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ANNOUNCE: The Haskell Platform
by Don Stewart 07 May '09

07 May '09

We're pleased to announce the first release of the Haskell Platform: a single, standard Haskell distribution for every system. Download the Haskell Platform 2009.2.0 installers and specification: http://hackage.haskell.org/platform/ The Haskell Platform is a blessed library and tool suite for Haskell culled from Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. What you get: http://hackage.haskell.org/platform/contents.html With regular time-based releases, we expect the platform will grow into a rich, indispensable development environment for all Haskell projects. Distro maintainers that support the Haskell Platform can be confident they're fully supporting Haskell as the developers intend it. Developers targetting the platform can be confident they have a trusted base of code to work with. *Please note that this is a beta release*. We do not expect all the installers to work perfectly, nor every developer need met, and we would appreciate feedback. You can help out by packaging the platform for your distro, or reporting bugs and feature requests, or installing Haskell onto your friends' machines. The process for adding new tools and libraries will be outlined in coming weeks. The Haskell Platform would not have been possible without the hard work of the Cabal development team, the Hackage developers and maintainers, the individual compiler, tool and library authors who contributed to the suite, and the distro maintainers who build and distribute the Haskell Platform. Thanks! -- The Platform Infrastructure Team

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beginner question: assigning local variable to a function
by Nico Rolle 07 May '09

07 May '09

hi why does this don't work? test = let a = (>) in 1 `a` 2 regards

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GLFW - Mac OS X
by Günther Schmidt 07 May '09

07 May '09

Hi, has anybody recently install the GLFW package on Mac OS X? It won't install on my machine. Günther

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Trying to contact authors of "Real World Haskell"?
by Itsme (Sophie) 07 May '09

07 May '09

I could not find any contact info for Brian O'Sullivan, Don Stewart, or John Goerzen on their book site. Any pointers to how I might locate any of them much appreciated. Thanks! Sophie

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Getting WriterT log lazily
by Magnus Therning 07 May '09

07 May '09

I've been playing around with (WriterT [Int] IO), trying to get the log out and map `print` over it... and do it lazily. However, I'm not really happy with what I have so far, since I've had to resort to `unsafePerformIO`. Any hints are welcome. What I have so far is: foo = let _tell i = do a <- return $ unsafePerformIO $ sleep 1 tell [a + 1 `seq` i] in do mapM_ _tell [1..10] main = do (_, ~res) <- runWriterT foo mapM_ print res Without the `seq` the call to sleep will simply be skipped (is there an easier way to force evaluation there?). Without `unsafePerformIO` all the sleeping is done up front, and all numbers are print at once at the end. The goal is of course to use code along the same shape to do something more useful, and then `unsafePerformIO` will really be unsafe... /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Operator precedence and associativity on happy grammars
by j.romildo＠gmail.com 07 May '09

07 May '09

Hello. I am learning how to use Happy (a LALR(1) parser generator) and I have a question on a grammar based on an example from the manual. The input file to Happy is attached to this message. The grammar is: Exp -> let var = Exp in Exp Exp -> Exp + Exp Exp -> Exp - Exp Exp -> Exp * Exp Exp -> Exp / Exp Exp -> ( Exp ) This grammar is ambiguous, but the conflicts that will appear in the parser table can be resolved with precedence and associativity declarations for the operators that surround expressions: %right in %left + - %left * / giving right associativity to the in token, and left associativity to the arithmetic operators, and giving lower precedence to in, followed by + and -, and then by * and /. I see that if the token in is not given a precedence declaration, the grammar still is ambiguous, as can be seen in the example of parsing the expression: let v = e1 in e2 + e3 which could be parsed as (let v = e1 in e2) + e3 or let v = e1 in (e2 + e3) Giving the token in a lower precedence makes the parser choose the last option. My question is about the associativity of the token in? Does it make any difference giving it left associativity, right associativity, or making it non associative? Regards. José Romildo

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traversing a tree using monad.cont
by Anatoly Yakovenko 07 May '09

07 May '09

So I am trying to traverse a tree in a specific order, but i have no idea where the things that i am looking for are located, and i want to avoid explicit backtracking. I was thinking i could do it with the continuation monad. Here is what i have module TestCont where import Control.Monad.Cont import Control.Monad.Identity import Control.Monad.State.Lazy --our stupid tree data Tree a = Tree [Tree a] | Leaf a --traverse all the branches search (Tree ts) next = do mapM_ (\ ti -> (callCC (search ti))) ts next $ () search tt@(Leaf a) next = do cur <- lift get case ((cur + 1) == a) of True -> do --the current leaf is what we want, update the state and return lift $ put a return $ () False -> do --the current leaf is not what we want, continue first, then try again next () search tt (\ _ -> error "fail") t1 = Leaf 1 t2 = Leaf 2 t3 = Tree [t1,t2] t4 = Leaf 3 t5::Tree Int = Tree [t4,t3] run = runIdentity (runStateT ((runContT $ callCC (search t5)) return) 0) it seems like next isn't quite doing what i want, because i don't think I ever try again after i call next $ () in the second clause. Any ideas? Thanks, Anatoly

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IO help
by applebiz89 07 May '09

07 May '09

I havent done much IO at all in haskell, only within the function itself. However I want to get the input from the interface for the function and havent done this before. In my main function, I want to ask the user what they would like to do 'become fan' for example, then with their choice initiate the function and ask for the appropriate input for that function. This is the code below: main :: IO() do putStr "Hi there! what is your name: " fanName = getLine do putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans of a film, 4 = Film released in a year: " input = getLine read input :: Int (if input == 1 then main x = insertFilm [] else if input == 2 then main x = becomeFan [] else if input == 3 then main x = numberOfFans []) Say they choose film in a given year function function: filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear films = [ title | (Film title director year fans) <- films, year == filmYear] I need to ask the user what filmYear they want to insert. But i need to do this from the main function...I chose to do an if statement to choose what function they want, but i dont know where to go from there? any help will be greatly appreciated. thanks apple -- View this message in context: http://www.nabble.com/IO-help-tp23423403p23423403.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Interesting Thread on OO Usefulness (scala mailing list)
by Paolo Losi 07 May '09

07 May '09

Hi all, I'm following an interesting thread on the scala mailing list: http://www.nabble.com/-scala--usefulness-of-OOP-td23268250.html Martin Odersky advocates the OO features of the scala language proposing an interesting problem where the OO approach seams valuable. I would be very much interested in seeing an Haskell solution to that problem. Any haskell guru want to take a stub at it or give an opinion from a pure FP point of view? Thanks Paolo

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calling a variable length parameter lambda expression
by Nico Rolle 07 May '09

07 May '09

Hi everyone. I have a problem. A function is recieving a lambda expression like this: (\ x y -> x > y) or like this (\ x y z a -> (x > y) && (z < a) my problem is now i know i have a list filled with the parameters for the lambda expression. but how can i call that expression? [parameters] is my list of parameters for the lambda expression. lambda_ex is my lambda expression is there a function wich can do smth like that? lambda _ex (unfold_parameters parameters) best regards

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