June 2008 - Haskell-Cafe

Why the exception?
by Thomas M. DuBuisson 24 Jun '08

24 Jun '08

Cafe I'm a bit lost on this exception and curious about what's going on. Is there a valid reason for this exception that I am missing? Note the hard-coded [0..100] could be any Word8 list you want (generated via arbitrary, [], or other) and it gives the same result. Load the module and perform: :break prop_LPS quickCheck prop_LPS :step :force ps *** Exception: Prelude.head: empty list > import qualified Data.ByteString as L > import Test.QuickCheck > > instance Arbitrary L.ByteString where > arbitrary = do > return $ L.pack [0..100] > > prop_LPS :: L.ByteString -> Bool > prop_LPS ps = ps `seq` True P.S. Same result with GHCi 6.8.2 and 6.8.3.

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Fwd: Cabalized, Self-Demoing, HStringTemplate-using HAppS Tutorial
by tphyahoo 24 Jun '08

24 Jun '08

---------- Forwarded message ---------- From: tphyahoo <thomashartm...(a)googlemail.com> Date: Jun 24, 7:46 pm Subject: Cabalized, Self-Demoing, HStringTemplate-using HAppS Tutorial To: HAppS There are some issues with the cabalized project -- templates are missing. Doh. Use darcs for now. Thomas. On Jun 24, 7:26 pm, tphyahoo <thomashartm...(a)googlemail.com> wrote: > Hello HAppSers! > I have created a self-demoing, HStringTemplate-using intro to HAppS. > There is a live demo at >http://rippledeals.com:5001 > It is cabalized at >http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happs-tuto... > and should install fine with sudo cabal install happs-tutorial. > There is a darcs repo at >http://code.haskell.org/happs-examples > And a google project at >http://code.google.com/p/happs-tutorial/ > which currently isn't used, but who knows, maybe for the wiki or issue > tracking. (I started at google hosting before migrating to darcs.) > I have all kinds of additional lessons planned, but under the release > early/release often motto I'm letting it out in its embryonic state. > Enjoy! > Thomas.

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Fwd: Cabalized, Self-Demoing, HStringTemplate-using HAppS Tutorial
by tphyahoo 24 Jun '08

24 Jun '08

---------- Forwarded message ---------- From: tphyahoo <thomashartm...(a)googlemail.com> Date: Jun 24, 7:26 pm Subject: Cabalized, Self-Demoing, HStringTemplate-using HAppS Tutorial To: HAppS Hello HAppSers! I have created a self-demoing, HStringTemplate-using intro to HAppS. There is a live demo at http://rippledeals.com:5001 It is cabalized at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happs-tuto... and should install fine with sudo cabal install happs-tutorial. There is a darcs repo at http://code.haskell.org/happs-examples And a google project at http://code.google.com/p/happs-tutorial/ which currently isn't used, but who knows, maybe for the wiki or issue tracking. (I started at google hosting before migrating to darcs.) I have all kinds of additional lessons planned, but under the release early/release often motto I'm letting it out in its embryonic state. Enjoy! Thomas.

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Bug in Emacs Haskell Mode
by Ronald Guida 24 Jun '08

24 Jun '08

Emacs Haskell Mode has the following useful feature: when "Haskell -> Load File" is used to load a file into GHCi from Emacs, Haskell Mode automatically looks for a "*.cabal" file in an attempt to find the project directory. When Haskell Mode finds the *.cabal file, it fails to check whether it has found a file or a folder. As a result, if I want to load a Haskell file that has no corresponding .cabal file, then Haskell Mode locates my "~/.cabal" *folder* and assumes that it's my project directory. I have a "~/.cabal" folder because Cabal-Install puts it there. Cabal-Install is part of Haskell's new package management system. Now I don't know very much Emacs Lisp, but I figured out a fix for the problem. In Haskell-Mode-2.4, in the file haskell-cabal.el, I changed the function "haskell-cabal-find-file" so that it explicitly checks to make sure that any *.cabal file is in fact a file and not a folder. I don't know if comparing the first letter of the file mode string to 'd' is the right way to diferentiate a folder from a file, but so far it appears to work for me, so I thought I'd share it. (defun haskell-cabal-find-file () "Return a buffer visiting the cabal file of the current directory, or nil." (catch 'found (let ((user (nth 2 (file-attributes default-directory))) ;; Abbreviate, so as to stop when we cross ~/. (root (abbreviate-file-name default-directory)) files) (while (and root (equal user (nth 2 (file-attributes root)))) (let ((files (directory-files root 'full "\\.cabal\\'"))) (if (and files (not (equal 100 (aref (nth 8 (file-attributes (car files))) 0)))) (throw 'found (find-file-noselect (car files))) (if (equal root (setq root (file-name-directory (directory-file-name root)))) (setq root nil)))) nil))))

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sdl-mixer loading issue
by Norbert Wojtowicz 24 Jun '08

24 Jun '08

Hello, I am trying to use the SDL-mixer bindings to play wav files. I've hit a wierd error and I can't figure out how to solve it: Prelude> import Graphics.UI.SDL.Mixer.Music Prelude Graphics.UI.SDL.Mixer.Music> loadMUS "test.wav" Loading package SDL-0.5.4 ... linking ... done. Loading package SDL-mixer-0.5.2 ... linking ... <interactive>: /home/norbert/bin/lib/SDL-mixer-0.5.2/ghc-6.8.2/HSSDL-mixer-0.5.2.o: unknown symbol `Mix_LoadWAV' ghc-6.8.2: unable to load package `SDL-mixer-0.5.2' I've looked up the source code; Mix_LoadWAV seems to be #defined, so I suspect it's some kind of library path issue but I don't grok the whole system well enough to figure it out; maybe someone could point me in the right direction? Have the issue now on Ubuntu 8.04 and previously had the same issue when I tested it on a FreeBSD box. I did install all of the "libsdl-*-dev" in the Ubuntu repo. - Norbert

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Template Haskell ListT wrinkle
by Robert Greayer 24 Jun '08

24 Jun '08

In messing around with TH, I noticed (what I consider to be an) odd wrinkle in the handling of list types within TH's syntax meta-data. For example, given the program at the end of this email, which prints out the TH representation of the types 'Ints' and '[Int]', where 'Ints' is just a type-alias for '[Int]', the following results are generated: Ints is: AppT (ConT GHC.Base.[]) (ConT GHC.Base.Int) [Int] is: AppT ListT (ConT GHC.Base.Int) type0, 'Ints' resolves to the application of the named type constructor "GHC.Base.[]" to the named type constructor "GHC.Base.Int" (which makes perfect sense). type1, '[Int]' resolves to the application of the 'built-in' ListT type constructor to the named type constructor "GHC.Base.Int" (which also makes perfect sense). What's odd (to me) is that in one situation, the 'named' constructor 'GHC.Base.[]' appears, and in the other, the 'built-in' constructor appears. Does anyone have insight into why this might be? (These results were obtained with GHC 6.8.2, btw). > {-# OPTIONS_GHC -XTemplateHaskell #-} > module Main where > import Language.Haskell.TH > > type Ints = [Int] > > type0 = $( reify ''Ints >>= (\ (TyConI (TySynD _ _ t)) -> return $ show t) >>= \s -> [| s |]) > type1 = $( [t| [Int] |] >>= return . show >>= \s -> [| s |]) > > main = do > putStrLn ("Ints is: " ++ type0) > putStrLn ("[Int] is: " ++ type1) Thanks rcg

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safe vs unsafe
by Galchin, Vasili 24 Jun '08

24 Jun '08

Hello, At runtime, I am getting .... aio: schedule: re-entered unsafely. Perhaps a 'foreign import unsafe' should be 'safe'? I want to understand from an implementation viewpoint .. deeper. This is a GC issue yes? Kind regards, Vasili

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Re: number-parameterized types and heterogeneous lists
by oleg＠okmij.org 24 Jun '08

24 Jun '08

Luke Palmer wrote in response to Harald ROTTER > > I also wonder if there is some kind of "generalized" foldr such that, e.g. > > D1 $ D0 $ D0 $ Sz = specialFoldr ($) Sz [D1,D0,D0] > > I think that this foldr must be some "special" foldr that augments the data > > type of the result in each foldr step. > > Would this be possible or am I just chasing phantoms ? > Mostly I believe you are. What you are describing is firmly in the > realm of dependent types, far beyond Haskell's type system. See > Epigram or Agda for languages which have attempted to tackle this > problem. First of all, Haskell (as implemented by released versions of GHC for at least two years) already permits dependent types, in the strict sense of a type of an expression being determined by the value of one argument of that expression: http://okmij.org/ftp/Computation/tagless-typed.html#tc-GADT-tc Second, (values of) types like D0 and D1 can be collected in a heterogenous list (HList), which does have the corresponding fold operator, called HFoldr in http://darcs.haskell.org/HList/Data/HList/HListPrelude.hs

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"C" buffer suggestions??
by Galchin, Vasili 23 Jun '08

23 Jun '08

Hello, I want to model an I/O read/write buffer in Haskell and marshall down into ANSI C. 1) ByteString? Or Ptr Word8? 2) What are example values of the types in 1)? Kind regards, Vasili

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Re: [Haskell-cafe] number-parameterized types and heterogeneous lists
by Harald ROTTER 23 Jun '08

23 Jun '08

Hello, sorry for the late answer, I was off for the weekend :-) The paper "Number-parameterized types" by Oleg Kielyov is located at http://okmij.org/ftp/papers/number-parameterized-types.pdf It impressively shows what one can do with Haskell's type system. What I am after is the replacement of a "chain" of digits, like e.g. D1 $ D0 $ D0 $ Sz by a list [D1,D0,D0] so I can effectively use list operations on those "typed numbers". I also wonder if there is some kind of "generalized" foldr such that, e.g. D1 $ D0 $ D0 $ Sz = specialFoldr ($) Sz [D1,D0,D0] I think that this foldr must be some "special" foldr that augments the data type of the result in each foldr step. Would this be possible or am I just chasing phantoms ? Thanks Harald. " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."

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