November 2010 - Haskell-Cafe

install GHC 7.0.1 RC on OS X?
by Edward Amsden 02 Nov '10

02 Nov '10

I'd like to install the GHC 7.0.1 RC on my mac (Snow Leopard, x86_64), but I don't want it to run over my current GHC 6.12.2 install. If I use the .pkg installer, it doesn't allow me to select the destination, and I worry that it will overwrite my 6.12.2 install. If I try to build from source, that requires me to use my current GHC, which (as I understand) would build for 32 bit. What other options do I have, or am I misunderstanding something? ------ Edward Amsden Undergraduate Computer Science Rochester Institute of Technology

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Haskellers browse users form
by Yitzchak Gale 02 Nov '10

02 Nov '10

I'm using Safari on Snow Leopard. On the "Browse Users" page on Haskellers, there are little buttons that look like number spinners on the two numerical fields, like "Using Haskell since..." When I press one of those, the number -1.7976931348623157e+308 appears in the field. That must be Lennart. Thanks, Yitz

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Generating random tuples
by Jacek Generowicz 01 Nov '10

01 Nov '10

I'm toying with generating random objects (for example tuples) and started wondering what pearls of wisdom Cafe might have on the matter. Two obvious points (relating to my toy code, shown below) are 1) The meaning of the limits required by randomR is not obvious for types such as tuples (you could come up with some definition, but it wouldn't be unique: how would you allow for different ones?[*]; you might decide that having such limits is nonsensical and not want to provide a randomR: would you then leave it undefinded?). [*] I guess this is related to issues such as Num being both a sum and and product monoid. 2) I'm sure there are at least half a dozen prettier definitions of random. But I suspect that the juicy bits will be in offerings about issues that I haven't even dreamed about. Presumably QuickCheck's test-data generation mechanism would be interesting to look at in this context. Is there a gentle explanation of how it works somewhere? Here's my initial effort: import Control.Monad import System.Random main :: IO (Int, Int) main = randomIO instance (Random a, Random b) => Random (a, b) where randomR = undefined random g0 = let (i1,g1) = random g0 (i2,g2) = random g1 in ((i1,i2), g1)

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commutativity
by Patrick Browne 01 Nov '10

01 Nov '10

Hi, Below are two questions on commutative operations in Haskell. infixl 5 `com` com :: Int -> Int -> Int x `com` y = (x + y) commutative com a b = (a `com` b) == (b`com`a) -- 1 + 3 == 3 + 1 -- This gives true by virtue of the value of LHS and RHS being equal after the plus operation -- Question 1 -- commutative com 1 3 -- This also gives true. Is it because of commutative equation or because of the plus operation? -- Question 2 -- In Haskell can commutativity be specified as a property of infix operations? This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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[ANNAUNCE] ghcjs-0.1.0 Haskell to Javascript compiler
by Victor Nazarov 01 Nov '10

01 Nov '10

I've been working on this for some month and I think now I'm ready to share the results. http://github.com/sviperll/ghcjs Haskell to Javascript translator ================================ Project aims to provide solution to * compile modern Haskell libraries to Javascript files and use them in Ajax applications or * develop entire Ajax application in Haskell language Building -------- Code builds as standard haskell package $ runghc Setup configure $ runghc Setup build $ runghc Setup install Usage ----- To compile Haskell module to Javascript use `ghcjs` command. $ ghcjs Test.hs This command is merely equivalent to the following $ ghc --make Test.hs but it compiles to Javascript instead of native code. See examples folder for an example of loading and running haskell code from browser. Status ------ The code is in alpha stage. Feel free to experiment with it as you wish. Implementation -------------- Compiler is implemented as [GHC](http://www.haskell.org/ghc/) backend using GHC API. And been tested with GHC 6.12.1. -- Victor Nazarov

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Edit Hackage
by Andrew Coppin 01 Nov '10

01 Nov '10

Today I uploaded a package to Hackage, and rediscovered something that you already know: I'm an idiot. More specifically, I copied the Cabal description from another package and then updated all the fields. Except that I forgot to update one. And now I have a package which I've erroneously placed in completely the wrong category. Is there any danger that at some point in the future, it might be possible to edit a package /after/ it has been released on Hackage? There are several reasons why you might wish to do this (beyond realising you did something wrong five minutes after hitting the upload button): The maintainer might change. The homepage might move. A better package might come along, making the existing one obsolete. Or you might want to stick a message on the package saying "hey, this version has a serious bug, please use the next version up instead!" (I guess you wouldn't want to actually change the code content of a package. That leaves the potential for several /different/ variants of "the same" package floating around, depending on when people installed it. That would be Bad.) Is there any danger that we might be able to do these kinds of things sometime soon?

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Forall and type synonyms in GHC 7.0
by Mario Blažević 01 Nov '10

01 Nov '10

Before uploading a new version of my project on Hackage, I decided to future-proof it against GHC 7.0. I ran into several compile errors caused by the changes in let generalization, but these were easy to fix by adding extra type annotations. But then I ran into another problem that I can't fix so easily. Here is its trimmed-down reproduction: > {-# LANGUAGE RankNTypes #-} > > module Test where > > data Component c = Component {with :: c} > > pair1 :: (Bool -> c1 -> c2 -> c3) -> Component c1 -> Component c2 -> Component c3 > pair1 combinator (Component c1) (Component c2) = Component (combinator True c1 c2) > > type PairBinder m = forall x y r. (x -> y -> m r) -> m x -> m y -> m r > > pair2 :: Monad m => (PairBinder m -> c1 -> c2 -> c3) -> Component c1 -> Component c2 -> Component c3 > pair2 combinator = pair1 (combinator . chooseBinder) > > chooseBinder :: Monad m => Bool -> PairBinder m > chooseBinder right = if right then rightBinder else leftBinder > > leftBinder :: Monad m => PairBinder m > leftBinder f mx my = do {x <- mx; y <- my; f x y} > > rightBinder :: Monad m => PairBinder m > rightBinder f mx my = do {y <- my; x <- mx; f x y} The general idea here, if you're intrigued, is that pair1 belongs to a generic module that packages things it knows nothing about into Components. The remaining definitions belong to a client of the generic module, and pair2 is a specialization of pair1 to components that have something to do with monads. Now this little test compiles fine with GHC 6.12.1, but GHC 7.0.0.20101029 reports the following error in the pair2 definition: TestForall.lhs:13:42: Couldn't match expected type `forall x y r. (x -> y -> m r) -> m x -> m y -> m r' with actual type `(x -> y -> m1 r) -> m1 x -> m1 y -> m1 r' Expected type: Bool -> PairBinder m Actual type: Bool -> (x -> y -> m1 r) -> m1 x -> m1 y -> m1 r In the second argument of `(.)', namely `chooseBinder' In the first argument of `pair1', namely `(combinator . chooseBinder)' I've tried adding extra type annotations without making any progress. At this point I'm beginning to suspect I ran into a bug in GHC 7.0, but I can't find it in GHC Trac; the only ticket that looks similar is #4347, but that one works for me. Is this a bug? If not, how do I make my code compile?

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Regex lib doesn't load
by Dan Bensen 01 Nov '10

01 Nov '10

Hi Haskellers. I'm trying to read a text file using GHCi on Haskell Platform 2010.2.0.0. The import line in my program is "import Text.Regex". When I run :main it says "unknown symbol `_regerror'" and "unable to load package `regex-posix-0.94.2'". Any clues on how fix this problem? Please let me know if there's a better place to ask about it. --Dan Bensen

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Red links in the new haskell theme
by Victor Oliveira 01 Nov '10

01 Nov '10

Hi Cafe, I really liked the new colors of haskell theme, but... Is really red a good color for links? At least for me, red links looks like broken or already visited ones. And the worst is hackage docs. It is really eye tiring to read. It's just a thought. Maybe it just with me. What about you? []s Victor

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Tsuru Capital Tokyo is looking for Southern Hemisphere summer (Dec 10 - Feb 11) interns
by employment 01 Nov '10

01 Nov '10

We are looking for an intern that has experience programming in a functional language, and is familiar with Haskell and web development. Some experience with Linux/Unix system administration is preferred. Familiarity with Git would be useful. The position is in Tokyo in a small English speaking team. We do algorithmic options market-making. The exact role is not fixed yet but we are looking particularly for someone to help with making a simulation scheduling framework and results visualisation in a web application. Compensation would be a small package including flights and accommodation. If interested please send a resume to jobs(a)tsurucapital.com You can see more details about the company at tsurucapital.com and at the company page on LinkedIn <http://tsurucapital.com/en/jobs.html#programmer>

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