- Haskell-Cafe - Haskell.org

Fwd: cabal-install sends invalid proxy password
by Valery V. Vorotyntsev 17 Dec '08

17 Dec '08

Hello, > I was surprised to discover that `cabal-install' -- a popular utility > for installing Hackage packages -- cannot work with HTTP proxies. > Despite all the necessary code linked in. > > `cabal update' command returns HTTP 407 (Proxy Authentication Required) > error. The problem is explained below and patches follow. See http://article.gmane.org/gmane.comp.lang.haskell.libraries/10420 No response so far. (I start to suspect that there is just one "proxied haskeller" subscribed to `libraries' and `cabal-dev' lists:) -- vvv

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How to think about this? (profiling)
by Patrick Perry 16 Dec '08

16 Dec '08

I agree with everyone else who has said that the better solution is Lemmih's. It is simple, fast, and does not use much memory. On the other hand, here is a more faithful implementation of what you were trying to write. To use mutable arrays, you need to work in either the IO or the ST monad. The advantage of ST is the function "runST", which is roughly equivalent to "unsafePerformIO", but is much safer. I hope this helps, Patrick \begin{code} import Control.Monad import Control.Monad.ST import Data.Array.ST import System.Environment -- | Create an array for storing the results of foo 0 through foo n. -- Initialize foo 0, foo 1, and foo 2, but set all other values to -- 'missing'. newMemo :: Int -> ST s (STUArray s Int Int) newMemo n = do arr <- newArray (0,n) missing :: ST s (STUArray s Int Int) writeArray arr 0 0 writeArray arr 1 1 writeArray arr 2 2 return arr -- | Code to indicate a missing value in the memo array. missing :: Int missing = -1 -- | Compute the value of the function at @i@, using the memo-ed results -- in the array. fooWithMemo :: STUArray s Int Int -> Int -> ST s Int fooWithMemo arr i = do x <- readArray arr i if x /= missing then return x else do r <- liftM3 (\ a b c -> a + b + c) (fooWithMemo arr $ i - 1) (fooWithMemo arr $ i - 2) (fooWithMemo arr $ i - 3) writeArray arr i r return r foo :: Int -> Int foo n = runST $ do arr <- newMemo n fooWithMemo arr n main = do (n:_) <- liftM (map read) getArgs print $ foo n \end{code}

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Reader monad, implicit parameters, or something else altogether?
by Bjorn Buckwalter 16 Dec '08

16 Dec '08

All, I have a growing amount of astrodynamics code that relies on various physical "constants". The problem with these so called "constants" are that they either aren't really constants or aren't well known. An example is the leap second table (see Data.Time.Clock.TAI). I'd like to be able to fetch current values of these constants at runtime and make them accessible to my astrodynamics functions by some means. To clarify, once initialized the constants will be considered constant for the remainder of the program. I'd store the constants in a data structure along the lines of: > data AstroData a = AstroData > { mu_Earth :: GravitationalParameter a > , leapSeconds :: LeapSecondTable > } I would like to know if there is any consensus on what is the best way to make such a data structure accessible in pure functions. Passing it explicitly would be a mess. It seems that two options are to use either a Reader monad or implicit parameters. Using a Reader monad is straight forward enough though it requires writing/converting code in/to monadic style and adds some clutter to the formulae. It seems implicit parameters could be cleaner but I've seen them referred to as everything from evil to "just what you need" and rendering the Reader monad obsolete... What do you people recommend? Thanks, Bjorn Buckwalter

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Random language humour
by Andrew Coppin 16 Dec '08

16 Dec '08

http://www.aegisub.net/2008/12/if-programming-languages-were-religions.html Seems pretty accurate, actually...

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How to think about this? (profiling)
by Magnus Therning 16 Dec '08

16 Dec '08

This behaviour by Haskell seems to go against my intuition, I'm sure I just need an update of my intuition ;-) I wanted to improve on the following little example code: foo :: Int -> Int foo 0 = 0 foo 1 = 1 foo 2 = 2 foo n = foo (n - 1) + foo (n - 2) + foo (n - 3) This is obviously going to run into problems for large values of `n` so I introduced a state to keep intermediate results in: foo :: Int -> State (UArray Int Int) Int foo 0 = return 0 foo 1 = return 1 foo 2 = return 2 foo n = do c <- get if (c ! n) /= -1 then return $ c ! n else do r <- liftM3 (\ a b c -> a + b + c) (foo $ n - 1) (foo $ n - 2) (foo $ n - 3) modify (\ s -> s // [(n, r)]) return r Then I added a convenience function and called it like this: createArray :: Int -> UArray Int Int createArray n = array (0, n) (zip [0..n] (repeat (-1))) main = do (n:_) <- liftM (map read) getArgs print $ evalState (foo n) (createArray n) Then I thought that this still looks pretty deeply recursive, but if I call the function for increasing values of `n` then I'll simply build up the state, sort of like doing a for-loop in an imperative language. I could then end it with a call to `foo n` and be done. I replaced `main` by: main = do (n:_) <- liftM (map read) getArgs print $ evalState (mapM_ foo [0..n] >> foo n) (createArray n) Then I started profiling and found out that the latter version both uses more memory and makes far more calls to `foo`. That's not what I expected! (I suspect there's something about laziness I'm missing.) Anyway, I ran it with `n=35` and got foo n : 202,048 bytes , foo entries 100 mapM_ foo [0..n] >> foo n : 236,312 , foo entries 135 + 1 How should I think about this in order to predict this behaviour in the future? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts

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haskell compiler never comes back
by Greg Meredith 16 Dec '08

16 Dec '08

Haskellians, The simple-minded and smallish code sample at this link<http://paste.pocoo.org/show/95503/>causes the compiler to go off into never-never land. Any clues would be greatly appreciated. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell] Missing Documentation
by Neil Mitchell 16 Dec '08

16 Dec '08

Hi Ron > This may very well be a FAQ, but I tried to search the archives and could not find a post... > Anyway, some documentation seems to be missing, which was there before I thought. For example: > http://www.haskell.org/ghc/docs/latest/html/libraries/haskell98/Random.html That's a bug, I've reported it here: http://hackage.haskell.org/trac/ghc/ticket/2886 > Thanks, and sorry if this lowers the S/N We usually use haskell@ for annoucements, and haskell-cafe@ for general discussion/questions etc, so in future these kinds of posts should go on that mailing list. It's a confusing system! Any follow ups to this post should also only go to haskell-cafe@ Thanks Neil

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MVar and Par ..
by Mozhgan Kabiri 16 Dec '08

16 Dec '08

Hi .. Hope you are doing well . I've just joined this group. Recently, I am struggling to do some simple experiment with haskell language about parallelism and wrong answers that we can get while using a shared variable . I tried to write a simple program, for example calculationg 'n=n+1' few times.And then I tried to do it in parallel by using 'par' and 'pseq' . The aim was to get the wrong answer because we have to share a variable here,and without using 'MVar' function we will get the wrong answer for the calculation .I don't know how to write it in parallel in order to get a wrong answer when we don't use MVar,because we have a shared variable here. I read about MVars as well,but also I don't know how to combine MVar and Par together to get the program to work.I wrote this :module Main where f :: Int -> Int -> Int f i n = g 1 i n where g x i n | x <= i = g (x+1) i (n+1) | otherwise = n main :: IO () main = do putStrLn "starting..." let r = f 10 5 putStrLn (show r) putStrLn "finished" I want to make to work in parallel by using 'Par'.And also use MVar for this simple example to work.All of the example about MVar are a little bit complicated and I couldn't figure it that how can I write one,the same !Can any one help me with this ? I want a simple example that I can feel the need of MVar when I run my program in parallel and while I am using a shared variable.Regards; Mozhgan _________________________________________________________________ News, entertainment and everything you care about at Live.com. Get it now! http://www.live.com/getstarted.aspx

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MVar and Par ..
by Mozhgan Kabiri 16 Dec '08

16 Dec '08

Hi .. Hope you are doing well . I've just joined this group. Recently, I am struggling to do some simple experiment with haskell language about parallelism and wrong answers that we can get while using a shared variable . I tried to write a simple program, for example calculationg 'n=n+1' few times.And then I tried to do it in parallel by using 'par' and 'pseq' . The aim was to get the wrong answer because we have to share a variable here,and without using 'MVar' function we will get the wrong answer for the calculation .I don't know how to write it in parallel in order to get a wrong answer when we don't use MVar,because we have a shared variable here. I read about MVars as well,but also I don't know how to combine MVar and Par together to get the program to work.I wrote this :module Main where f :: Int -> Int -> Int f i n = g 1 i n where g x i n | x <= i = g (x+1) i (n+1) | otherwise = n main :: IO () main = do putStrLn "starting..." let r = f 10 5 putStrLn (show r) putStrLn "finished" I want to make to work in parallel by using 'Par'.And also use MVar for this simple example to work.All of the example about MVar are a little bit complicated and I couldn't figure it that how can I write one,the same !Can any one help me with this ? I want a simple example that I can feel the need of MVar when I run my program in parallel and while I am using a shared variable.Regards; Mozhgan Get news, entertainment and everything you care about at Live.com. Check it out! _________________________________________________________________ Explore the seven wonders of the world http://search.msn.com/results.aspx?q=7+wonders+world&mkt=en-US&form=QBRE

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Re: [Haskell-cafe] Multi-parameter type class woes
by Mario Blažević 16 Dec '08

16 Dec '08

> I think that http://www.haskell.org/pipermail/haskell-cafe/2008-April/041461.html > may be relevant. It's a design decision. Thanks for the link. I've read through the thread, but rather than try to figure out if it's the same issue and whether it's a design decision or a historical accident, I've decided to create a ticket (#2885) and let GHC developers decide if it's valid or not.

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