April 2006 - Haskell-Cafe

Funct dependency conflict when using types
by Frank 30 Apr '06

30 Apr '06

I have observed the error message (ghci 6.4) Functional dependencies conflict between instance declarations: LocTest.hs:16:0: instance B S Bool LocTest.hs:22:0: instance (F f, B a b) => B (f a) (f b) when I changed in an instance from a simple type to a type with a parameter (i.e. from Int to (Int, Bool)). The following minimal program demonstrates the problem: an instance with type S (= T Float) conflicts with B (f a) (f b) whereas the comparable instance with data R (= R Float) does not. Is this intentional? How can it be avoided? Help is greatly appreciated! Andrew ---------------------------------------- class F f where m2 :: (a -> b -> c) -> f a -> f b -> f c --zipWith class B a b | a -> b where dist2 :: a -> a -> b instance B S Bool where -- line 16 dist2 a d = True instance B R Float where dist2 a d = 1 instance (F f, B a b) => B (f a) (f b) where -- line 22 dist2 a b = m2 dist2 a b data T f = T f type S = T Float data R = R Float

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Current situation regarding global IORefs
by Brian Hulley 29 Apr '06

29 Apr '06

Hi - I've run into the global mutable state problem described in http://www.haskell.org/hawiki/GlobalMutableState Since the page was last edited in March last year, I'm wondering if there have been any developments or further thoughts on how to safely create top level IORefs since they are absolutely essential for the library I'm writing. For my library, which implements a GUI, I have a Manager module which keeps track of which control currently has the keyboard focus etc, and I don't want to have to pass round the state of the manager to every control since this would be monstrously inconvenient and a total waste of space/time, so at the moment I'm reduced to: module Manager where keyboard :: IORef (Maybe Control) {-# NOINLINE keyboard #-} keyboard = unsafePerformIO $ newIORef Nothing The problem is that I don't know if this is guaranteed to be completely safe for all Haskell compilers or even for all future versions of ghc (?) Thanks, Brian.

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Newbie "Instance Of Floating Int" Error
by Aditya Siram 29 Apr '06

29 Apr '06

Hi all, I just started working with Haskell and am having some difficulties with its type system. Here is function that is supposed to calculate the distance between two coordinates: distBetween (x1,y1) (x2,y2) = sqrt((x2-x1)^2 + (y2-y1)^2) I am trying to explictly give it a type signature. Here is what I have tried and the errors generated by Hugs: type Point = (Int,Int) distBetween :: Point -> Point -> Float >>ERROR - Type error in explicitly typed binding *** Term : distBetween *** Type : Point -> Point -> Int *** Does not match : Point -> Point -> Float distBetween :: Point -> Point -> Int >>Instance of Floating Int required for definition of distBetween Any help is appreciated... Deech

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Saving a data structure to a file
by Neil Mitchell 28 Apr '06

28 Apr '06

Hi, I have a program which needs to cache intermediate data structures to a file (finite, non-cyclic, relatively simple, around ~100Kb when shown). Is there any easy way to acheive this? The current approach I am using is to add deriving Read, Show to all the data structures, and then writeFile "output.txt" (show x) read =<< readFile "output.txt" The problem is that this is too slow, and itsn't lazy. Is there any way to do better than this? I can imagine something like showBinary/readBinary which did this using binary files directly, but is there anything that does this? I'd like it to work with GHC and Hugs at least, and the less extra code I have to write the better :) Thanks Neil

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Billion-triple RDF store
by Joel Reymont 26 Apr '06

26 Apr '06

Folks, How would you go about implementing a RDF triple store in Haskell capable of handling a billion triples? I'm looking for ideas. For example, you could use tries to store all strings but you could easily get to the point where you can't load them all into, say, 1Gb of memory which is all I have on my machine. I'm thinking that the required data could be pulled from disk lazily (as needed) and laziness is a Haskell forte. Thanks, Joel P.S. I know about Swish but would go for a far more compact and efficient representation. -- http://wagerlabs.com/

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Re: Proposal for Number classes
by Serge D. Mechveliani 26 Apr '06

26 Apr '06

Dylan Thurston <dthurston(a)barnard.edu> wrote on 17 Apr 2006 > On Sat, Apr 08, 2006 at 10:16:53PM +0400, Serge D. Mechveliani wrote: >> I think that without dependent types for a Haskell-like language, >> it is impossible to propose any adequate and in the same time plainly >> looking algebraic class system. > Agreed. Is there anything really wrong with the Kiselyov-Shan > approach to dependent types? Does it look too bizarre? > > http://okmij.org/ftp/Haskell/types.html#Prepose > http://okmij.org/ftp/Haskell/number-parameterized-types.html I am out of this field since about 2001. The first paper mentions the foreign function interface, and transmitting integer by means of tricks with types (I recall difficulties with existential types), and so on. It also mentions passing vocabularies (maybe, via foreign interface extension). Of course, if a language extension allows to express the constructs like if isPrime n then (pass inv (the inversion operation) to the vocabulary of (ResidueDomain Integer n), with this implementation ... ) else do not pass it , then this is what is needed: an equivalent of a dynamic conditional instance. In this example, passing inv expresses actially dynamic and conditional definition of the Field instance. Field a <==> (CommutativeRing a, `inv' is in vocabulary of a). In Magma system, and in Aldor, it can be, probably, written plainly. But these systems are not functional, nor lazy, have not pattern matching, etc. Currently I am out of this field, have not time to study all this. For example, if Dylan Thurston tries to implement the Basic Algebra proposal basing on the approach of `configurations' of http://okmij.org/ftp/Haskell/types.html#Prepose, and with the functionality as of the BAL proposal, then we could observe whether the thing looks natural enough. I could also think of doing an attentive investigation on this subject, maybe, together with some qualified people, if I have this project as somehow a visiting research job, and paid. And there is no guarantee that the result occurs positive. ----------------- Serge Mechveliani mechvel(a)botik.ru

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Haskell.org and Google Summer of Code 2006
by Paolo Martini 25 Apr '06

25 Apr '06

Hello, It is now official. Haskell.org is a mentoring organization for this year's Summer of Code programme. You can see it listed on the official site <http://code.google.com/soc/> clicking on `Haskell.org (ideas)' you'll be directed to the trac site we are setting up. (It's 4 AM and I've just finished adding the projects, and cleaning things up a bit.) I am _very_ happy about seeing this project working out for real. I fear that I wouldn't be able to reach everybody even mailing all the important mailing list; I hope many students will be able to take advantage of this great occasion. Help me spread the word, tell everybody about the project :-) You can find the new site here <http://hackage.haskell.org/trac/summer-of-code/>, the StartPage will be updated with details during the program. Projects ideas and volunteers for mentoring are still very much welcome! students' application period starts on May 1. Thanks very much for the support, hope to hear thoughts and proposals, Paolo. --

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Project proposal
by michael＠schmong.org 25 Apr '06

25 Apr '06

Hi, All this chatter about the Summer of Code has inspired me to throw caution to the wind and become more committed to an idea I had for a Haskell project. I first thought to apply this to Python, but well, Haskell stole my heart. I wanted to throw the idea out there and see if anyone was interested in mentoring me through the process. I want to build a MUD engine written in Haskell with Haskell as the MUD development language. This is a great way to introduce kids to Haskell as their first programming language. And when they get bored and grow out of mudding, they will have knowledge of a real-world language instead of just a toy language particular to whatever MUD they were playing with. Getting a mentor, and completing the project is more important to me than officially becoming a Summer of Code project. But if they both happen then all the better. Any takers? Michael Litchard of Mission College Santa Clara , CA

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Google Summer of Code 2006
by Paolo Martini 25 Apr '06

25 Apr '06

Hello, Haskell.org is going to partecipate as mentoring organization to the Google Summer of Code programme of this year. We have formed a group of volunteers taking care of the administrative work as well as the mentoring part. The group is listed here: <http://haskell.org/haskellwiki/Summer_of_Code:_People>. We are now looking for projects, and mentors. We've reached number 24 but we are happily going to add more, Haskell Open Source paid work for the summer! If you run some active open source program you might want to mentor some student who will be working on your code, for example. Either just add your ideas here: <http://haskell.org/haskellwiki/Summer_of_Code:_Project_suggestions> (as well as your informations on the People's page) or contact me for details. I am also looking for students, I'd like to ear from them if they are going to partecipate with Haskell projects, last year I was alone! (I did write the bindings to the Cairo vector graphics library and integrated them in Gtk2Hs.) Details about the programme itself can be found on the official webpage: http://code.google.com/summerofcode.html Hoping you like the idea as much as me, Paolo. --

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Re: [Haskell-cafe] Google Summer of Code 2006
by Paolo Martini 25 Apr '06

25 Apr '06

Evan Martin ha scritto: > (This is Evan from Google.) > Just wanted to say that it's rad to see you making all this happen. Hi Evan! Fabulous to see you around! Thank you very much, we have done great work last year. Google is doing so well making students able to work on what they like most, during the summer, I couldn't resist about sharing this opportunity and making it available for the bigger number of people I could. Cheers, Paolo. --

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