October 2007 - Haskell-Cafe

building hslogger
by Chris Hayden 20 Oct '07

20 Oct '07

Hello! I'm trying to build the most recent version of hslogger and have run into some issues. Below is the error message I receive. (I see that the same error has come up elsewhere: http://haskell.galois.com/packages/archive/hslogger/1.0.2/log) The hslogger (http://software.complete.org/hslogger) website indicates that questions should be posted here - I apologize if this is the wrong venue. Thank you, Chris runhaskell Setup.hs configure Setup.hs:19:35: Couldn't match expected type `(Either GenericPackageDescription PackageDescription, HookedBuildInfo)' against inferred type `PackageDescription' In the first argument of `(confHook defaultUserHooks)', namely `mydescrip' In the expression: let mydescrip = case os of "mingw32" -> ... _ -> ... in (confHook defaultUserHooks) mydescrip flags In the definition of `customConfHook': customConfHook descrip flags = let mydescrip = case ... of "mingw32" -> ... _ -> ... in (confHook defaultUserHooks) mydescrip flags

3 2

Why doesn't Hackage link to Haddock documentation anymore?
by Johan Tibell 20 Oct '07

20 Oct '07

Maybe I'm seeing things but from what I remember packages that used to have links to Haddock documentation in their exported modules list no longer has. It's a super useful feature! What happened to those packages? -- Johan

4 4

On the verge of ... giving up!
by Vimal 19 Oct '07

19 Oct '07

Dear Haskellers, I have been trying my best to read about Haskell from the various tutorials available on the internet and blogs. I havent been following YAHT properly, so its always been learning from 'bits and pieces' scattered around. For most languages (like C/C++/Ruby/Python), the above approach has helped me. But for Haskell, it is failing. I like learning by comparison with other similar languages. This approach worked for me when I tried learning Python+Perl together. The nicer syntax and easier object-orientedness made me leave Perl behind and pursue Python. I also tried it for Haskell (Lisp+OCaml+Haskell together). The next step I usually take in learning a language is, not to go by the topics found in textbooks, but by taking real world examples and then blindly try to solve it using that language as a tool. For e.g, I tried writing a terminal GTalk client for Python when I was learning it, and learnt so many features that way. I used to call this 'learning by need', and it worked, to the extent that I never knew how to take 'input' from the user, but knew how to write Objects in Python! (Since I never used input in that example :) I didnt want to repeat that mistake, so I made sure I would learn IO in Haskell, which initially turned out to be a disaster, due to the 'Moands' which sounded like 'Go Mads' to me. Then, I set out to learn Monads + Category Theory from a Math perspective. And since I haven't been introduced to abstract math (like Groups, etc.), I found this a little difficult. However I tried my best to understand the tiniest bit and waited for the tiniest spark that would enlighten me. It didn't work out. --snip-- Okay, so you might be wondering as to whats the whole point of this mail? Well, I am almost on the verge of giving up on something I really like to learn, just because I didn't go in the right order! So, I requested my institute to buy Dr. Graham Hutton's book. I would be getting hold of that quite soon, and am willing to start from the beginning. Meanwhile, could anyone suggest if there was anything wrong in my approach to learning Haskell/the other languages? I agree that the learning methodology is something personal and I have to find out what best suits me, but I would like to hear something from you, Haskellers, too. I have no need to hurry anything at this point of time. But after being introduced to the tiniest bit of Haskell, and after seeing such a large and active community here, and #haskell, I had the plan of conducting a Haskell Workshop, in our department (sometime in Feb next year, the dates have not been finalized). I just hope that I should be able to reach a substantial amount of familiarity with Haskell by that time! Sorry for the long mail, and pardon me if it was too boring. I just put my thoughts on <strike>paper</strike> a mail :) I did have a couple of ambitious projects in my mind when to help me learn Haskell. They were: 1. An online judge system (Like http://spoj.pl) I had already done one for contests that were held during our Technical festival here, using php. The ideas are laid out. All I had to focus on was learning Haskell. 2. A solver for the Peg-Solitaire. This was more of an academic interest. I have seen Richard Bird's presentation on 'How to write a functional pearl, with an example' and was quite impressed by it. But the actual modelling might be slightly tricky here, and I am yet to start off with it. Many thanks for your patience, Cheers, -- ~Vimal IIT Madras RLE :) encode = map (length &&& head) . group decode = concatMap (uncurry replicate)

34 78

Filesystem questions
by Andrew Coppin 19 Oct '07

19 Oct '07

I notice that getDirectoryContents appears to return its results in alphabetical order. Is this behaviour actually guaranteed? Related: Is there a way to get rid of "." and ".." in the results? (Obviously this causes directory recusion to malfunction.) I can of course manually filter them out, but it's annoying. Finally, why do doesDirectoryExist "C:" doesDirectoryExist "C:\" both yield False? (This frequently trips up programs that try to check that a destination folder exists.)

11 19

Pixel plotter
by Andrew Coppin 19 Oct '07

19 Oct '07

> http://darcs.haskell.org/~lemmih/hsSDL/ <http://darcs.haskell.org/%7Elemmih/hsSDL/> How do I use this? I've seen instructions floating around somewhere that if a library has a "Setup.hs", you're supposed to do runhaskell Setup configure runhaskell Setup build runhaskell Setup install However, the first command prints a huge stream of text and then complains about "can't find sh". (Well, why would you? It's Windows...)

5 6

Re: [Haskell] Fingerprints and hashing
by Bulat Ziganshin 19 Oct '07

19 Oct '07

Hello Simon, Thursday, October 11, 2007, 1:42:31 PM, you wrote: > For various applications (including identifying common > sub-expressions, and version tracking in GHC), I'd like a Haskell > library that supports simple fingerprint operations. lots of hash-related links was collected at http://www.encode.ru/forums/index.php?action=vthread&forum=1&topic=413 -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

1 0

Suspected stupid Haskell Question
by Big_Ham 19 Oct '07

19 Oct '07

Is there a library function to take a list of Strings and return a list of ints showing how many times each String occurs in the list. So for example: ["egg", "egg", "cheese"] would return [2,1] I couldn't find anything on a search, or anything in the librarys. Thanks BH. -- View this message in context: http://www.nabble.com/Suspected-stupid-Haskell-Question-tf4639170.html#a132… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

12 19

Re: [Haskell-cafe] Polymorphic (typeclass) values in a list?
by TJ 19 Oct '07

19 Oct '07

Henning Thielemann: > > class Renderable a where > > render :: a -> RasterImage > > > > scene :: Renderable a => [a] > > This signature is valid, but it means that all list elements must be of > the same Renderable type. Yes, that's exactly the restriction I'm unhappy about. > You could let the user plug together the alternatives for Renderable. That > is, declare the class Renderable and let the user define and instantiate > > data Figure > = Point Something > | Line Something > | Polygon Something But if I already have the types Point, Line, and Polygon, and I want to create a "union type" Figure as above, then my code will look like this: data Point = Point Something data Line = Line Something data Polygon = Polygon Something data Figure = FPoint Point | FLine Line | FPolygon Polygon aFigure = FPoint Point Something aListOfFigures = [FPoint Point Something, FPolygon Polygon Something, FLine Line Something] > > Is there a way of achieving what I want to do? Existentials maybe? I'm > > still learning the basic stuff and don't grok existentials at all, but > > I even if I use those, I'll still have to wrap things up in a > > constructor, won't I? > > I assume, that you could use > http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.htm… That's a nice page :) From a quick reading, the best I came up with was this: data R = forall a. Renderable a => V a instance Show R where render (R a) = render a Which is precisely what I meant when I said that I'd still have to wrap things up in a constructor. Is this hidden type variable thing what "existential types" mean? OT: forall just introduces a new type variable, right? Thanks, TJ

3 2

Do you trust Wikipedia?
by PR Stanley 19 Oct '07

19 Oct '07

Hi Do you trust mathematical materials on Wikipedia? Paul

22 31

Re: [Haskell] Re: Trying to install binary-0.4
by Simon Marlow 19 Oct '07

19 Oct '07

Udo Stenzel wrote: > Simon Marlow wrote: >> So a package that depends on 'base' (with no upper version bound) *might* >> be broken in GHC 6.8.1, depending on which modules from base it actually >> uses. Let's look at the other options: >> >> - if we rename base, the package will *definitely* be broken >> >> - if the package specified an upper bound on its base dependency, it will >> *definitely* be broken > > - if you provide a 'base' configuration that pulls in the stuff that > used to be in base, the package will work I don't know of a way to do that. The name of the package is baked into the object files at compile time, so you can't use the same compiled module in more than one package. > I hate betting, but I'd like to know if... > > - it is okay to give GHC 6.4/6.6 a castrated configuration of the base > libraries to remove the conflict with recent ByteString? Sure, that's what I suggested before. Moving modules of an existing package from 'exposed-modules' to 'hidden-modules' is safe (I wouldn't recommend removing them entirely). > - when GHC 6.8 comes out containing base-3, will it be possible to > additionally install something calling base-2 with both being > available to packages? In theory yes - the system was designed to allow this. In practice we've never tried it, and base-2 might not compile unmodified with GHC 6.8. > - If so, will existing Cabal packages automatically pick up the > compatible base-2 despite base-3 being available? Only if they specify an upper bound on the base dependency, which most don't, but it's an easy change to make. Cheers, Simon

17 54