- Haskell-Cafe - Haskell.org

Squashing space leaks
by Greg Buchholz 09 May '05

09 May '05

I've been dabbling with implementing the "n-body" benchmark over at the "Great Computer Language Shootout"... http://shootout.alioth.debian.org/benchmark.php?test=nbody ...and I've stumbled on to a problem with space leaks. The code below works great for low numbers of iterations, but crashes and burns with large numbers. You can temporarily fix it up by specifying larger stack and heap sizes (with the +RTS -K & -H options), but this only gets you so far. I haven't been able to glean much information from the profiling reports. You can see the reports I generated (like the biography report) over at <http://sleepingsquirrel.org/nbody>. I've tried several different variations on the "advance" function, as well as trying "seq" in numerous spots, but I haven't stumbled on to the right combination yet. Any pointers? Thanks, Greg Buchholz >-- n-body simulation >-- compile: ghc -O2 -o nbody nbody.hs >-- run: nbody 100000 > >import System(getArgs) >import Numeric > >data Vec = V !Double !Double !Double deriving Show >instance Num Vec where > (V a b c) + (V x y z) = (V (a+x) (b+y) (c+z)) > (V a b c) - (V x y z) = (V (a-x) (b-y) (c-z)) > fromInteger 0 = V 0.0 0.0 0.0 -- for sum function >instance Eq Vec where (V a b c) == (V x y z) = (a==x) && (b==y) && (c==z) > >dot (V a b c) (V x y z) = a*x + b*y + c*z >scale (V a b c) n = V (n*a) (n*b) (n*c) >mag (V x y z) = sqrt (x*x + y*y + z*z) > >data Planet = Planet Vec Vec Double deriving Show --Position Velocity Mass >dist (Planet p1 _ _) (Planet p2 _ _) = mag $ p1 - p2 >mass (Planet _ _ m) = m >vel (Planet _ v _) = v >pos (Planet p _ _) = p > >main = do > [arg] <- getArgs > let iter = read arg > let n = 5::Int > let bodies = offset_momentum n [sun, jupiter, saturn, neptune, uranus] > let begin = energy n bodies > putStrLn $ showFFloat (Just 9) begin "" > let final = (iterate (advance n 0.01) bodies) > let end = energy n (final !! iter) > putStrLn $ showFFloat (Just 9) end "" > >days_per_year = 365.24 >solar_mass = 4.0 * pi * pi > >advance :: Int -> Double -> [Planet] -> [Planet] >advance n dt (p:ps) = take n ((Planet (pos p + delta_x) > (new_v) (mass p) ) : advance n dt (ps++[p])) > where > delta_v = sum (map (\q -> > (pos p - pos q) `scale` ((mass q)*dt/(dist p q)^3)) ps) > new_v = (vel p) - delta_v > delta_x = new_v `scale` dt > >energy:: Int -> [Planet] -> Double >energy n ps = kinetic - potential > where > kinetic = 0.5 * (sum (map (\q->(mass q)*((vel q) `dot` (vel q))) ps)) > potential = sum [(mass (ps!!i))*(mass (ps!!j))/(dist (ps!!i) (ps!!j)) > | i<-[0..n-1], j<-[i+1..n-1]] > >offset_momentum n ((Planet p v m):ps) = (Planet p new_v m):ps > where new_v = (sum (map (\n->(vel n) `scale` (mass n)) ps)) > `scale` ((-1.0)/solar_mass) > >{- >rotations 0 _ = [] >rotations n xss@(x:xs) = flipped:rotations (n-1) flipped > where flipped = xs++[x] > >advance :: Int -> Double -> [Planet] -> [Planet] >advance n dt pss = map (adv dt) (rotations n pss) > >adv dt (p:ps) = Planet (pos p + delta_x) (new_v) (mass p) > where > delta_v = sum (map (\q -> > (pos p - pos q) `scale` ((mass q)*dt/(dist p q)^3)) ps) > new_v = (vel p) - delta_v > delta_x = new_v `scale` dt >--} >jupiter = (Planet > (V 4.84143144246472090e+00 (-1.16032004402742839e+00) > (-1.03622044471123109e-01)) > (V ( 1.66007664274403694e-03 * days_per_year) > ( 7.69901118419740425e-03 * days_per_year) > ((-6.90460016972063023e-05) * days_per_year)) > (9.54791938424326609e-04 * solar_mass)) > >saturn = (Planet > (V 8.34336671824457987e+00 4.12479856412430479e+00 > (-4.03523417114321381e-01)) > (V (-2.76742510726862411e-03 * days_per_year) > (4.99852801234917238e-03 * days_per_year) > (2.30417297573763929e-05 * days_per_year)) > (2.85885980666130812e-04 * solar_mass)) > >uranus = (Planet > (V 1.28943695621391310e+01 (-1.51111514016986312e+01) > (-2.23307578892655734e-01)) > (V (2.96460137564761618e-03 * days_per_year) > (2.37847173959480950e-03 * days_per_year) > (-2.96589568540237556e-05 * days_per_year)) > (4.36624404335156298e-05 * solar_mass)) > >neptune = (Planet > (V 1.53796971148509165e+01 (-2.59193146099879641e+01) > 1.79258772950371181e-01) > (V (2.68067772490389322e-03 * days_per_year) > (1.62824170038242295e-03 * days_per_year) > (-9.51592254519715870e-05 * days_per_year)) > (5.15138902046611451e-05 * solar_mass)) > >sun = (Planet (V 0.0 0.0 0.0) (V 0.0 0.0 0.0) solar_mass)

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Monad transformers
by Max Vasin 08 May '05

08 May '05

Hello! Suppose we have functions f :: ReaderT env monad1 rtype g :: Reader env rtype and we need to call g from f. Currently I write something like f = do env <- ask let r = runReader g env doSmth r I don't like doing it this way (I need to get environment and explicitly pass it). Is there another way to get things done? -- WBR, Max Vasin.

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Signature browser
by Henning Thielemann 08 May '05

08 May '05

When searching a function in a library I usually scan manually for suitable types in the Haddock function overview. I wonder whether this can be automated or whether this is even implemented already. I want to enter search requests like "a function which takes a predicate and maybe other inputs and outputs at least a list". But I'm afraid formulating such a request takes more time than manual scan. :-(

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Compiling with NHC98
by Daniel Carrera 08 May '05

08 May '05

Hello, After days of effort, I finally managed to compile and install a Haskell compiler. The one I have is NHC98. So now, with my new toy, I am eager to compile my very first Haskell program (I've been using 'runhugs' so far). But I'm having problems: ---<quote>---- $ nhc98 prng.hs -o prng I/O error (user-defined), call to function `userError': In file ./RC4.hi: 1:1-1:6 Found _module_ but expected a interface ---<quote>---- Okay, so I need to learn what an interface is. It looks like Hugs and NHC98 have idfferent ideas of what's acceptable. The file prng.hs starts with: ---<quote>---- module Main where import RC4 .... ---<quote>---- And the file RC4.hs contains all the interesting code. So that's where I am. I'm not clear what it is NHC98 is complaining about or how to fix it. It looks like it doesn't like modules and it wants an interface instead. Can someone point me to where I can learn what an interface is? and how to turn my module into one? Or else, how to make NHC98 accept the module. Any help would be much appreciated. Cheers, Daniel.

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Re: [Haskell-cafe] explaining currying w/o math
by Donn Cave 08 May '05

08 May '05

Quoth Daniel Carrera <dcarrera(a)digitaldistribution.com>: ... | In your opinion, do you think Haskell is appropriate for someone with | zero math background? I can't imagine how I'd explain something like | currying to someone who doesn't know math. Then she may have to learn it from someone else, but there are resources out there. The first thing that came up for me in a web search: http://www.willamette.edu/~fruehr/haskell/lectures/tutorial4.html Haskell functions of several arguments are actually "curried", i.e., they are higher-order functions of successive arguments > :t take take :: Int -> [a] -> [a] > :t take 10 take 10 :: [a] -> [a] > :t take 10 ['a'..'z'] take 10 (enumFromTo 'a' 'z') :: [Char] I think the key is examples. Donn Cave, donn(a)drizzle.com

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Newbie : What does the sequence function make?
by mandziy＠web.de 06 May '05

06 May '05

Please,can anyone explain it to me? Cheers! ______________________________________________________________ Verschicken Sie romantische, coole und witzige Bilder per SMS! Jetzt bei WEB.DE FreeMail: http://f.web.de/?mc=021193

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supplying some of the arguments to a function
by Mark Goldman 06 May '05

06 May '05

if I had a function f that took x y and z in that order, is there some way that I can supply y and z and get back a function that takes x? This question comes about after talking with a prof about currying and wether it buys you anything. -mdg

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where versus let
by Scott J. 06 May '05

06 May '05

Hi, Instead of the following piece of code : data State s a = S(s -> (a,s)) instance Monad (State s) where return a = S(\s -> (a,s)) (S f ) >>= g = S(\s -> let (a,s1) = f s S fun = g a (b,s2) = fun s1 in (b,s2) ) I thought to write it in the following form using "where": data State s a = S(s -> (a,s)) instance Monad (State s) where return a = S(\s -> (a,s)) (S f ) >>= g = S(\s -> (b,s2) where (a,s1) = f s S fun = g a (b,s2) = fun s1 ) However the compiler does not accept this use of "where". What is the reason? Thanks Jan

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I love type declarations. :-)
by Daniel Carrera 06 May '05

06 May '05

Hey, Whooo hoo! I just finished my first non-trivial Haskell program. It's the pseudo-random number generator from RC4. And if it weren't for type declarations I'd still be bug-hunting. I've been trying to get the PRNG to work for two days, but Hugs kept wanting to use Integer where Int should be used, ugh. But while reading a Haskell tutorial I came accross type declarations: function :: Int -> Int -> [Int] This solved all my problems! I added proper type declarations to everything, and suddenly Hugs was happy. Type declarations also brought other benefits: 1) I quickly spoted two bugs I hadn't hit yet. 2) I removed the fromIntegral() functions, making the code cleaner. 3) And my code is self-documented. Before, when writing imperative code, I would add a comment for each function describing its input and output. Now type declarations provide that, so I merely describe what the function does. And type declarations are better than a comment because Haskell helps me make sure that they are right. Go Haskell! Cheers, Daniel.

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How to get an variable or its value?
by Michael Onofruk 06 May '05

06 May '05

Hi! >From this "monster" [([Int],Char)] I need to get Char variable. How do I do this? Or in general - how does this works in Haskell? Cheers, M. Yahoo! Mail Stay connected, organized, and protected. Take the tour: http://tour.mail.yahoo.com/mailtour.html

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RE: [Haskell-cafe] Conal Elliott's Vertigo, anyone get it to run?
by Atwood, John Wesley 05 May '05

05 May '05

> On 5/5/05, Atwood, John Wesley <john.atwood(a)hp.com> wrote: > > I tried to run Conal Elliott's Vertigo demos, on 3 diffferent XP > > boxes; it ran on none. The error is "Common Language Runtime Services > > - Appication has generated an exception that could not be handled. > > Process id=xxxx, Thread id=xxxx". Any suggestions? > > http://conal.net/Vertigo/ > > > On 5/5/05, Sebastian Sylvan [mailto:sebastian.sylvan@gmail.com] wrote: > Do you have .Net installed? > > I got it to run, don't know what you're doing wrong. > Yep, version 1.1; don't have the C# compiler installed, but it's my understanding that that's not needed for the demos. John

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Haskell vs OCaml
by Daniel Carrera 05 May '05

05 May '05

Hey, Marcin just mentioned OCaml as another functional programming language I should keep in mind. Can anyone offer an opinion on how Haskell and OCaml compare? Is OCaml as easy to learn as Haskell? Does it have much the same virtues? I'll go take a look at it. /daniel goes to Google. Cheers, Daniel.

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Re: [Haskell-cafe] How to show a variable with "show"?
by Michael Onofruk 05 May '05

05 May '05

> > > > And how do I see functions or variables defined > > inside of other functions? In example above - y > > > > Short answer -- you don't. > Thanks for explanations! Cheers, M. __________________________________ Do you Yahoo!? Yahoo! Mail - Helps protect you from nasty viruses. http://promotions.yahoo.com/new_mail

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Re: [Haskell-cafe] How to show a variable with "show"?
by Michael Onofruk 05 May '05

05 May '05

> module Main where > > main = do putStr (show (foo 3)) > putStr "\n" > putStr (show (foo 7)) > putStr "\n" > putStr (show y) > > foo x = (x + 2) / y where > y = 2 * 2 And how do I see functions or variables defined inside of other functions? In example above - y Yahoo! Mail Stay connected, organized, and protected. Take the tour: http://tour.mail.yahoo.com/mailtour.html

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How to show a variable with "show"?
by Michael Onofruk 05 May '05

05 May '05

Hi! In my program I would like to see an result of function or an variable. Wenn I use "show myfunction" or "show variable", I'm getting errors like "ERROR - Improperly terminated character constant" or " Syntax error in declaration (unexpected `;', possibly due to bad layout)" Any help would be appreciated! Yahoo! Mail Stay connected, organized, and protected. Take the tour: http://tour.mail.yahoo.com/mailtour.html

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Int vs Integer?
by Daniel Carrera 05 May '05

05 May '05

Hello, What's the difference between "Int" and "Integer"? How can I tell Haskell which one I want? I'm having trouble with a function: ints = 0 : map (1+) ints prng = map iter ints where iter 0 = [0,0] iter n = [i,j] where i = mod n 256 j = ints !! i (eventually 'iter' will be more a more interesting function). When I try to load this module in Hugs I get the error: ERROR "./PRNG.hs":24 - Type error in list *** Expression : [i,j] *** Term : j *** Type : Integer *** Does not match : Int Line 24 is the 'prng' line. I've spent all day trying to fix this, but for the life of me I can't figure out why it happens or how to make it stop. Haskell just doesn't seem to like the 'iter !! i' construct. It's interesting that if you replace 'i' by (say) 3, Haskell runs the program happily. But, if you replace 'i = mod n 256' by 'i=3' you get the same error. Any words of wisdom would be appreciated. Cheers, Daniel.

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Where do you use Haskell?
by Daniel Carrera 05 May '05

05 May '05

Hi all, Again, I'm the new guy slowly learning this "fuctional programming" thing. :-) I've been reading, and I'm really liking the elgance of Haskell, and FP as a whole. But I wonder about the range of applicability. You see, one of the things about FP is that there are no side-effects and the same function on the same parameters always returns the same value. But any program that interacts with the real world cannot meet those properties. The function ask_user_name() will not return the same value each time. If write a chess program, the function get_user_move() will be different each time. And every time you update the screen, you're having a side-effect. So, I figure that to do these tasks you heed that "do ... <-" work around. But that kills the whole point of using FP in the first place, right? So, I'm tempted to conclude that FP is only applicable to situations where user interaction is a small part of the program. For example, for simulations. Now, I'm sure I'm not the first person to have this train of thought. And I'm sure there is a good answer why I'm wrong. :-) I'm eager to hear what that might be. Cheers, Daniel.

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Conal Elliott's Vertigo, anyone get it to run?
by Atwood, John Wesley 05 May '05

05 May '05

I tried to run Conal Elliott's Vertigo demos, on 3 diffferent XP boxes; it ran on none. The error is "Common Language Runtime Services - Appication has generated an exception that could not be handled. Process id=xxxx, Thread id=xxxx". Any suggestions? http://conal.net/Vertigo/ John Atwood

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Comparison with Clean?
by Daniel Carrera 04 May '05

04 May '05

Hi all, Anyone here familiar with the Clean programming language? http://www.cs.ru.nl/~clean/ It looks /very/ similar to Haskell, both in functionality and syntax. I would be grateful for any sort of comparison. I'm trying to decide which language I should try to learn. Cheers, Daniel.

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Speed comparison?
by Daniel Carrera 04 May '05

04 May '05

Hi all, Thank you for all the information on my previous question. I learned a lot, and good pointers to more info. My next question is about speed. How fast would you consider Haskell? (say, for computational work). How would you compare it to C, Python and Ruby? I suggest C, Python and Ruby as a basis of comparison because: 1) C is what I'd use if I wanted something fast. 2) Python or Ruby is what I'd use if I wanted clean code. I know that Haskell can be compiled, thanks to ghc. Ruby is interpreted and I'm pretty sure Python is quasi-interpreted (it's "compiled" on the fly to Java-style bytecode and a VM runs the bytecode). Furthermore, the Haskell page says that ghc produces fast programs. So I would guess that Haskell is faster than Python, but not as fast as C. Would that be correct? For computational work (e.g. a simulation, factoring primes, whatever) does it approach C speed? I would be happy to hear a comparison with any other language you find interesting.:-) As well as any other information you'd like to share. Cheers, Daniel.

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Re: [Haskell-cafe] Speed comparison?
by John Hughes 04 May '05

04 May '05

>>Furthermore, the Haskell page says that ghc produces fast programs. >>So I would guess that Haskell is faster than Python, but not as fast >>as C. >> >>Would that be correct? >> >> > >Usually yes. > > > In a sense. I think it's important to remember that what matters is performance of a whole application, not of highly tuned benchmark code, and in writing applications with good performance, then the quality of profiling tools, and (especially) the amount of time the programmer has available for tuning, can be far more important than the compiler. There have certainly been occasions where Haskell entries to the ICFP programming contest have beat OCaml or even C/C++ entries on performance, by a wide margin. Likewise Ericsson's ATM switch, which is controlled by a million lines plus of Erlang (running on a byte code interpreter!), performs better than all its competitors. Benchmarks give a very one-sided view, ignoring the large effect that ease of programming can have on the final system's performance. John Hughes

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RE: [Haskell-cafe] Mutual recursive modules with GHC: Classes, instances
by Simon Peyton-Jones 04 May '05

04 May '05

I'm afraid so, as of today. It's the kind of thing that could get fixed if enough people wanted it, but at the moment it's pretty low on the list. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Henning Thielemann | Sent: 04 May 2005 10:31 | To: Simon Peyton-Jones | Cc: Haskell Cafe | Subject: RE: [Haskell-cafe] Mutual recursive modules with GHC: Classes,instances | | | On Wed, 4 May 2005, Simon Peyton-Jones wrote: | | > hi-boot files can't contain class declarations. This should be checked, | > but isn't, hence puzzling message. | > | > GHC 6.4 has a better setup. Actually, it still doesn't check for | > absence of class decls, but it will shortly. | | This means it is impossible to declare mutually dependent classes where | each class resides in a separate module? | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Comparison with Clean?
by Krasimir Angelov 04 May '05

04 May '05

If you want to compare Haskell and Clean then take a look at Object I/O library. It is ported to Haskell and you can learn a lot comparing both implementations. I spent a lot of time translating Clean functions to Haskell and my own impression is that Haskell's do notation is much more easier that uniqueness typing. In Clean you have to explicitly maintain the state while in Haskell it is hidden. In order to preserve uniquenes property of state, quite often you have write uggly functions like: getXYZ :: State -> (XYZ, State) getXYZ st = (f st, st) while in Haskell the above function should be: getXYZ :: State -> XYZ getXYZ st = f st It is more clean and from the type signature you can see that this is readonly access to the state. Cheers, Krasimir On 5/4/05, Daniel Carrera <dcarrera(a)digitaldistribution.com> wrote: > Hi all, > > Anyone here familiar with the Clean programming language? > > http://www.cs.ru.nl/~clean/ > > It looks /very/ similar to Haskell, both in functionality and syntax. > > I would be grateful for any sort of comparison. I'm trying to decide > which language I should try to learn. > > Cheers, > Daniel. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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RE: [Haskell-cafe] Mutual recursive modules with GHC: Classes, instances
by Simon Peyton-Jones 04 May '05

04 May '05

hi-boot files can't contain class declarations. This should be checked, but isn't, hence puzzling message. GHC 6.4 has a better setup. Actually, it still doesn't check for absence of class decls, but it will shortly. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Henning Thielemann | Sent: 04 May 2005 10:10 | To: Haskell Cafe | Subject: [Haskell-cafe] Mutual recursive modules with GHC: Classes, instances | | | I tried to set up mutual recursive modules according to section 4.6.10. of | the manual of GHC-6.2.2. I illustrate my problems with a simple fictitious | example. | | ---- A.hi-boot ---- | | module A where | | | class GHC.Num.Num a => C a where | answer :: a | | f :: B.T -> GHC.Num.Int | | | ---- A.hs ---- | | module A where | | import B | | class Num a => C a where | answer :: a | | instance C Int where | answer = 42 | | f :: B.T -> Int | f (B.Cons x) = x | | | ---- B.hs ---- | | module B where | | import {-# SOURCE #-} A | | | data T = Cons Int | | newAnswer :: Int | newAnswer = A.answer + 1 | | -------------- | | | $ ghc --make A.hs B.hs | Chasing modules from: A.hs,B.hs | Compiling B ( B.hs, B.o ) | | B.hs:1: | Failed to find interface decl for `A.$dmanswer' | from module `A' | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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[Haskell-cafe] Mutual recursive modules with GHC: Classes, instances
by Henning Thielemann 04 May '05

04 May '05

I tried to set up mutual recursive modules according to section 4.6.10. of the manual of GHC-6.2.2. I illustrate my problems with a simple fictitious example. ---- A.hi-boot ---- module A where class GHC.Num.Num a => C a where answer :: a f :: B.T -> GHC.Num.Int ---- A.hs ---- module A where import B class Num a => C a where answer :: a instance C Int where answer = 42 f :: B.T -> Int f (B.Cons x) = x ---- B.hs ---- module B where import {-# SOURCE #-} A data T = Cons Int newAnswer :: Int newAnswer = A.answer + 1 -------------- $ ghc --make A.hs B.hs Chasing modules from: A.hs,B.hs Compiling B ( B.hs, B.o ) B.hs:1: Failed to find interface decl for `A.$dmanswer' from module `A'

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Array functions?
by Daniel Carrera 04 May '05

04 May '05

Hello, I hope these don't turn out to be RTFM questions, but I can't find them in my FM :-) 1) Is there a function to get the ith element from an array? 2) Is there a function to get the "index" of an entry in an array? I've implemented these two functions below: 1) find 0 (x:xs) = x find n (x:xs) = find (n-1) xs 2) index i (x:xs) = if i == x then 0 else 1 + index a xs This was a fun exercise, but I can't shack off the feeling that I just re-invented the wheel. I need these because I want to be able to swap any two elements from an array. This is my swap function: -- swap i j array => swaps the ith and jth elements of 'array'. -- swap i j arr = a_head ++ [item_j] ++ a_midd ++ [item_i] ++ a_tail where a_head = [a | a <- arr, index a arr < i] item_i = find i arr a_midd = [a | a <- arr,(index a arr > i) && (index a arr < j)] item_j = find j arr a_tail = [a | a <- arr, index a arr > j] I'm sure this was a poor way to accomplish this, but it was a learning experience. If anyone would like to show me a more elegant solution, I would be happy to see it. Cheers, Daniel. Haskell newbie.

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Clarification on proof section of HS: The Craft of FP
by Echo Nolan 04 May '05

04 May '05

Hello all, My copy of HS: The Craft Of FP just arrived and I was reading section 8.5 about induction. On page 141, Simon Thompson gives a method of proving properties of functions on lists: 1) Prove that the property holds for the null list 2) Prove that the property holds for (x:xs) under the assumption that the property holds for xs. My question is this: what is the proof of (P([]) and ( P(xs) => P(x:xs))) => P(xs)? In other words, how does proof of the property on empty lists and proof that the property holds on (x:xs) under the assumption that it holds on xs give proof that it holds on all lists? Isn't there a recursive dependency here? You are defining the proof for lists with at least one element based on the assumption that it holds for all lists, right? How does this get you a proof of the general property if it depends on the assumption of the general property? Regards and thanks, Echo Nolan.

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How to join two lists of lists?
by Khrystyna Mandziy 03 May '05

03 May '05

Hi all! I'm trying to join to lists of lists. The problem is, i would like to get a new list of lists of tuples and not list of tuples of lists (that what zip makes). list1 = [[1,2],[3,4],[5,6]] list2 = [[a,b],[c,d],[e,f]] desiredlist = [[(1,a),(2,b)],[(3,c),(4,d)],[(5,e),(6,f)]] Thanks for any help!

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Here's a curve ball for Haskell.
by Daniel Carrera 03 May '05

03 May '05

Alright, in Haskell there are no side-effects when you call a function twice on the same data you get the same result... I just decided I'll try to write a good pseudo random number generator in Haskell :-) I'm off to class now, but I'll try it tonight. Cheers, Daniel.

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Re: [Haskell-cafe] Speed comparison?
by Daniel Carrera 03 May '05

03 May '05

Greg Buchholz wrote: > You might find the "The Great Computer Language Shootout" > informative... > > http://shootout.alioth.debian.org/ Thanks! That's a great resource. At first glance, Haskell seems to do well over-all. Cheers, Daniel.

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 03 May '05

03 May '05

On 01 May 2005 19:28, Sven Panne wrote: > Gour wrote: >> Simon Marlow (simonmar(a)microsoft.com) wrote: >>> Great news, thanks John. >>> >>> Is it possible to set up a two-way synch so we can move over to >>> darcs gradually? It's not really practical for us to move over in >>> one go, we've simply accumulated too many dependencies on CVS, and >>> there are lots of people using the repo with CVS. If we had a >>> two-way synch, we can experiment with darcs non-destructively. >> >> Great news, thanks Simon. >> >> Nice to hear you are considering to move to darcs. > > Hmmm, > > I don't want to spoil all the enthusiasm for a new tool, especially a > Haskell-based one, but being an old CVS veteran who recently fell into > love with subversion, I'd really like to know the benefits switching > to darcs. First of all, let me say that we haven't committed to darcs in any way - we're just planning to experiment with it. Besides, it's not my decision alone - lots of people (yourself included) have code in the CVS repository, so pulling the rug out is not an option. On to darcs vs. Subversion. I too was excited by Subversion until quite recently: it does a great job of being a "better CVS than CVS", but I don't think it goes far enough. The lack of distribution and good support for merging are the two main drawbacks, as I see it (I'm aware of SVK, though I haven't tried it). Fundamentally, darcs is built on a model that is far simpler than SVN: a repository is just a set of patches, a branch is just a repository. I think having a simpler model will have benefits in terms of accessibility - I've met several people who were just not comfortable with the CVS model, and couldn't get to grips with it. Distribution is useful, as others have mentioned. Having a local copy of the repo that I can commit to before pushing patches to the main repo is a big win, as is allowing other developers to have their own local branches, and the convenience of 'darcs send' is going to make it easier for people to contribute to GHC, and for us to accept contributions. Last I checked, SVN's branching support was lacking: making a branch is easy, but merging between branches is hard, just like CVS. Sure they plan to fix this (or maybe they've already improved things), but my guess is that darcs' support for merging will always be superior because It's What Darcs Does. Good support for merging just falls out of darcs' simpler model. And I aggree with Manuel's point: we should support darcs because it's written in Haskell! Cheers, Simon

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A little bit OT: Google as a global directory/resolver for cabalized packages?
by Dimitry Golubovsky 03 May '05

03 May '05

If one tries to search Google: http://www.google.com/search?hl=en&lr=&c2coff=1&q=exposed-modules+version+f… it returns URLs of all Google-indexed .cabal files (currently 7- just this few?) So if one makes such a request from a program, then parses the resulting HTML extracting <a> tags containing links ending with ".cabal", it would result in a directory of .cabal files locations suitable for furter processing. Similarly, if all locations of a particular package need to be found, package name needs to be added to the query string. Using Google API might be an alternative, but it returns only up to 10 results (per their docs) which might be enough to find all mirrors of a single package, but not the whole list of available packages. Publishing in such a directory might be simple: just place one's .cabal file in a place where Google can index it. One weakness: too easy to spam. Could this be a (cheap) way to set up package name -> URL resolution? -- Dimitry Golubovsky Anywhere on the Web

3 2

Random State Monad and Stochastics
by Dominic Steinitz 02 May '05

02 May '05

I don't think they are in the standard libraries but there was some discussion about them a few months ago but I couldn't find a reference. Peter, Can you supply one? I think you were a participant in the discussion. Did you put a library of this sort of thing together? Here's my tuppenceworth which I used to send a sequence of ip packets (as actions) and to stop when I got a destination unreachable: sequenceWhile_ :: Monad m => (a -> Bool) -> [m a] -> m () sequenceWhile_ p [] = return () sequenceWhile_ p (x:xs) = x >>= \c -> if (p c) then sequenceWhile_ p xs else return () Dominic.

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RE: [Haskell-cafe] Typeclasses and instances
by Simon Peyton-Jones 02 May '05

02 May '05

I've finally gotten around to fixing this. The fix won't be in 6.4 (ever) as mentioned below, but it's done in the HEAD. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Simon Peyton-Jones | Sent: 18 March 2005 13:01 | To: Mark Carroll; haskell-cafe(a)haskell.org | Subject: RE: [Haskell-cafe] Typeclasses and instances | | Strange. This is a bug in GHC that crept in when I implemented GADTs. | It turns out to be slightly structural; fixing it will change the | interface-file format. So it won't get fixed in 6.4; but will in the | HEAD. That doesn't bother me; the context on a data/newtype decl is a | strange thing anyway; only useful for generating extra errors... but | yes, it's a bug thanks for pointing it out. | | Simon | | | -----Original Message----- | | From: haskell-cafe-bounces(a)haskell.org | [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Mark | | Carroll | | Sent: 17 March 2005 19:58 | | To: haskell-cafe(a)haskell.org | | Subject: Re: [Haskell-cafe] Typeclasses and instances | | | | Another note, with more help from friends: | | | | It turns out that GHC 6.4 will let me do, | | | | newtype Floating a => Test a = Test [a] deriving Show | | | | x = Test [False, True] | | | | but, if I change "newtype" to "data", it then says, | | | | No instance for (Floating Bool) | | | | I'm not sure I quite understand what's going on. | | | | -- Mark | | _______________________________________________ | | Haskell-Cafe mailing list | | Haskell-Cafe(a)haskell.org | | http://www.haskell.org/mailman/listinfo/haskell-cafe | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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HGL documentation
by Peter Padawitz 02 May '05

02 May '05

Is there a HGL documentation in one volume (e.g. pdf file)? Printing all pieces of http://www.haskell.org/ghc/docs/latest/html/libraries/HGL is a little tedious... Peter

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lhs2TeX and line numbers in error messages produced by LaTeX
by Stefan Wehr (formerly Heimann) 02 May '05

02 May '05

Hi! I use lhs2TeX for typesetting literate haskell code. The problem is that LaTeX uses the line numbers from the .tex file generated by lhs2TeX and not the line number from the original .lhs file when producing error messages. Does anyone have a solution for this problem? Cheers, Stefan -- Stefan Wehr (formerly Heimann) Web: http://www.stefanwehr.de PGP: Key is available from pgp.mit.edu, ID 0B9F5CE4

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 02 May '05

02 May '05

On 28 April 2005 04:52, John Goerzen wrote: > I am pleased to announce that I have used tailor.py to successfully > convert the entire history of fptools HEAD branch, dating back to > 1996, from CVS to darcs. For those of you that don't know, fptools > represents the development area for the GHC Haskell compiler as well > as many other related projects. It contains 21,949 changesets. Great news, thanks John. Is it possible to set up a two-way synch so we can move over to darcs gradually? It's not really practical for us to move over in one go, we've simply accumulated too many dependencies on CVS, and there are lots of people using the repo with CVS. If we had a two-way synch, we can experiment with darcs non-destructively. Off the top of my head, a few other things we need before we can even think of switching over: - split up the full fptools repo into pieces (as we discussed on #haskell). - a web interface to the repo - commit mails to the cvs-<blah>@haskell.org lists Cheers, Simon

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 01 May '05

01 May '05

On 28 April 2005 16:02, John Goerzen wrote: >>> 3. Rename the smaller project's files as appropriate >>> >>> 4. Checkpoint here >> >> Won't that leave a lot of useless history in each individual project? >> When I do 'darcs get' locally to check out a few sub-repositories, >> won't I get N copies of all the old history? > > Yes, but they'll all be hardlinked together, so no matter how many > copies you get, the old history is only stored on disk once. If I do 'darcs get' to get a bunch of different repositories from cvs.haskell.org to my local filesystem, they won't all end up hard-linked together, surely? > I bet some darcs hacker could come up with a script that would even > let us remove the patches from a descended repo that don't apply > anymore. Now that sounds like a plan. > I know it's not ideal, but trying to manually convert each part of the > CVS repo, one at a time, is far, far less ideal. Not something I care > to attempt, anyway :-) I don't understand why converting each part of the repo separately is so hard? (but I've never used the tools - I'm just curious about why it's so difficult). > The other thing is that having a big repo in darcs may turn out to be > not as bad as we all thought at first. The only really annoying thing > there is how long it takes to do an initial darcs get (~5 min for me > if I use --partial). Other than that, darcs performs quite nicely. I agree with your list of pros/cons, and just to be clear, I'm not stuck on the idea of having a split repo. In fact, the simplicity of a single repo is very attractive. But what worries me is: if I just want to check out e.g. Haddock, I have to get the entire fptools repo (350M+, wasn't it?). I can build a source distribution with just the bits I want, but I can't get a darcs tree with anything but the whole lot. So, here's two potential solutions: 1. Make it possible to 'darcs get' just part of a tree. Patches that don't touch any files in the "live" parts of the tree are discarded. (I don't know if this is possible, or how difficult it is). 2. Create separate repositories for GHC, Happy, Haddock etc., and duplicate the shared fptools structure in each project. Each time we modify something in the shared part of the tree, we pull the patch into the other trees. (is it possible to cherry-pick from a tree that doesn't have a common ancestor? If not, can we make the repositories appear to have common ancestry?). Cheers, Simon

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Random State Monad and Stochastics
by Lemming 01 May '05

01 May '05

When working with QuickCheck (the framework for tests with randomised input) I encountered some interesting connections between random variables in stochastics and their implementation in Haskell. I see the following correspondence between notions in programming and in mathematics: random generator ~ random variable / probabilistic experiment result of a random generator ~ outcome of a probabilistic experiment Thus the signature rx :: (Random a, RandomGen g) => State g a can be considered as "rx is a random variable". In the do-notation the line x <- rx means that "x is an outcome of rx". In a language without higher order functions and using a random generator "function" it is not possible to work with random variables it is only possible to compute with outcomes, e.g. rand()+rand(). In a language where random generators are implemented as objects computing with random variables is possible but still cumbersome. In Haskell we have both options either computing with outcomes do x <- rx y <- ry return (x+y) or computing with random variables liftM2 (+) rx ry This means that liftM like functions convert ordinary arithmetic into random variable arithmetic. But there is also some arithmetic on random variables which can not be performed on outcomes. For example, given a function that repeats an action until the result fulfills a certain property (I wonder if there is already something of this kind in the standard libraries) untilM :: Monad m => (a -> Bool) -> m a -> m a untilM p m = do x <- m if p x then return x else untilM p m we can suppress certain outcomes of an experiment. E.g. if State (randomR (-10,10)) is a uniformly distributed random variable between -10 and 10, then untilM (0/=) (State (randomR (-10,10))) is a random variable with a uniform distribution of {-10, ..., -1, 1, ..., 10}.

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combining HUnit and QuickCheck
by Lemming 30 Apr '05

30 Apr '05

I want to run randomized tests and others and I thought it would be a good idea to run QuickCheck tests in the HUnit framework. Eventually if a test fails the program shall fail. I couldn't figure out how to obtain the test result and a meaningful diagnostic message from QuickCheck. The functions I've found (quickCheck, test, runTests) report everything to the standard output. There is also QuickCheck.Batch.run which returns a TestResult. I wonder if there is already a function which decides whether the TestResult is a success or not and a function which turns the TestResult into a error string.

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Writing functions in Haskell.
by Daniel Carrera 29 Apr '05

29 Apr '05

Hello, I'm trying to get started with Haskell. I must say that as good as the language must be, the documentation was been a source of frustration. Only one document actually showed me how to get started (ie. run hugs or ghci), and I was asked to give out my email address before getting it. Sigh... Well, in any event, I did get that document in the end and I've been reading through it. I've compiled Hugs on my Solaris workstation. There were no errors at compilation. Yet, I think there's something seriously wrong with my installation. I can't seem to be able to defin functions: my/prompt $ hugs .... Hugs.Base> fibs 1 = 1 ERROR - Syntax error in input (unexpected `=') And some functions seem to be missing. Hugs.Base> Char.toUpper 'a' ERROR - Undefined qualified variable "Char.toUpper" I was hoping that someone could confirm or deny that my Hugs installation is broken. Thank you for your time. Cheers, Daniel.

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RE: [Haskell-cafe] RFE: Extensible algebraic user-defined data types?
by Simon Peyton-Jones 29 Apr '05

29 Apr '05

| Suppose I want to extend this datatype to the one including | possibility of neither Left or Right (i. e. None). Currently I have to | use Maybe Either, so my options are: | | Just (Left a) | Just (Right b) | Nothing | | If I could extend the Either datatype I might have (syntax may be | different, this is just an example) | | data NEither a b = <Either a b> | None As Ben says, this is altogether a bigger deal. You can imagine two main approaches 1. NEither simply "macro-includes" the constructors of Either in NEither, but they are different constructors. So Either.Left and NEither.Left would be different, and incompatible constructors. This is simple, but clunky; for example, now you'd need to use a qualified name to disambiguate which you meant, whenever you used Left. 2. Use sub-typing, so that a value (Left x) is *both* in type Either and in type NEither. This opens a very large and complicated design space, as Ben mentioned. Simon

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RFE: Extensible algebraic user-defined data types?
by Dimitry Golubovsky 29 Apr '05

29 Apr '05

Hi, Is it possible to have such a feature in the future versions of the Haskell language? For example, there is an Either datatype which is Left a| Right b. Suppose I want to extend this datatype to the one including possibility of neither Left or Right (i. e. None). Currently I have to use Maybe Either, so my options are: Just (Left a) Just (Right b) Nothing If I could extend the Either datatype I might have (syntax may be different, this is just an example) data NEither a b = <Either a b> | None where datatype in angles is a parent datatype, and all its possible data constructors are included, and their list is extended with None. which gave me possibilities: Left a | Right b | None Probably I wouldn't expect to be able to reuse Nothing here (although I might want to) because Nothing already has been defined to be of the type Maybe. This is just a suggestion, that's why it is posted in the Cafe. PS Or is there a similar feature in the language already? Regarding reusing constructor names across several datatypes: is it possible to qualify them with their enclosing datatype name, like Maybe.Nothing where there is a name conflict? Then I might reuse Nothing in my hypothetical data type, and it would be NEither.Nothing if conflicting with Maybe.Nothing PPS I may be missing something again, as always ;) -- Dimitry Golubovsky Anywhere on the Web

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about Ghci
by SCOTT J. 29 Apr '05

29 Apr '05

Hi, I use Windows XP and I like to start Ghci in extended mode. What do I have to do in order not having to type always :set -fglasgow-exts thanks Jan

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RE: [Haskell-cafe] How to speed things up
by Simon Marlow 28 Apr '05

28 Apr '05

On 28 April 2005 19:21, Greg Buchholz wrote: > Dmitry Vyal wrote: >> By the way, how to use Unboxed arrays and unsafeAccumArray Greg >> Buchholz mentioned? I can't find them in GHC 6.2 documentation. > > > http://www.haskell.org/~simonmar/haddock-example/Data.Array.Base.html Please use the documentation on the GHC site. The latest is here: http://www.haskell.org/ghc/docs/latest/html/libraries/ if you specifically need documentation for the libraries that came with GHC 6.2, it can be found here: http://www.haskell.org/ghc/docs/6.2/html/libraries/ Cheers, Simon

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How to speed things up
by Dmitry Vyal 28 Apr '05

28 Apr '05

Hello everybody. I have a long list consisted of a small number (about a dozen) of elements repeating in random pattern. I know all the possible elements. I need to count number of occurences of each particular element and to do i quickly. For example quick_func Eq a => [a] -> [(a,Int)] quick_func [1,2,3,1,2,9,1,9] == [(1,3),(2,2),(3,1),(9,2)] According to profiler this function is the bottle-neck in my sluggish program so I really need to speed it up. Any proposals?

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Newbie question -- fromInt
by Tim Rowe 28 Apr '05

28 Apr '05

I'm working through "The craft of functional programming" , and I've come to excercise 3.14: "Give a function to return the average of three integers". My attempt at an answer is: averageThree :: Int -> Int -> Int -> Float averageThree a b c = fromInt(a + b + c) / 3 but when I try to load this file I get an error, "Undefined variable fromInt". I get the error on winHugs version Nov 2003 and on Hugs version Nov 2003 (in both cases in Hugs mode). Curiously, when I found a machine with an old (2002) version of Hugs on it the file loaded fine. Has something changed between versions of Hugs, is my code reasonable, and how /do/ I convert an Int to a Float in Hugs version Nov 2003? Thanks in advance, -- Tim Rowe

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 28 Apr '05

28 Apr '05

On 28 April 2005 14:26, John Goerzen wrote: > To do that though, we should really identify a permanent home for the > canonical fptools darcs repo. I'm not really set up to provide > accounts for those that would need write access, and I don't want to > be the gatekeeper (I suspect nobody else wants that either <g>). > > If cvs.haskell.org is up to the task, that would be an ideal location > IMHO. I haven't yet convinced tailor.py to work with the pserver for > fptools, so if it can access the repo on the local filesystem, that > would be ideal. Plus, one could then cron it to run frequently. cvs.haskell.org is the way to go - we already have all the required accounts set up on that machine, and you get local access to the CVS repo. I've asked the owner of the box if it can be upgraded. Failing that, you'll just have to make darcs more efficient ;-) There's already an old version of darcs installed (0.9.18), and darcs repos in /home/darcs. Cabal is kept there, for example. We need to upgrade that version of darcs, I presume - can you point me at an appropriate RPM? > Now, having said that, I did keep the request in mind. I figure that > this big repo can be split up into smaller ones at any time after the > CVS mirroring is stopped. For each smaller one, the process would be: > > 1. Branch (darcs get) the master repo > > 2. Delete all the files that don't apply to the smaller project > > 3. Rename the smaller project's files as appropriate > > 4. Checkpoint here Won't that leave a lot of useless history in each individual project? When I do 'darcs get' locally to check out a few sub-repositories, won't I get N copies of all the old history? >> - commit mails to the cvs-<blah>@haskell.org lists > > I figure a cron job can do this. Every x minutes, run darcs changes > -s, and send copies of never-before-seen logs to the list. Should be > fairly trivial. I can do this too. (But only after the CVS gateway > is disabled; otherwise, you'd get two copies for every commit) Keep the cvs gateway for the time being; at some point we can seamlessly change to generating commit mails from the darcs repository. BTW, one thing I've always wanted is a hyperlink in the commit mail that takes me straight to the patch. Cheers, Simon

2 1

Ambiguous type signature in class declaration
by Bo Herlin 28 Apr '05

28 Apr '05

Hi I am trying to make a class like this: > class CRank a b where > rank :: a -> b -> Maybe Integer -- Nothing means b is out of range or badly constructed > unrank :: a -> Integer -> Maybe b -- Nothing means rank is out of range > count :: a -> Maybe Integer -- Nothing means infinity with possible instances like > data DPrime = Prime deriving (Show) > > instance CRank DPrime Integer where > rank Prime x = Nothing -- to be implemented: (rank Prime 11) should give (Just 4) > unrank Prime r = Nothing -- to be implemented: (unrank Prime 4) should give (Just 11) > count _ = Nothing -- Nothing means infinity and > data DFibonacci = Fibonacci deriving (Show) > > instance CRank DFibonacci Integer where > rank Fibonacci x = Nothing -- to be implemented > unrank Fibonacci r = Nothing -- to be implemented > count _ = Nothing -- Nothing means infinity but all i get is ERROR "./Cafe.lhs":8 - Ambiguous type signature in class declaration *** ambiguous type : CRank a b => a -> Maybe Integer *** assigned to : count Any suggestions anyone? Thanks in advance /Bo

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REMINDER: Contributions to the HC&A Report (May 2005 edition)
by Andres Loeh 28 Apr '05

28 Apr '05

Dear Haskellers, the deadline for the May 2005 edition of the Haskell Communities and Activities Report is only a few days away -- but this is still enough time to make sure that the report contains a section on *your* project, on the interesting stuff that you've been doing; using or affecting Haskell in some way. Many projects that have been included in former reports have not yet updated their entries. Please have a look at http://haskell.org/communities/topics.html and at the November 2004 edition for reference. * Has your project been listed in previous Reports, but is not yet updated? Please write a short update! * Are you no longer working on a project that was included in the Report? Write up what you are working on instead, and tell me if someone else has picked up the project. * Is some project you have heard about not included in any previous Report? Please let me know ... There is still time to write a completely new entry, on a new compiler, tool, library, company, user group, idea, ... -- as long as there is a connection to the Haskell language, there is a place for it in the Report! Submissions are due by next Tuesday, that is Tuesday, 03 May 2005. Please mail your entries to <hcar at haskell dot org>, in plain text or pseudo-(La)TeX format. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2005-April/015688.html I look forward to receiving your contributions. Thanks a lot, Andres (current editor) -- Haskell Communities and Activities Report (http://haskell.org/communities) 05/2005 edition submission deadline: 03 May 2005 <hcar(a)haskell.org>

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