December 2008 - Haskell-Cafe

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 15 Dec '08

15 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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Minimal complete definitions
by George Pollard 15 Dec '08

15 Dec '08

Good afternoon Café, I've written a little bit of code to calculate minimal complete definitions for a class given which of its functions use which other functions. As an example: doDependencies ord = ([],[["<="],["compare"]]) doDependencies num = (["plus","times","abs","signum","fromInteger"],[["minus"],["negate"]]) The first part of the pair is those functions which must *always* be implemented, the second part is a list of possible minimal complete definitions available for the provided list. This can help catch mistakes; a comment in the GHC source for GHC.Classes notes that compare must be implemented using (<=) and not (<) in order to give the minimal complete definition (<= OR compare). If we use the incorrect (<) then my code calculates the MCD as: doDependencies wrongOrd = ([],[["<"],["<","<=","compare"],["compare"]]) That is, the MCD is (< OR (< AND <= AND compare) OR compare). Now I have two questions: 1) Is my code correct? ;) 2) Could this be incorporated into GHC in order to detect when someone hasn't provided a sufficient definition for a class? As an example, it could detect this: > ~$ cat test2.hs > data Die d = Die d > instance Eq (Die d) where > main = do > let i = Die "stack overflow" > print (i == i) > ~$ ghc -Wall test2.hs --make > ~$ ./test2 > Stack space overflow: current size 8388608 bytes. > Use `+RTS -Ksize' to increase it. Given the following: doDependencies [("==", Just ["/="]),("/=", Just ["=="])] = ([],[["/="],["=="]]) GHC could warn that either (==) or (/=) must be implemented. Thanks, - George

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Re: Re: [Haskell-cafe] Multi-parameter type class woes
by Mario Blažević 15 Dec '08

15 Dec '08

> I'll take a swing at this one: > > instance Container (Maybe x) [x] where > wrapper = isNothing > . . . > > That isn't a sensible definition of 'wrapper', but I believe without > trying to compile it is completely legal. Which wrapper do you use? > > You /don't/ have a different matching Container instance, but without the > functional dependency you /might/, and ghc barfs. But liftWrap doesn't require any particular instance, it's a generic function accepting any pair of types for which there is an instance of Container. Instance selection (as I understand it) shouldn't come into play until one applies liftWrap to a particular type, and indeed it does cause problems there: note the type annotations on the last line. That part I understand and accept, or at least have learned to live with. > On Sun, 14 Dec 2008, Mario Bla?evi? wrote: > >> I have, for a change, a relatively simple problem with >> type classes. Can somebody explain to me, or point me to an explanation of >> the behaviour I see? >> >> Here is a short and useless example: >> >> {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} >> >> import Data.Maybe >> >> class Container x y where >> wrapper :: x -> Bool >> unwrap :: x -> y >> rewrap :: y -> x >> >> liftWrap :: Container x y => (y -> y) -> (x -> x) >> liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x >> >> instance Container (Maybe x) x where >> wrapper = isJust >> unwrap = fromJust >> rewrap = Just >> >> main = print (liftWrap (succ :: Int -> Int) (Just 1 :: Maybe Int)) >> >> GHC 6.10.1 refuses to typecheck the 'wrapper' function >> in definition of 'liftWrap', with the following error message: >> >> Could not deduce (Container x y) from the context (Container x y1) >> arising from a use of `wrapper' at Test.hs:11:22-30 >> Possible fix: >> add (Container x y) to the context of >> the type signature for `liftWrap' >> In the expression: wrapper x >> In the expression: >> (if wrapper x then rewrap . f . unwrap else id) x >> In the definition of `liftWrap': >> liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x >> >> Let me clarify that I'm aware that in this particular >> example a functional dependecy should be used. Also, I can think of a few >> workarounds for my actual problem, so I'm not asking for any solutions. I'm >> looking for an explanation. It bugs me that my intuition of how this type >> class should have worked is completely wrong. The error message does not >> help, to put it mildly. Where should I go, what should I read? >> _______________________________________________ >> 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: haskell compiler never comes back
by Greg Meredith 15 Dec '08

15 Dec '08

George, Thanks for the response. If i take out the AllowUndecidableInstances i get no complaints and the compiler hangs. See this<http://paste.pocoo.org/show/95523/>. Thus, i can find no observable difference for this flag in this particular code sample. Best wishes, --greg On Mon, Dec 15, 2008 at 2:34 PM, George Pollard <porges(a)porg.es> wrote: > This is precisely what AllowUndecidableInstances allows; the type > checking becomes possibly non-terminating. It should *eventually* > terminate because the stack depth is restricted. > > - George > -- 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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Data.List.Split
by Brent Yorgey 15 Dec '08

15 Dec '08

Today yet another newbie in #haskell asked about a 'split' function for lists, and I got fed up with saying 'no one can agree on the right interface so it doesn't exist, just write it yourself', because it's a really dumb answer, even if it's true. Instead of trying to get a 'split' function added to Data.List (which will never ever happen), I thought, why not create a module Data.List.Split which contains implementations of every conceivable way to split a list? Of course, people will probably still argue over what should go in such a module, what to name the various functions, etc., but hopefully we can converge on something useful. I've created a Data.List.Split page on the wiki: http://haskell.org/haskellwiki/Data.List.Split Please add code to it! Once something useful is hashed out we can upload it to hackage. Perhaps eventually some of it can be folded into the standard libraries, but this seems like a much more lightweight way to get started. -Brent

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Re: [Haskell-cafe] Time for a new logo?
by Brettschneider, Matthias 15 Dec '08

15 Dec '08

If there is someone interested in playing around with the logo Don posted, I made a gimp-version out of it. http://frosch03.de/haskell/Haskell.xcf http://frosch03.de/haskell/Haskell.png -- Matthias

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