August 2008 - Haskell-Cafe

Currying function using values from array
by Sukit Tretriluxana 08 Aug '08

08 Aug '08

Dear Haskell experts, I am currently studying Groovy language. An experiment I did with its closure is to perform closure/function "curry" using an array containing the values for the parameter binding. See the sample below. int addThemUp(a,b,c,d,e) { a+b+c+d+e } def arrayCurry(arr, cls) { arr.inject(cls) { c, v -> c.curry(v) } } println addThemUp(1,2,3,4,5) println arrayCurry([1,2,3,4,5], this.&addThemUp)() println arrayCurry([1,2,3,4], this.&addThemUp)(5) The printouts from the above code are the same, verifying that the code works fine. Then I come to ask myself how I can do the same in Haskell. I'm not a Haskell expert so I couldn't figure it. I wonder if you guys could shed some light on this. Thanks, Ed

6 7

Building fastcgi from hackage on windows.
by Edward Ing 08 Aug '08

08 Aug '08

I am using the cabal system to build the hackage version of fastcgi. I get the below messages with the "runghc Setup.lhs build" command. Network\FastCGI.hsc:59:21: fcgiapp.h: No such file or directory Network\FastCGI.hsc: In function `main': Network\FastCGI.hsc:200:0: error: `FCGX_Request' undeclared (first use in this function) It looks like I need the fastcgi sources. Where does this source come from? I didn't get any dependency messages when I did the setup config. Edward Ing

3 5

Re: [Haskell-cafe] Phantoms
by Trevor Elliott 08 Aug '08

08 Aug '08

Hi Andrew, Here's an example that might help: > import Data.Typeable > > data Test a = Test Int > > instance Typeable a => Show (Test a) where > show t@(Test i) = "Test " ++ show (typeOf (ty t)) ++ " " ++ show i > where > ty :: Test a -> a, > ty = undefined and its use: *Main> show (Test 10 :: Test Int) "Test Int 10" *Main> show (Test 10 :: Test String) "Test [Char] 10" --trevor On Wed, 2008-08-06 at 19:09 +0100, Andrew Coppin wrote: > I just (re)discovered that I can do things like > > data Foo x = Foo Int Int > > Now "Foo Int" and "Foo Double" are, as far as the type checker cares, > two completely different types, even though in fact they are the same. > This is actually Quite Useful, in the particular case I'm working on. > > Here's the thing though... How do I get it so that "Foo Int" and "Foo > Double" produce slightly different strings when printed? > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Show me
by Andrew Coppin 08 Aug '08

08 Aug '08

Suppose we have the following: data Foo x list_foo :: [x] -> Foo x foo_list :: Foo x -> [x] What would be the best way to write a Show instance? The thing that I came up with is instance (Show x) => Show (Foo x) where show foo = "list_foo " ++ show (foo_list foo) But apparently you're supposed to use the strange "showsPrec" method that I don't understand. So can somebody show me a correct instance definition for this type please?

3 4

USB and serial port
by Maurício 08 Aug '08

08 Aug '08

Hi, What are the alternatives to read a USB port (actually, a COM serial port with an USB adapter) in Haskell, running in Linux? Thanks, Maurício

5 8

Last Call for the Italian Haskellers Summer Meeting
by Titto Assini 08 Aug '08

08 Aug '08

As usual we will now switch to Dante's "bella lingua". Ottimissimi, mancano pochi giorni al primo incontro estivo/balneare degli haskeller italiani. Per informazioni e registrarsi date uno sguardo a: http://www.haskell.org/haskellwiki/ItaloHaskell/Summer_2008 Oppure contattate il sottoscritto via email o per telefono al 0584 791669. A presto ragazzi, titto -- Pasqualino "Titto" Assini, Ph.D. 25 Heath Road - CO79PT Wivenhoe - Colchester - U.K.

4 3

adjunction of product and exponentiation
by Jason Dusek 08 Aug '08

08 Aug '08

The problem as stated is to find the unit for the adjunction: ((- x A), (-)^A x A) The latter functor takes an arrow f to (f . -) x id_A and does the obvious thing for objects. The co-unit diagram is given as: B^A x A ---- eval_AB ----> B ^ ^ | | | | curry(g) x id_A g : C x A -> B | | | | | | C x A --------------------+ This diagram is somewhat puzzling, because it seems the second functor has turned into (-)^A ! Continuing in with that, we get a unit diagram like this: C ---- magic ----> (C x A)^A | | | | | | curry(g) (g . -) | | | | | v +------------------> B^A So what is the magic? It is an arrow that takes a C to an arrow that takes an A and makes the product C x A. I want to write curry(C x A) but that is ridiculous looking. What's the right notation for this thing? -- _jsn

2 4

instance deduction and phantom types
by Jason Dagit 07 Aug '08

07 Aug '08

Hello, I recently had a problem where GHC couldn't deduce some type classes from my constraints. I would get error messages like: Could not deduce (Conflict (PatchInfoAnd p), Patchy (PatchInfoAnd p)) from the context (RepoPatch p) But, I had some instances like this (minus the function definitions): class (Patchy p, Effect p, FromPrims p, Conflict p) => RepoPatch p instance Effect p => Effect (PatchInfoAnd p) instance Conflict (p x y) => Conflict (PatchInfoAnd (p x y)) instance RepoPatch (p x y) => Patchy (PatchInfoAnd (p x y)) The x and y in the last two are just phantom types on p. When I noticed that the phantom types were missing from the instance definition of Effect (PatchInfoAnd p), I removed them from the last two definitions. This made the "Could not deduce..." error message disappear. Why do the phantom types not belong in the instance definition? I'd like to understand the problem more fully, but I'm not sure what the problem is or how to get started researching it, so if someone could point me in the right direction I'd be very happy! Thanks! Jason

2 3

ANN: FTGL 1.0. Portable truetype font rendering in OpenGL
by Jefferson Heard 07 Aug '08

07 Aug '08

Hi folks. I've just released some Haskell bindings to FTGL at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FTGL . FTGL is an easy to use library for portable rendering of TrueType fonts in OpenGL, with functions for creating bitmapped fonts, texture-mapped fonts, buffered fonts, polygonal 2D fonts, and extruded polygonal 3D fonts. The code works on Windows and Linux. I've not tested it yet on the Mac. To use FTGL in your programs, just import Graphics.Rendering.FTGL and use do font <- createTextureFont "font.ttf" setFontFaceSize font 24 72 renderFont font "some string" More documentation can be found at the FTGL homepage at http://ftgl.sourceforge.net -- Jeff

3 3

ANN: Haddock version 2.2.1
by David Waern 07 Aug '08

07 Aug '08

-------------------------------------------- -- Haddock 2.2.1 -------------------------------------------- A new version of Haddock, the Haskell documentation tool, is out. The homepage will be updated soon, containing a windows binary release. Please use the bug tracker to submit bug reports and feature requests. -------------------------------------------- -- Changes since version 2.1.0 -------------------------------------------- * Support for GHC 6.8.3 * The Hoogle backend is back * Show associated types in the documentation for class declarations * Show type family declarations * Show type equality predicates * Major bug fixes (#1 and #44) * It is no longer required to specify the path to GHC's lib dir * Remove unnecessary parenthesis in type signatures * Minor bug fixes -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haddock-2.2.1 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haskelldoc(a)haskell.org Code repository: http://code.haskell.org/haddock -------------------------------------------- -- Details -------------------------------------------- This release contains experimental and incomplete support for GHC's Type Families extension. We have basic support for rendering of type family declarations and associated types in the documentation. Type/Data instances are not shown in the documentation at all, and associated types are not shown for class instances. Feedback is appreciated, especially for ideas of how to render type family instances. Use either the bugtracker, or send a mail to the mailing list. We have also got the Hoogle backend back, thanks to Neil Mitchell! The plan is to be compatible with the upcoming pre-release of Hoogle 4, which is being developed by Neil as a Summer of Code project. -------------------------------------------- -- Get Involved -------------------------------------------- Haddock is an important project, as it is part of the Haskell infrastructure core. The code base is gradually becoming easier to read, better documented and more stable, but more work is needed. To get involved, start by grabbing the code at http://code.haskell.org/haddock then take a look at the bug and feature tracker at http://trac.haskell.org/haddock for things to work on. David

4 7