- Haskell-Cafe - Haskell.org

Re: [Haskell] possible bug in pretty-1.0.1.0
by Duncan Coutts 18 Dec '08

18 Dec '08

On Mon, 2008-12-15 at 08:17 -0800, John MacFarlane wrote: > I noticed a difference in how "hang" works between pretty-1.0.0.0 and > pretty-1.0.1.0. I think it's a bug. If this isn't the right place to > report it, please let me know where I should. (Maintainer is listed > as libraries(a)haskell.org, but that is a closed mailing list. Perhaps > Cabal should include a report-bugs-at field?) I'd just like to advertise the fact that as of Cabal-1.6 you can put a bug-reports field in your .cabal file and it will be displayed by hackage. We would like to encourage all package authors to use this. It can be a mailto: url to a maintainer or mailing list or it can be a http: url to a bug tracker website. (Cabal-1.6 also supports specifying darcs/git/whatever repos) Duncan

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Re: [Haskell-cafe] Functional MetaPost in 5 Steps
by Stefan Endrullis 18 Dec '08

18 Dec '08

I have the same problem with FuncMP. I've also replaced the "generate" function by a custom "toMPFile" as you suggested, but mpost still generates only an empty white sheet. I've used the simple example posted in the first message of this thread. The generated .mp file is also relatively simple (see attachment http://www.nabble.com/file/p21070707/Test.mp Test.mp ) The only problem is the line "input FuncMP", which produces additional code in the ps file and causes the white page. If you remove this line everything is fine again and you can see the text "blah". Here's the additional code FuncMP produces in the .ps file: ----------------------------------------------------------------- /xdvi$run where {pop errordict begin /undefined {} bind def end} if /fmp1 /fmp1 def /fmp8 /fmp8 def /fmp24 /fmp24 def /bitline1 {gsave pop dup scale dup length 8 mul 1 true currentpoint translate [66.66666 0 0 66.66666 0 0] 4 index imagemask pop grestore} bind def /bitline8 {gsave pop dup scale dup length 1 8 currentpoint translate [2.8 0 0 2.8 0 0] 4 index image pop grestore} bind def /bitline24 {gsave pop dup scale dup length 3 idiv 1 8 currentpoint translate [2.777777 0 0 2.777777 0 0] 4 index false 3 colorimage pop grestore} bind def /XDVIfshow {findfont exch scalefont setfont show} bind def /DVIPSfshow {exch gsave 72 TeXDict /Resolution get div -72 TeXDict /VResolution get div scale 1 DVImag div dup scale get cvx exec show grestore} bind def /fshowText {/xdvi$run where {pop XDVIfshow} {DVIPSfshow} ifelse} def /fshow { exch dup /fmp1 eq {bitline1} {dup /fmp8 eq {bitline8} {dup /fmp24 eq {bitline24} {fshowText} ifelse} ifelse} ifelse} def ----------------------------------------------------------------- You can also fix the .ps file by removing the "/fshow ..." part from the file and the text "blah" will appear in your ghostscript viewer. Well, this is my first test with FuncMP and that's why I'm asking this silly question now: Is the "fshow" function really necessary or can I remove it from every generated .ps file without problems? :) Or you know another solution? Does your generated .ps files also contain this code? If no, what version of FuncMP do you use? -- View this message in context: http://www.nabble.com/Functional-MetaPost-in-5-Steps-tp20144360p21070707.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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excessive usage of CPU by threadDelay in GHCi
by Jeremy Shaw 18 Dec '08

18 Dec '08

Hello, I have the following simple program: import Control.Concurrent main = threadDelay (10^6) >> main If I run it in GHCi it requires 2-5% of my CPU. If i compile it, it takes 0% of my CPU. It does not matter if I compile -O0, -O2, -threaded, it always uses 0% (which is good). Is it expected that threadDelay should be really expensive in GHCi? Am I doing something wrong? Or should I file a bug? Thanks. - jeremy

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How to choose an arbitrary Arbitrary?
by Bas van Dijk 18 Dec '08

18 Dec '08

Hello, I was playing with the following tree type (attached below) which can be seen as the reification of an applicative. I wondered if I could define a QuickCheck Arbitrary instance for it. The only way I got it to type check however, was to give 'arg' a monomorphic type (for example: 'Gen (Tree ())'). If I left it polymorphic I got a "Ambiguous type variable in constraints" error. This is understandable because if the subtrees have polymorphic types they can be of any type: Tree Int, Tree Float, Tree String, etc. The system then doesn't know which Arbitrary to choose. My question is, is it possible to keep 'arg' polymorphic (i.e. 'Gen (Tree b)') and let the system somehow choose an arbitrary Arbitrary? I guess not, however I like to be proven wrong. regards, Bas ---------------------------------------------------- {-# LANGUAGE ExistentialQuantification #-} module Tree where import Control.Monad import Test.QuickCheck -- I'm using QuickCheck-2.1.0.1 data Tree a = Val a | forall b. Tree (b -> a) :*: Tree b instance Arbitrary a => Arbitrary (Tree a) where arbitrary = sized sizedTree sizedTree :: (Arbitrary a) => Int -> Gen (Tree a) sizedTree n | n <= 0 = liftM Val arbitrary | otherwise = liftM2 (:*:) func arg where m = n `div` 2 func = sizedTree m arg :: Gen (Tree ()) -- how to make this: -- Gen (Tree b) ??? arg = sizedTree m ----------------------------------------------------

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Implementing PacMan
by Andrew Coppin 18 Dec '08

18 Dec '08

What do we think of this, folks? http://prog21.dadgum.com/23.html (And the rest in the series, obviously.) To me, it seems that this plan would *work*... but it wouldn't be very eligant. You'd have the code to respond to user input and move PacMan in one place, the code for collision detection in other place, the code that decides what to *do* about a collision somewhere else, the code for drawing the animations in yet another place... If you wanted to change one game element, you'd have to make changes scattered all over the place. The whole thing seems messy, non-modular and non-composible to my ears. Thoughts, people?

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Re: [Haskell-cafe] Re: Time for a new logo?
by Jeff Wheeler 18 Dec '08

18 Dec '08

On Dec 16, 2008, at 10:08 PM, Ryan Grant wrote: > nice. > the first is better. > in the second, i don't even see the lambda. Thanks the feedback. I just uploaded a new version [1] that is icon- sized, although the font used is Helvetica Neue, which is non-free. I have no free fonts on my Mac, unfortunately, so it'll stay for the moment. Jeff Wheeler [1] http://media.nokrev.com/junk/haskell-logos/logo4.png

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Please tell me this function exists
by Brian Hurt 18 Dec '08

18 Dec '08

I know it's not hard to write, but still: concat :: String -> [String] -> String concat _ [] = "" concat _ [x] = x concat sep x:xs = x ++ sep ++ (concat sep xs) I've got to be stupid and missing it in the standard libraries. Please tell me where. Thanks. Brian

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monadic versus aplicative style. These two snips seem to do the same thing yet one has a bug.
by Thomas Hartman 17 Dec '08

17 Dec '08

Hi, I rewrote a piece of code that used applicative to use instead monadic style, basically because I wanted to adapt it for my own purposes but hadn't wrapped my head around applicative yet. Unfortunately my rewrite had a bug, which I'm still not completely clear on. (I am guessing it's a laziness mixed with IO issue.) I discarded my own version and am using the original now of course, but I'm curious if anyone has anything to say on the difference between these two bits of code and, in general if there is any way to "think" about the difference between monadic and applicative. For what it's worth, the code comes from HSTringTemplate (on hackage). The bug is described in http://groups.google.com/group/HAppS/browse_thread/thread/70d4b1fbe8a4c7ac Basically, in happs, this bit of code caused an unrelated handler to not fully pages correctly, where there was more than one image to load. It took forever to diagnose and track this down. Ugh. Thanks, thomas. -- incorrect behavior directoryGroup' :: (FilePath -> FilePath -> IO (StringTemplate a)) -> FilePath -> IO (STGroup a) directoryGroup' templatereader path = do fs <- return . ( filter ((".st" ==) . takeExtension) ) =<< getDirectoryContents path templates <- mapM (templatereader path) fs stmapping <- return $ zip fs templates return $ groupStringTemplates stmapping -- correct behavior --directoryGroup2' :: (Stringable a) => FilePath -> IO (STGroup a) directoryGroup'2 templatereader path = groupStringTemplates <$> (fmap <$> zip . (map dropExtension) <*> mapM ( templatereader path ) =<< filter ((".st" ==) . takeExtension) <$> getDirectoryContents path)

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Threads not running under GHC 6.10?
by Gwern Branwen 17 Dec '08

17 Dec '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 So, the Hint library was recently updated and I was editing Mueval to run with (i386) 6.10, when I discovered that for some reason, DoS'ing expressions were succeeding in rendering my machine unusable. Eventually, I discovered that my watchdog thread didn't seem to be running. But with +RTS -N2 -RTS all my tests did pass! Here's a simple example of what I mean; the following is basically a very lightly adapted version of the actual Mueval code: - ---------------- import Control.Concurrent (forkIO, killThread, myThreadId, threadDelay, throwTo, yield, ThreadId) import System.Posix.Signals (sigXCPU, installHandler, Handler(CatchOnce)) import Control.OldException (Exception(ErrorCall)) main :: IO () main = do tid ThreadId -> IO () watchDog tout tid = do installHandler sigXCPU (CatchOnce $ throwTo tid $ ErrorCall "Time limit exceeded.") Nothing forkIO $ do threadDelay (tout * 100000) -- Time's up. It's a good day to die. throwTo tid (ErrorCall "Time limit exceeded") yield -- give the other thread a chance killThread tid -- Die now, srsly. error "Time expired" return () -- Never reached. Either we error out in -- watchDog, or the evaluation thread finishes. - -------------- Now, from the looks of it, this should always error out with "Time limit exceeded." And the threading is done via forkIO, so it shouldn't matter how it's compiled (be it threaded or no) or run; nor do we need to worry about optimizations, since x has no sensible value for any input - it's always bottom. But the results are very different: gwern@craft:31542~>=ghc -fforce-recomp -O0 example.hs && ./a.out ^C^C gwern@craft:31532~>=ghc -fforce-recomp -O0 -threaded example.hs && ./a.out ^C^C gwern@craft:31536~>=ghc -threaded -fforce-recomp -O0 -threaded example.hs && ./a.out +RTS -N1 -RTS [a minute later] ^C^C^C^C^C^C gwern@craft:31540~>=ghc -threaded -fforce-recomp -O0 -threaded example.hs && ./a.out +RTS -N2 -RTS a.out: Time limit exceeded gwern@craft:31543~>=ghc -fforce-recomp -O2 example.hs && ./a.out a.out: Time limit exceeded gwern@craft:31544~>=ghc -threaded -fforce-recomp -O2 example.hs && ./a.out a.out: Time limit exceeded gwern@craft:31545~>=ghc -threaded -fforce-recomp -O2 example.hs && ./a.out +RTS -N1 -RTS a.out: Time limit exceeded gwern@craft:31546~>=ghc -threaded -fforce-recomp -O2 example.hs && ./a.out +RTS -N2 -RTS a.out: Time limit exceeded So it seems that without optimizations, or without explicit/multiple OS threads, the watchdog thread never gets called! I'm not any sort of expert on parallelism or GHC, but this seems like a bad thing to me. One final note: I suspect there are further ways this can manifest. When compiled with -O2, example.hs terminates (as it should). But Mueval does in fact set -O2 as a ghc-option:, and yet I still found it looping. So, does anyone know whether this is an already known bug or whether it's something else? - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkk63+EACgkQvpDo5Pfl1oITZgCdG2p4VcB6m5IhfqzOT0fi5qrI VagAnRILZSxBzadSj2wlzoOWfBuwdbJo =X61b -----END PGP SIGNATURE-----

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Type wildcards
by Eyal Lotem 17 Dec '08

17 Dec '08

Martin Foster (aka. EvilTerran) suggested an interesting idea, and I decided it was too nice to ignore/be forgotten inside Martin's head... So I'd like to try and suggest it. Type wildcards that allow partially specifying types, e.g: f :: _ -> String f x = show x This will instruct the type-inferrer to "fill out" the wild-card part only (e.g: Show a => a). This feature can be used for a couple of benefits: A. It allows hard-coding part of a type, rather than the all-or-nothing situation we have now (I want to force a single parameter to be more specific, but I'd still prefer to infer the whole type). B. Without scoped type variables, there are currently situations you cannot provide a type signature at all. This may mean that you cannot resolve type ambiguities in some cases, or make types more specific for some purpose. A type wild-card, in addition to scoped type variables, will allow to only specify the part we want. C. Its a nice way to kill the -Wall warnings about missing type declarations, if you want to: f :: _ f = "I am explicitly asking to infer this type, rather than specifying it" Eyal

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