programming/LanguageNiches 122. http://overwatering.blogspot.com/2007/08/thoughts-on-scheme.html 123. http://www.blurty.com/users/claudiusmaximus/day/2007/08/18 124. http://www.present-tense.org/2007/08/18/playing-with-propositional-logic/ 125. http://pinderkent.blogsavy.com/archives/107 126. http://paulspontifications.blogspot.com/2007/08/anatomy-of-new-monad.html 127. http://polyomino.blogspot.com/2007/08/welcome.html 128. http://blogs.msdn.com/lukeh/archive/2007/08/19/monadic-parser-combinators-u… 129. http://lukeplant.me.uk/blog.php?id=1107301674 130. http://r6.ca/blog/20070820T175938Z.html 131. http://notes-on-haskell.blogspot.com/2007/08/universal-floating-point-error… 132. http://grahamis.com/blog/2007/08/21/polyglot-programming-is-it-all-greek-to… 133. http://useless-factor.blogspot.com/2007/08/factors-curry.html 134. http://madhadron.auditblogs.com/2007/08/21/how-to-learn-to-program/ 135. http://importantshock.wordpress.com/2007/08/21/haskell-curry-yes-i-dated-hi… 136. http://balasubramanians.com/blog/?p=312 137. http://www.oreillynet.com/onlamp/blog/2007/08/a_beautiful_regex_matcher_in_… 138. http://kfahlgren.com/blog/2007/08/21/san-francisco-bay-area-fp-group/ 139. http://neilmitchell.blogspot.com/2007/08/anlgohaskell-aftermath.html 140. http://blog.jbapple.com/2007/08/leibniz-equality-decomposition-and.html * [141]Word ladder in Haskell * [142]Existential Data Constructors in Haskell and Qi * [143]Sun slots transactional memory into Rock * [144]Haskell for pluggable apps, with the GHC API * [145]Yoda Speaks Haskell: A Tutorial * [146]Haskell Syntax Gem * [147]ClusterBy: a handy little function for the toolbox * [148]Squarefree numbers in Haskell * [149]Overloading functional references * [150]Haskell PDF 1.0 * [151]More fun with randoms * [152]Learn Haskell in 5 minutes a day: lesson 1 * [153]Destructive Quicksort in Haskell * [154]Tries and their derivatives * [155]Haskell HMAC * [156]Solving the word numbers problem: part 2 * [157]Solving the word numbers problem: part 3 * [158]Functional Forth * [159]F#'s monadic syntax * [160]The Actors Model and Haskell 141. http://gbacon.blogspot.com/2007/08/word-ladder-in-haskell.html 142. http://programmingkungfuqi.blogspot.com/2007/08/existential-data-constructo… 143. http://www.theregister.com/2007/08/21/sun_transactional_memory_rock/ 144. http://austin.youareinferior.net/?q=node/29 145. http://www.kirit.com/Yoda%20speaks%20Visual%20Haskell 146. http://www.joachim-breitner.de/blog/archives/262-Haskell-Syntax-Gem.html 147. http://blog.moertel.com/articles/2007/09/01/clusterby-a-handy-little-functi… 148. http://byorgey.wordpress.com/2007/09/01/squarefree-numbers-in-haskell/ 149. http://twan.home.fmf.nl/blog/haskell/overloading-functional-references.deta… 150. http://www.alpheccar.org/en/posts/show/80 151. http://osfameron.vox.com/library/post/more-random-fun.html 152. http://learnhaskell.blogspot.com/2007/09/lesson-1-hello-world.html 153. http://jputnam.livejournal.com/42065.html 154. http://sigfpe.blogspot.com/2007/09/tries-and-their-derivatives_08.html 155. http://scsibug.com/2007/09/16/haskell-hmac/ 156. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/WordNumbers2/ 157. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/WordNumbers3/ 158. http://wiki.forthfreak.net/index.cgi?FunForth 159. http://blogs.msdn.com/dsyme/archive/2007/09/22/some-details-on-f-computatio… 160. http://lstephen.wordpress.com/2007/09/08/the-actors-model-and-haskell/ * [161]Haskell and the South African Computing Olympiad * [162]The Power of Folds * [163]Folding Incremental Averages in Haskell * [164]OpenGL tetris in Haskell * [165]Learn Haskell in 5 minutes a day: Lesson 1: Hello, World! * [166]Learn Haskell in 5 minutes a day: Lesson 2: Input and Output, Variable Binding, and more * [167]Learn Haskell in 5 minutes a day: Lesson 3: case * [168]Break a string into groups of characters * [169]A news aggregator * [170]Haskell elevator pitch * [171]Do Notation and Sequence Operator * [172]Part 1: How sequential languages obscure parallelism * [173]Part 2: How to achieve parallel execution * [174]Part 3: Explicit parallel programming with threads and locks * [175]Part 4: Explicit parallelism: message-passing programming * [176]Part 5: Implicit parallel programming: Declarative languages * [177]Part 6: So, why aren't we using functional languages yet? * [178]More articles about multicores and multiprocessors 161. http://squing.blogspot.com/2007/09/haskelling-saco-1.html 162. http://www.sftank.net/?q=node/11 163. http://kraeutler.net/vincent/essays/Folding%20Incremental%20Averages%20in%2… 164. http://myawesomeblag.blogspot.com/2007/03/opengl-tetris-in-haskell.html 165. http://learnhaskell.blogspot.com/2007/09/lesson-1-hello-world.html 166. http://learnhaskell.blogspot.com/2007/09/lesson-2-input-and-output-variable… 167. http://learnhaskell.blogspot.com/2007/09/lesson-3-case-3.html 168. http://www.perlmonks.org/?node_id=632023 169. http://www.b7j0c.org/content/haskell-newspage.html 170. http://nickmudge.info/?post=51 171. http://nickmudge.info/?post=56 172. http://www.embedded.com/design/multicore/201500267 173. http://www.embedded.com/showArticle.jhtml?articleID=201801070 174. http://www.embedded.com/showArticle.jhtml?articleID=201802337 175. http://www.embedded.com/design/multicore/201803783?_requestid=708481 176. http://www.embedded.com/design/multicore/201804960?_requestid=708724 177. http://www.embedded.com/design/multicore/201806715 178. http://www.embedded.com/design/multicore/173400008 Quotes of the Week * nomeata: Haskell is basically Swiss: Small, Efficient, and it is fun to explore the higher parts. * Tom Moertel: In the not-too-distant future, perhaps, we might look back in amazement at the days when important security properties were neither free nor guaranteed but expensive and uncertain, underwritten only by the heroic efforts of individual programmers, struggling against impossible odds to achieve inhuman perfection. * Adam Turoff: In any case, Simon Peyton Jones is right -- the way forward involves functional programming, whether it means choosing a language like Haskell, or integrating ideas from Haskell into your language of choice. * bootslack: Once, around the time of the discovery of fire, there was a large population of people that thought mastering fire would be too difficult so they didn't. The rest of us killed them, cooked them and ate them. Code Watch Notable new features and bug fixes to the Haskell compilers. Thu Sep 6 09:19:48 PDT 2007. Norman Ramsey . [179]massive changes to add a 'zipper' representation of C-- 179. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/23114 Tue Aug 14 03:36:23 PDT 2007. Ben Lippmeier. Add graph coloring register allocator. Refactored linear allocator into separate liveness annotation and allocation stages. Added graph coloring allocator, use -fregs-graph to enable. New dump flags are -ddump-asm-native -- output of cmm -> native transform. -ddump-asm-liveness -- code annotated with register liveness info -ddump-asm-coalesce -- output of register move coalescing (this is a separate pass when using the coloring allocator) (this could change in the future) -ddump-asm-regalloc -- code after register allocation -ddump-asm-regalloc-stages -- blocks after each build/spill stage of coloring allocator -ddump-asm-conflicts -- a global register liveness graph in graphviz format The new register allocator will allocate some registers, but it's not quite ready for prime-time yet. The spill code generator needs some work. About the Haskell Weekly News New editions are posted to [180]the Haskell mailing list as well as to [181]the Haskell Sequence and [182]Planet Haskell. [183]RSS is also available, and headlines appear on [184]haskell.org. Headlines are available as [185]PDF. To help create new editions of this newsletter, please see the [186]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [187]http://www.cse.unsw.edu.au/~dons/code/hwn References 180. http://www.haskell.org/mailman/listinfo/haskell 181. http://sequence.complete.org/ 182. http://planet.haskell.org/ 183. http://sequence.complete.org/node/feed 184. http://haskell.org/ 185. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20070923.pdf 186. http://haskell.org/haskellwiki/HWN 187. http://www.cse.unsw.edu.au/~dons/code/hwn

1 0

Re: [Haskell-cafe] what is f=f (not) doing ?
by Ronald Guida 24 Sep '07

24 Sep '07

Miguel Mitrofanov wrote: >> A deadlock happens whenever two (or more) threads are blocked on each >> other. Deadlocks can be extremely hard to detect, especially if they >> occur intermittently. > > Isn't that so much different from garbage collection? Replace > "thread" with "chunk of data", and "waits for" with "has a pointer > to" and these two problems look very similar. And we all know there > ARE efficient garbage collectors. Actually, I think the two problems look /extremely/ similar. When doing garbage collection, one BIG problem is determining what's really a pointer and what's just data. There are conservative garbage collectors that assume just about anything can be a pointer, and then there are precise garbage collectors that use knowledge of the layout of "chunks of data" to identify the pointers to follow. All garbage collectors have one thing in common, though. They all depend on being able to identify, for each chunk of data, what else it has pointers to. If all else fails, a conservative GC can simply assume that everything is potentially a pointer and try to follow it. Now, a deadlock detector would need to be able to identify, for each blocked thread, what that thread is waiting for. The OS already tracks this information and uses it to determine when a blocked thread becomes un-blocked and ready to run. Now, if we could make sure that /all/ locks go through the OS, then the OS would be able to build a complete dependency table. When a deadlock is about to occur, the OS would be able to detect it immediately. Back to reality: Not all locks go through the OS. Suppose I'm using fibers. That means I have one OS thread running many routines at the same time, and I'm doing my own cooperative multitasking. GHC does this. The OS has no idea what's going on. Even if I had an OS that could track all locks, I can still get into deadlock situations. Suppose I have a programming error that causes two of my threads to deadlock. Furthermore, suppose that when the deadlock is detected, I make both threads roll back and start again with the same pre- conditions. Now they will go back into the same deadlock. This cycle will repeat indefinitely, and it's beyond the OS's ability to detect. Regarding deadlock detection, I think the real questions to ask are (1) Do deadlocks occur often enough for us to care? (2) When they do occur, are they severe enough for us to care? If the answers are both "no", then deadlock detection is not worth the trouble of implementation. My opinion is that for typical commercial end-user software, the answers are both "no". For "critical" computing systems, like flight computers and life support systems, the answers are obviously "yes"; a deadlock that occurs once per million years is still a problem if it can bring down a multi-million dollar aircraft with hundreds of passengers on board. For these types of systems, the developers tend to spend extra development resources to try to design fool-proof systems. In some cases, they may even go to heroic lengths to /prove/ that their software is correct. It /is/ possible to design a system that cannot deadlock. It turns out that one of the necessary conditions for deadlock is the hold and wait condition. If a process that's already holding resources is /not/ allowed to request additional resources, then deadlocks are impossible. If we really, absolutely don't want any deadlocks, then we can design a system in which a process that needs a resource is required to release all its resources and then re-allocate its updated list of resources. -- Ron

1 0

what is f=f (not) doing ?
by Pasqualino 'Titto' Assini 24 Sep '07

24 Sep '07

Hi, if I define: f = f and then try to evaluate 'f' in GHCi, as one would expect, the interpreter never returns an answer. The funny thing is that, while it is stuck in an infinite loop, GHCi doesn't seem to use any CPU time at all. How is this possible? Thanks titto

9 15

are some of these "reverse" algos better than others? is there a quick and dirty way to reveal this fact?
by Thomas Hartman 24 Sep '07

24 Sep '07

I came up with the following functions to do "reverse." The first one is obviously bad, because of the expensive list concat. The last one, "myreverse", I think is the reference implementation you get in the prelude. The ones between, I'm not so sure. I suspect they're "naive" as the name indicates, but in practice they stop working at the same place as myreverse, at least on hugs. If versions "naive 2-5" are indeed inferior to myreverse, is there a quick and dirty way to reveal which ones are better via a profiling tool, or some trick like that? Something easier than doing the space/time complexity analysis by hand I mean? By the way, on ghc they all work out to [1..1000000] and beyond. -- fails on [1..100000] on win/hugs naivereverse [] = [] naivereverse (x:xs) = naivereverse xs ++ [x] -- fails on [1..1000000] on win/hugs naivereverse2 [] = [] naivereverse2 (x:xs) = ( last xs ) : ( naivereverse2 ( init xs ) ++ [x] ) naivereverse3 [] = [] naivereverse3 ( x : xs ) = ( last xs ) : naivereverse3 ( init xs ) naivereverse4 xs = myreverse' [] xs where myreverse' reversed [] = reversed myreverse' reversed xs = myreverse' ( (head xs) : reversed ) ( tail xs ) naivereverse5 xs = myreverse' [] xs where myreverse' reversed [] = reversed myreverse' reversed (x:xs) = myreverse' ( x : reversed ) xs -- this is the usual implementation right? myreverse xs = foldl f [] xs where f accum el = el : accum t.

5 5

Composition Operator
by PR Stanley 23 Sep '07

23 Sep '07

Hi (.) :: (b -> c) -> (a -> b) -> (a -> c) While I understand the purpose and the semantics of the (.) operator I'm not sure about the above definition. Is the definition interpreted sequentially - (.) is a fun taht takes a fun of type (b -> c) and returns another fun of type (a -> b) etc? Any ideas? Thanks, Paul

5 5

Win32 Open GL / Glut Applications
by John Wicket 22 Sep '07

22 Sep '07

I can take any of these opengl applications or other examples on the web, but I can't get the application to run on Win32? They will compile(except for the GLU ones) but when I launch them, the windows just closes? Is HOpenGL supported well on Win32? http://www.haskell.org/HOpenGL/ I have ghc 6.6

5 4

Sequencing Operations in a Monad
by SevenThunders 22 Sep '07

22 Sep '07

I have a matrix library written in C and interfaced into Haskell with a lot of additional Haskell support. The C library of course has a lot of side effects and actually ties into the BLAS libraries, thus at the present time, most of the interesting calls are done in the IO monad. I have no intention of rewriting what I've done so far or using someone elses Matrix library. (Mine is tuned somewhat for my application). I attempted to extend my Haskell matrix interface using type classes (real and complex matrices) and have run into a conceptual problem. I would like to be able to use operator notation for matrix arithmetic. e.g. R = Q * (A + B) Unfortunately if I wrap my matrix references in the IO monad, then at best computations like S = A + B are themselves IO computations and thus whenever they are 'invoked' the computation ends up getting performed repeatedly contrary to my intentions. For example I might have some code like this, let S = A += B in do (r,c) <- size S k <- matindex S ..... code of this nature results in S being applied twice and if the operator += has side effects, those side effects will be applied twice. Even if there are no side effects the computation will unnecessarily be applied twice. What I need is a way to force a single execution of the IO action without losing the syntax sugar. If you arrange the types to try to do all the operations inside the IO monad you can't chain together more than 1 binary operation. eg. do S <- A + B Z <- Q * S vs do S <- Q * (A + B) Are there any suggestions for this dilemma? Am I using the wrong monad for this task? -- View this message in context: http://www.nabble.com/Sequencing-Operations-in-a-Monad-tf4446047.html#a1268… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 10

Re: [Haskell-cafe] reverse with foldl
by Miguel Mitrofanov 21 Sep '07

21 Sep '07

> reverse = foldl (\x -> \xs -> xs:x) [] Doesn't typecheck.

2 1

Interesting folds over bytestring lists?
by Justin Bailey 21 Sep '07

21 Sep '07

I have a data structure which is a list of bytestrings, but externally it looks like one big string. One of the operations I want to support takes a section of the string, starting at some arbitrary index and ending somewhere further down the line. In implementing the function I came up with the two functions below, dropTo and takeTo. In my mind, dropTo moves over the list of bytestrings until it reaches the starting point, and then returns the rest. takeTo, in contrast, scans over the list until it has seen enough bytes to return the amount requested. In both cases I am trying to share structure as much as possible in order to avoid unnecessary copying and space leaks. I thought these two functions were interested and am looking for feedback, comments, improvements, etc. Thanks! import qualified Data.ByteString as S import qualified Data.ByteString.Lazy as L import qualified Data.ByteString.Base as B import Test.QuickCheck import Data.List (foldl') import Data.Word dropTo :: Int -> [L.ByteString] -> [L.ByteString] dropTo _ [] = [] dropTo amt strs = let dropTo' :: (Int, [L.ByteString]) -> L.ByteString -> (Int, [L.ByteString]) dropTo' (rem, acc) ss | rem == 0 = (0, acc) | otherwise = let chunks = L.toChunks ss in case foldl' dropStricts (rem, chunks) chunks of (!n, rest) | null rest -> (n, drop 1 acc) | otherwise -> (0, L.fromChunks rest : drop 1 acc) dropStricts :: (Int, [S.ByteString]) -> S.ByteString -> (Int, [S.ByteString]) dropStricts (rem, acc) str | rem == 0 = (0, acc) | rem - S.length str == 0 = (0, drop 1 acc) | rem - S.length str < 0 = (0, S.drop rem str : (drop 1 acc)) | otherwise = (rem - S.length str, drop 1 acc) (_, rest) = foldl' dropTo' (amt, strs) strs in rest takeTo :: Int -> [L.ByteString] -> [L.ByteString] takeTo _ [] = [] takeTo amt strs = let countLazies :: (Int, Int, L.ByteString) -> L.ByteString -> (Int, Int, L.ByteString) countLazies (rem, !total, lazyLeftover) ss | rem == 0 = (0, total, lazyLeftover) | otherwise = let chunks = L.toChunks ss in case foldl' countStricts (rem, 0, S.empty) chunks of (!n, amt, strictLeftover) | S.null strictLeftover -> (n, total + 1, L.empty) | otherwise -> (n, total, L.fromChunks (take amt chunks ++ [strictLeftover])) countStricts :: (Int, Int, S.ByteString) -> S.ByteString -> (Int, Int, S.ByteString) countStricts (rem, !total, leftover) str | rem == 0 = (0, total, leftover) | rem - S.length str == 0 = (0, total + 1, S.empty) | rem - S.length str < 0 = (0, total, S.take rem str) | otherwise = (rem - S.length str, total + 1, S.empty) in case foldl' countLazies (amt, 0, L.empty) strs of (_, total, leftover) | L.null leftover -> take total strs | otherwise -> take total strs ++ [leftover] prop_dropToNonEmpty :: [[Word8]] -> Int -> Property prop_dropToNonEmpty strs amt = amt >= 0 && (all (not . null) strs) ==> all (not . L.null) (dropTo amt (map (L.pack) strs)) prop_dropToCorrect :: [[Word8]] -> Int -> Property prop_dropToCorrect strs amt = let lazyStr = L.drop amt64 (toLazyBS strs) amt64 = fromIntegral amt in amt >= 0 && (all (not . null) strs) ==> (L.concat (dropTo amt (map (L.pack) strs))) == lazyStr prop_takeToNonEmpty :: [[Word8]] -> Int -> Property prop_takeToNonEmpty strs amt = amt >= 0 && (all (not . null) strs) ==> all (not . L.null) (takeTo amt (map (L.pack) strs)) prop_takeToCorrect :: [[Word8]] -> Int -> Property prop_takeToCorrect strs amt = let lazyStr = L.take amt64 (toLazyBS strs) amt64 = fromIntegral amt in amt >= 0 && (all (not . null) strs) ==> (L.concat (takeTo amt (map (L.pack) strs))) == lazyStr -- Functions and instances for testing purposes toLazyBS :: [[Word8]] -> L.ByteString toLazyBS = L.concat . map L.pack instance Arbitrary Word8 where arbitrary = elements [minBound .. maxBound] coarbitrary = undefined main = do putStrLn "prop_takeToNonEmpty" quickCheck prop_takeToNonEmpty putStrLn "prop_takeToCorrect" quickCheck prop_takeToCorrect putStrLn "prop_dropToCorrect" quickCheck prop_dropToCorrect putStrLn "prop_dropToNonEmpty" quickCheck prop_dropToNonEmpty

4 5

Graphics.SOE
by Peter Padawitz 21 Sep '07

21 Sep '07

Can scrollbars be attached to windows created with openWindow?

1 0

Slightly Off-topic
by PR Stanley 21 Sep '07

21 Sep '07

Hi Does anyone know of a good active list for students and experts in discrete mathematics, in particular, logic? Thanks, Paul

1 0

Accumulator value for "and" and "or"
by PR Stanley 21 Sep '07

21 Sep '07

Hi or = foldl (||) False and = foldl (&&) True I can understand the rationale for the accumulator value - True && [] where [] = True and True || [] where [] = False Other than the practical convenience is there a reason for having the empty list in and and or equating to True and False? Thanks, Paul

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How can I stop GHCi from calling "show" for IO actions?
by Ryan Ingram 21 Sep '07

21 Sep '07

Prelude> let inf = repeat 1 Prelude> inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I expect this to happen) Prelude> let x = inf (no output here!) Prelude> :t x x :: [Integer] Prelude> return inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I also expect this to happen) Prelude> y <- return inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I do not expect this to happen here!) Prelude> :t y <interactive>:1:0: Not in scope: 'y' Is this a bug? Why does "y <- return exp" have different behavior than "let y = exp"? Is there a way to make GHCi not print the result of an action but still make my variables get bound? -- ryan

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interaction between OS processes
by Andrea Rossato 21 Sep '07

21 Sep '07

Hi, there's something I don't get about interaction among OS processes and Haskell handles/channels. Suppose I have a very small program that takes a line and prints it till you write "quit": main = do s <- getLine case s of "quit" -> putStrLn "quitting" >> return () _ -> loop s where loop s = do putStrLn s main This is a small interactive process I would like to talk to from another Haskell program, like the following one which, indeed, is just a wrapper around the first. Now, if I write a single line with "quit", I get "quitting" back, otherwise the program doesn't work. I think I need some direction in order to understand how handles work. The same with channels, I'm afraid. Could you please point me in the right direction? Thanks for your kind attention. Andrea The not working code: import Control.Concurrent import System.Process import System.IO main = do c <- runInteractiveCommand "./main2" loop c loop c@(i,o,e,p) = do s <- getLine hPutStrLn i s hFlush i -- now "i" is closed, right? s' <- hGetLine o putStrLn s' loop c

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reverse with foldl
by PR Stanley 21 Sep '07

21 Sep '07

Hi reverse = foldl (xs x . x:xs) [] (xs ++ ) = foldl (\ys y -> ys ++ [y]) xs If I were to define reverse with foldl I would do it this way: reverse = foldl (\x -> \xs -> xs:x) [] Any idea what the first code frag is suposed to achieve? Thanks, Paul

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