December 2015 - Haskell-Cafe

Storable types with zero size
by Douglas McClean 19 Dec '15

19 Dec '15

Can the sizeOf a Storable type be 0? As far as I can see the documentation doesn't say. I wouldn't expect there to be a problem, but there also isn't an instance Storable (), which I was expecting to see. If so, should its alignment be 1 or 0? -Doug McClean

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No instance of ... error with Statistics.Sample.Histogram
by Sigrid Keydana 19 Dec '15

19 Dec '15

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Stackage update and plans for LTS Haskell 4
by Adam Bergmark 19 Dec '15

19 Dec '15

Hi café, This is an update on Stackage moving to GHC 7.10.3 and towards LTS Haskell 4. I especially encourage maintainers with packages on stackage to read the post since some packages have been disabled in the process. http://andnam.tumblr.com/post/135510474243/stackage-update-and-plans-for-lt… All the best, Adam Discussion on Reddit: https://www.reddit.com/r/haskell/comments/3xh5uq/stackage_update_and_plans_…

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Preventing sharing
by David Turner 19 Dec '15

19 Dec '15

I have been playing around with this definition of fib with a colleague: fib n = fibs !! n where fibs = 0 : 1 : zipWith (+) fibs (tail fibs) My initial expectation was that this should take linear time and constant space, but this proved not to be the case: it leaks (a linear amount of) space. We fixed this by replacing (!!) with a version that is strict in the head of its first argument, effectively forcing the list to be strict as far as is needed. With that fix we could quite happily do { print $ fib 1000000 } and all was well, so I tried to write a Criterion benchmark to show that the time taken was linear in n. And the space leak reappeared! Looking at the core, I think what had happened was that GHC had spotted that the inner definition of fibs didn't depend on n and floated it out to the top level so it could be shared between calls. Normally a good move, but in this case it's a disaster as it's much quicker to recalculate the list as needed than to keep it around for next time, for sufficiently large values of n. Now I'm a bit stuck: how do you prevent this from happening? Obviously here we could just implement fibs differently, but as a more general question, how would you prevent unwanted sharing from taking place? Cheers, David

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documentation of Haskell's hierarchical libraries
by s nedunuri 18 Dec '15

18 Dec '15

I am wondering where I might be able to get a reasonably complete documentation of Haskell's libraries. I was using the documentation at https://downloads.haskell.org/~ghc/latest/docs/html/libraries/ but I noticed that it was missing hash tables, which are located in Data.HashTable. I am not sure what other modules might be missing from there. Is there some place where everything (up to some release) is documented? thanks PS the documentation bundled with the Haskell Platform (7.10.2-a) is even worse as its missing large sections of what you find at the online documentation

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Re: [Haskell-cafe] Canned routines for the first say thousand digits of pi, e, sqrt 2, etc?
by Olaf Klinke 18 Dec '15

18 Dec '15

As it happens, I am just studying a presentation [1] Martin Escardo gave to students at the University of Birmingham. It contains Haskell code for exact real number computation. Among other things, there is a function that computes a signed digit representation of pi/32. It computes several thousand digits in a few seconds. I did not try it yet, but many irrational numbers are fixed points of simple arithmetical expressions. For example, the golden ratio is the fixed point of \x -> 1+1/x. Infinite streams of digits should be a type where such a fixed point is computable. Or you could use a sufficiently precise rational approximation and convert that do decimal in the usual way. import Data.Ratio import Data.List (iterate) -- one step of Heron's algorithm for sqrt(a) heron :: (Fractional a) => a -> a -> a heron a x = (x+a/x)/2 -- infinite stream of approximations to sqrt(a) approx :: (Fractional a) => a -> [a] approx a = iterate (heron a) 1 -- Find an interval with rational end-points -- for a signed-digit real number type SDReal = [Int] -- use digits [-1,0,1] interval :: Int -> SDReal -> (Rational,Rational) interval precision x = let f = foldr (\d g -> (a d).g) id (take precision x)) a d = \x -> ((fromIntegral d)+x)/2 in (f(-1),f(1)) Cheers, Olaf [1] www.cs.bham.ac.uk/~mhe/.talks/phdopen2013/realreals.lhs

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Canned routines for the first say thousand digits of pi, e, sqrt 2, etc?
by Jeffrey Brown 18 Dec '15

18 Dec '15

Do there exist libraries for finding the first thousand or so digits of irrational numbers? In base 12? I searhed all the "Numeric" and "Numerical" packages on Hackage and found nothing appropriate. I did figure out how to change the base of a number: https://github.com/JeffreyBenjaminBrown/play/blob/master/digits/chBase.hs but that method appears limited to numbers only a dozen or so digits long. My motivation is this video, in which James Zamerski puts the first couple hundred digits of pi to music. I've listened to it maybe 15 times in two days. https://www.youtube.com/watch?v=AOaR4NS7ObI It makes me want to try playing the first few hundred digits of other irrational numbers on the piano and try to harmonize them. -- Jeffrey Benjamin Brown

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haskell GIS projects
by Noon Silk 18 Dec '15

18 Dec '15

Hello Cafe, Is anyone using any nice modern GIS libraries in Haskell? Say for opening various geographic file formats, and doing geometric queries, etc? Searching often leads me to TerraHS: https://wiki.haskell.org/TerraHS but it's pretty out of date. Is there some cool library I'm missing? -- Noon Silk, ن "Every morning when I wake up, I experience an exquisite joy — the joy of being this signature."

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Need ideas how to model the lack of something
by martin 17 Dec '15

17 Dec '15

Hello all, here is a problem where I did not manage to find a suitable abstraction. The main idea goes like this: a List (and many other containers) can be seen as something containing "stuff". There is a function (:) that unconditionally adds an element to the container and returns a new container Now suppose the container has the possiblility to refuse having another element added to it, e.g. because it has only limited "space". In that case the corresponding function would have a signature of insert :: a -> C a -> Maybe (C a). If an item can successfully be added, then the returned container will be less space avaiable. I'd like stuff and space to be symmetrical (maybe there lies the first flaw, because I can enumerate the elements, but I cannot enumerate the space). A symmetry like electrones and holes. I started like this data C a = C { insert :: a -> Maybe (C a), remove :: Maybe (a, C a) } but I could not implement anything sensible on top of this. I'd be happy to hear any comments on this, including loud thinking and random ramblings.

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Haskell safe ccalls
by Valentino Picotti 17 Dec '15

17 Dec '15

Hi everybody, I’m new on this list and have a newbie question about Haskell FFI and safe calls. I have written the following lines of C code that rely on the RTS: Condition* init_cond(){ Condition* c = malloc(sizeof(Condition)); initCondition(c); return c; } Mutex* init_mutex(){ Mutex* m = malloc(sizeof(Mutex)); initMutex(m); return m; } void wait_condition(Condition* condition, Mutex* mutex) { ACQUIRE_LOCK(mutex); if(!waitCondition(condition, mutex)){ fprintf(stderr, "A: Wait failed!"); }; RELEASE_LOCK(mutex); } And the Haskell program is written as follow: data Condition = Condition data Mutex = Mutex foreign import ccall "init_cond" init_cond :: IO (Ptr Condition) foreign import ccall "init_mutex" init_mutex :: IO (Ptr Mutex) foreign import ccall "wait_condition" wait_condition:: Ptr Condition -> Ptr Mutex -> IO () main :: IO () main = do c <- init_cond m <- init_mutex wait_condition c m putStrLn “Hello!” The behaviour that i want from my application is the same as the following C program: int main(int argc, char** argv){ pthread_cond_t c = PTHREAD_COND_INITIALIZER; pthread_mutex_t m; pthread_mutex_init(&m, 0); pthread_mutex_lock(&m); pthread_cond_wait(&c, &m); printf(“I didn’t block!\n"); pthread_mutex_unlock(&m); return 0; } What I expect is that the program doesn’t reach the putStrLn line, but it prints “Hello!” and terminates with exit code 0. I can’t figure out what’s going on under the hood. Can someone explain me what i’m missing? (I’m familiar with Capabilities, Tasks, Haskell threads) Thanks, Valentino P.

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