- Haskell-Cafe - Haskell.org

Silly question
by Joel Reymont 31 Oct '05

31 Oct '05

How do I run z "a" from ghci or the IO monad? module Foo where import qualified Data.Map as M import Control.Monad import Control.Monad.List import Prelude hiding ((!!)) (!!) :: (Ord k, MonadPlus m) => [M.Map k v] -> k -> m v (!!) maps key = msum $ map (M.lookup key) maps x :: M.Map String Int = M.empty y :: M.Map String Int = M.fromList [("a", 1)] z m = [x, y] !! m Thanks, Joel -- http://wagerlabs.com/

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RE: [Haskell-cafe] FFI and modifying haskell memory
by Simon Marlow 29 Oct '05

29 Oct '05

On 21 October 2005 14:40, Joel Reymont wrote: > I stumbled upon this when binding the Zlib library and want to make > sure I'm doing this right. > > foreign import ccall unsafe "uncompress" uncompress_ :: > Ptr Word8 -> Ptr CULong -> Ptr Word8 -> CULong -> IO CInt > > You give this function the address of your size as second argument > and it updates size for you. basic uncompress(char* buf, unsigned > long *size, ...) for you. > > Is it correct that I need to allocate a chunk of 4 bytes to be passed > into C and updated there and then need to peek my value from that > chunk of memory? yes, something like with $ \pculong -> r <- uncompress_ buf pculong ... size <- peek pculong > I thought StablePtr would give me the address of a Haskell value but > it gives me an offset into some table instead. That's right, the reason being that Haskell values move around when GC happens. Cheers, Simon

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hugs maintenance
by Iavor Diatchki 29 Oct '05

29 Oct '05

hello, i was wondering if anyone is maintaining hugs. in particular is the hugs-bugs list still operational? i tried to post a bug-report, but it seems that it never got there, but it also didn't bounce back... and the last post in the archive is from 10 sept 2005. -iavor

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RE: [Haskell-cafe] lockFile: fd out of range
by Simon Marlow 28 Oct '05

28 Oct '05

Yes, it's hardcoded. FD_SETSIZE should be the maximum number of file descriptors you can ever have open. On my Linux box here, FD_SETSIZE is 1024, and I can't ulimit -n any more than this. Cheers, Simon On 27 October 2005 15:18, Joel Reymont wrote: > Actually, I think I was just hitting the top range of FD_SETSIZE, > about 8000 on that machine. > > Does FD_SETSIZE get hardcoded into ghc-built binaries? That is if I > increase the available descriptors per process with uname -n, will it > be taken into account? > > Thanks, Joel > > On Oct 26, 2005, at 10:04 AM, Simon Marlow wrote: > >> lockFile uses FD_SETSIZE as its idea of the maximum value of a file >> descriptor. If you can get file descriptors outside this range, then >> the above error would ensue. Not sure if this is the case on >> FreeBSD... seems odd if it is. Can you try a truss/ktrace and see >> the values of your file descriptors?

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LINQ
by Niklas Broberg 28 Oct '05

28 Oct '05

Why is it that everything that OO "steals" from the functional paradigm is always marketed as something new that will revolution the way we program? Can't they at least give some credit where credit is due? :-p http://msdn.microsoft.com/netframework/future/linq/default.aspx?pull=/libra… /Niklas

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Quickcheck examples and Data.Word32
by Joel Reymont 28 Oct '05

28 Oct '05

Folks, Does anyone have QuickCheck examples they could send me? Also, how can I check Word32, Word64, etc. properties? It looks like the built-in random generator only works with Int values and for the life of me I cannot figure out how to extend it. Last but not least, I'm trying to figure out how I can use QuickCheck with tests against a network server. I would want to make sure that a certain packet sequence always produces a particular packet sequence in response. Is this a good area to apply QC? Thanks, Joel -- http://wagerlabs.com/

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hugs type inference
by Christian Maeder 28 Oct '05

28 Oct '05

Hi, Hugs leaves an odd context when showing the type of the following expression. Is this a bug or just a harmless flaw? Christian Hugs.Base> :t toRational 1 / 2 toRational 1 / 2 :: Real a => Ratio Integer ghci yields: Prelude> :t toRational 1 / 2 <interactive>:1:0: Warning: Defaulting the following constraint(s) to type `Integer' `Real a' arising from use of `toRational' at <interactive>:1:0-9 In the first argument of `(/)', namely `toRational 1' toRational 1 / 2 :: Rational

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Nice way to calculate character frequency in a string
by Scherrer, Chad 28 Oct '05

28 Oct '05

Sorry to drag this thread out, but here's one more thing you might try... I was thinking, if we just wanted something like intTable :: [Int] -> [(Int, Int)] we could just replace Map with IntMap in the previous solution: intTable xs = IntMap.assocs $! foldl' f IntMap.empty xs where f m x = let m' = IntMap.insertWith (+) x 1 m Just v = IntMap.lookup x m' in v `seq` m' To get another polymorphic version, we could just write this wrapper: freq :: (Enum a) => [a] -> [(a,Int)] freq = map fstToEnum . intTable . map fromEnum where fstToEnum (x,y) = (toEnum x, y) This seems to run faster than the other polymorphic version on my machine. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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Monads as control structures?
by Creighton Hogg 28 Oct '05

28 Oct '05

Hi, so I'm a newbie getting used to Haskell. I'm writing some simple things like genetic algorithms in it for practice, and I keep coming across something that really bugs me: are there any standard libraries that allow you to do imperative style for or while loops using monads to keep track of state? I know there's things like "until", but honestly that's not quite what I'm looking for. I just think there should be a simple way to say "execute this block of code 10 times" without having to wrap it up in recursion. Haskell seems to me to be a very powerful language, and it looks like it should be possible to define control structures such as for loops using monads.

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Re: [Haskell-cafe] Monads as control structures?
by Creighton Hogg 28 Oct '05

28 Oct '05

On Thu, 27 Oct 2005, Creighton Hogg wrote: > On Thu, 27 Oct 2005, Robert Dockins wrote: > > > > > On Oct 27, 2005, at 11:54 AM, Creighton Hogg wrote: > > > > > Hi, > > > so I'm a newbie getting used to Haskell. I'm writing some > > > simple things like genetic algorithms in it for practice, > > > and I keep coming across something that really bugs me: > > > are there any standard libraries that allow you to > > > do imperative style for or while loops using monads to keep > > > track of state? > > > One way is to create a list of the actions you want to execute, and > > then use one of the sequence family of functions. > > The actions can share state with an IORef or STRef or whatever. > > Another option is to use a fold with >>= to allow > > actions to pass their results directly to the next action. This > > works even in "stateless" monads like the list monad. > > > > Some examples using sequence: > > > > > > forMonad :: Monad m => a -> (a -> Bool) -> (a -> a) -> (a -> m ()) -> > > m () > > forMonad init cond inc f = sequence_ $ map f $ takeWhile cond $ > > iterate inc init > > > > xTimes :: Monad m => Int -> (Int -> m ()) -> m () > > xTimes x f = sequence_ $ map f [0..(x-1)] > > > > main = do { forMonad 0 (<10) (+1) (putStrLn . show); xTimes 10 (\_ -> > > putStrLn "hi") } > > Thanks to all who answered, this gives me a lot to chew on. > I especially like these examples, because it makes me > realize that I've been thinking about monads and evaluation > wrong. > I think doing genetic programming in Haskell is going to be > easier than I thought. >

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