May 2010 - Haskell-Cafe

www.haskell.org web server down?
by Sean Leather 31 May '10

31 May '10

It responds to pings but not http. http://downforeveryoneorjustme.com/www.haskell.org Sean

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Why Either = Left | Right instead of something like Result = Success | Failure
by Ionut G. Stan 31 May '10

31 May '10

Hi, I was just wondering if there's any particular reason for which the two constructors of the Either data type are named Left and Right. I'm thinking that something like Success | Failure or Right | Wrong would have been a little better. I've recently seen that Scala uses a similar convention for some error notifications so I'm starting to believe there's more background behind it than just an unfortunate naming. Thanks, -- Ionuț G. Stan | http://igstan.ro

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Improving Haskell on Windows by adding it to TakeoffGW
by Hamish Mackenzie 30 May '10

30 May '10

Finally someone has created a working MinGW distribution with a package manager! http://sourceforge.net/apps/mediawiki/takeoffgw/index.php?title=Main_Page Pier11 has done a great job of making MinGW libraries, tools and apps easy to install. It may seem limited at this stage (compared to what is available on linux), but it is a darn site better than fishing around for a combination of prebuilt binaries or building everything from source. I think we should embrace this project and try to add support for installing Haskell using it (GHC, Haskell Platform, Darcs, Gtk2Hs, Leksah, ...). You can read the comments here to get a feel for just how Windows users feel about TakeoffGW... http://www.reddit.com/r/programming/comments/c9duj/takeoffgw_mingw_based_di… I tried it out myself and had Gtk2Hs and Leksah up and running in no time using TakeoffGW to install all the prerequisites. Hamish

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Re: Proposal to solve Haskell's MPTC dilemma
by Isaac Dupree 30 May '10

30 May '10

On 05/26/10 15:42, Carlos Camarao wrote: > What do you think? I think you are proposing using the current set of instances in scope in order to remove ambiguity. Am I right? ..I read the haskell-cafe thread so far, and it looks like I'm right. This is what I'll add to what's been said so far: Your proposal appears to allow /incoherent/ instance selection. This means that an expression can be well-typed in one module, and well-typed in another module, but have different semantics in the two modules. For example (drawing from above discussion) : module C where class F a b where f :: a -> b class O a where o :: a module P where import C instance F Bool Bool where f = not instance O Bool where o = True k :: Bool k = f o module Q where import C instance F Int Bool where f = even instance O Int where o = 0 k :: Bool k = f o module Main where import P import Q -- (here, all four instances are in scope) main = do { print P.k ; print Q.k } -- should result, according to your proposal, in -- False -- True -- , am I correct? Also, in your paper, example 2 includes > m = (m1 * m2) * m3 and you state > In Example 2, there is no means of specializing type variable c0 occurring in the > type of m to Matrix. I suggest that there is an appropriate such means, namely, to write m = (m1 * m2 :: Matrix) * m3 . (Could the paper address how that solution falls short? Are there other cases in which there is more than just a little syntactical convenience at stake?, or is even that much added code too much for some use-case?) -Isaac

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How to fix this problem in FreeBSD ?
by zaxis 30 May '10

30 May '10

/usr/ports/devel/hs-ghc-paths>sudo make install clean ===> hs-ghc-paths-0.1.0.5_1 depends on executable: ghc - found ===> Configuring for hs-ghc-paths-0.1.0.5_1 [1 of 1] Compiling Main ( Setup.hs, Setup.o ) Setup.hs:18:22: Not in scope: `preMakefile' *** Error code 1 Stop in /usr/ports/devel/hs-ghc-paths. Sincerely! ----- e^(π⋅i) + 1 = 0 -- View this message in context: http://old.nabble.com/How-to-fix-this-problem-in-FreeBSD---tp28714271p28714… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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gemcutter src?
by Jens Petersen 30 May '10

30 May '10

On 16 May 2010 05:13, Henning Thielemann <schlepptop(a)henning-thielemann.de> wrote: > http://resources.businessobjects.com/labs/cal/gemcutter-techpaper.pdf Does anyone have the url to the source code? (I guess businessobjects was acquired by SAP.) Jens

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Reproducible Bug in Emacs' haskell-mode
by aditya siram 29 May '10

29 May '10

Hi all, I have a reproducible bug in Emacs' Haskell mode where Emacs gets stuck in an endless loop. I am using GNU Emacs 23.0.93.1. To reproduce: 1. Use a bad Language pragma eg. {-# LANGUAGE FlexibleTypes #-} 2. Load a GHCI instance with C-c C-l. GHCI outputs the following error: unsupported extension: FlexibleTypes 3. Fix the pragma: {-# FlexibleInstances -#} 4. Try to load the file again with C-c C-l and Emacs gets stuck. -deech

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Work on Video Games in Haskell
by Ryan Trinkle 29 May '10

29 May '10

iPwn Studios is seeking Haskell developers for its debut title, BloodKnight. * No prior game development experience is required, but you must be very comfortable working in Haskell. * Compensation is negotiable; profit-sharing may be available in some cases. * To apply, or for more information, contact me at ryan(a)ipwnstudios.com. BloodKnight is an action-roleplaying game inspired by games like Diablo and Fallout. It is currently in the final stages of development, and will be released later this year on a variety of smartphone platforms, including iPhone and Android. iPwn Studios is a start-up company located in Boston, MA. We believe in giving back to the Haskell community, so we've open-sourced our ghc-iphone project, which allows GHC to produce binaries for the iPhone. Check it out at http://projects.haskell.org/ghc-iphone/. Ryan Trinkle iPwn Studios

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Math questions
by Mujtaba Boori 29 May '10

29 May '10

Hello I am try to solve this equation Define a higher order function that tests whether two functions , both defined on integers , coincide for all integers between 1 and 100 how can I solve this ? is there any thing in Haskell library to solve this kind ? -- Mujtaba Ali Alboori

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Existentials and SYB [Was: GADTs and Scrap your Boilerplate]
by oleg＠okmij.org 28 May '10

28 May '10

Oscar Finnsson wrote: > I got the GADT > > data DataBox where > DataBox :: (Show d, Eq d, Data d) => d -> DataBox > > and I'm trying to get this to compile > > instance Data DataBox where > gfoldl k z (DataBox d) = z DataBox `k` d > gunfold k z c = k (z DataBox) -- not OK As has been pointed out, DataBox is an ordinary existential data type, only written using the new-style notation. For clarity, let us define our DataBox with the conventional notation: > data DataBox = forall d. (Show d, Eq d, Data d) => DataBox d So, the question becomes of using existentials with Scrap your Boilerplate. On one hand, it is unnervingly trivial to incorporate existentials into the SYB framework. Let us consider the signature of gunfold, the problematic member of the (Data a) type class. -- | Unfolding constructor applications gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c a We have to eventually produce a value of the type (c DataBox). We observe that our work is done if we manage to produce one value of the type DataBox. We then apply the second argument to gunfold (which `lifts' from any type r to the type c r), and we are done. To produce an existential value, we need one witness of the constraints Show, Eq and Data. For example, the type Int is such witness. Thus, we are done indeed. Here is the complete code: > {-# LANGUAGE ExistentialQuantification, Rank2Types #-} > > module F where > > import Data.Generics > > data DataBox = forall d. (Show d, Eq d, Data d) => DataBox d > > instance Typeable DataBox where > typeOf _ = mkTyConApp (mkTyCon "DataBox") [] > > instance Data DataBox where > gfoldl k z (DataBox d) = z DataBox `k` d > gunfold k z c = z (DataBox (42::Int)) One may complain that we have fixed not only the type of the argument to DataBox but also the value. We should let gunfold to `produce' the value. Here is a sketch: > enDataI :: (Int -> DataBox) > enDataI = DataBox > > enDataB :: (Bool -> DataBox) > enDataB = DataBox > > instance Data DataBox where > gfoldl k z (DataBox d) = z DataBox `k` d > gunfold k z c = (if True then k (z enDataI) else k (z enDataB)) Now, the particular Int value to supply to DataBox will be produced by the function k. The above code used the constant True to fix the choice of the type to Int. Generally, one could use the third argument of gunfold to help make the choice. That is, we could incorporate the choice of the type in the value of Constr.

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