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ok, someone check me on this (type unification from the (>>=)/fmap thread)
by Brandon S. Allbery KF8NH 10 May '09

10 May '09

I can't tell where I'm making the mistake here. In the thread where (>>=) and fmap were being confused, the error cited a type (Maybe a) which was supposed to be in typeclass Num. As far as I can tell, if the typechecker gets to the point where Num and Maybe are both present and (m) is Maybe, it has to (1) be focused on the (m b) part of (a -> m b), and therefore (2) must have already unified (a) and (b). But that means (m b) must unify with (Num a => a), which is unified with (b), resulting in the attempt to unify (Num a => a) with (Maybe a); since we get the error about (Maybe a) not being a Num, it must not have gotten there. But that means it can't have related Num to (m a) with (m) bound to Maybe, which is why I assumed it had unified (m) with ((->) r) instead. Can the typechecker really get the Num to the other end of (a -> m b) without also getting the (a) there? Or is it checking for the Num constraint before it has fully evaluated the type of (m b)? I thought typeclass constraints happened later than basic type unification. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

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Heads up: Conflicting versions of network-2.2.1
by Johan Tibell 10 May '09

10 May '09

Hi, The version of network-2.2.1 that shipped with GHC 6.10 differs from the one on Hackage [1]. The version that was released with GHC was never uploaded to Hackage and was never tagged in the darcs repository. When making a new release of network I forgot to check the version number on the version of network that shipped with GHC. My appologies. If you want the API additions that are present in network-2.2.1 on Hackage use network-2.2.1.1 instead to make sure that e.g. cabal-install doesn't use the version of network that comes with GHC. In the future there won't be any releases directly from the darcs repo. All releases will be made on Hackage to later be picked up by the Haskell Platform. Cheers, Johan 1. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-2.2.1

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Just 3 >>= (1+)?
by michael rice 10 May '09

10 May '09

Why doesn't this work? Michael ================ data Maybe a = Nothing | Just a instance Monad Maybe where return = Just fail = Nothing Nothing >>= f = Nothing (Just x) >>= f = f x instance MonadPlus Maybe where mzero = Nothing Nothing `mplus` x = x x `mplus` _ = x ================ [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> Just 3 >>= (1+) <interactive>:1:0: No instance for (Num (Maybe b)) arising from a use of `it' at <interactive>:1:0-14 Possible fix: add an instance declaration for (Num (Maybe b)) In the first argument of `print', namely `it' In a stmt of a 'do' expression: print it Prelude>

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Haskell IO problem
by applebiz89 10 May '09

10 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. // Say I have this as my data type and list of films data Film = Film String String Int [String] -- List of films testDatabase :: [Film] testDatabase = [(Film "Casino Royale" "Martin Campbell" 2006 ["Garry", "Dave", "Zoe"]) ] // with functions such as: becomeFan :: String -> String -> [Film] -> [Film] becomeFan _ _ [] = [] becomeFan filmName fanName ((Film title director year fans):xs) | filmName == title = (Film title director year fanName:fans) : xs | otherwise = (Film title director year fans) : becomeFan filmName fanName xs filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear films = [ title | (Film title director year fans) <- films, year == filmYear] // I want to ask the user what function they want to use, I have this so far 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 []) // I thought using the if statement would work, but now I cant think how to gather the needed input for the function they have chosen? thanks apple -- View this message in context: http://www.nabble.com/Haskell-IO-problem-tp23404351p23404351.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Is Haskell a Good Choice for Web Applications? (ANN: Vocabulink)
by jekor＠jekor.com 10 May '09

10 May '09

I decided to find out for myself. You can find the results at http://jekor.com/article/is-haskell-a-good-choice-for-web-applications Included is the source code for the web application powering http://www.vocabulink.com/ The source is roughly 2,000 lines of Haskell, along with some SQL and JavaScript. It's written in literate style and includes a 75-page PDF. It demonstrates and explains how to: * use FastCGI to communicate with a web server (nginx in this case) * move data to and from a PostgreSQL database (HDBC) * authenticate users with cookies * interact with users via asynchronous JavaScript and JSON (AJAJ) * collect data with HTML forms (formlets) * communicate with users via email * cache with memcached * implement a custom forums system (with threaded comments) I make no claims that the code is elegant or idiomatic. It is however real code that's running "in the wild". And I hope it's useful to anyone else considering doing web development in Haskell. I welcome and encourage your feedback!

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Re: [Haskell-cafe] cabal parse problems
by Vasili I. Galchin 09 May '09

09 May '09

sorrily nope, Brian ... Vasili On Wed, May 6, 2009 at 6:06 PM, <brian(a)lorf.org> wrote: > On Wednesday, 06.05.09 at 18:05, Vasili I. Galchin wrote: > > Exposed-modules: Swish.HaskellRDF.BuiltInDatatypes, > > ... > > Swish.HaskellRDF.GraphMatch, > > Think it probably doesn't like that trailing comma. >

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haskell - main function
by applebiz89 09 May '09

09 May '09

Could anyone look at this segment of code; it's not compiling wondering if anyone could correct me as to why. Thanks Code: -- Film as datatype type Title = String type Director = String type Year = Int type Fan = String data Film = Film Title Director Year [Fan] -- List of films testDatabase :: [Film] testDatabase = [(Film "Casino Royale" "Martin Campbell" 2006 ["Garry", "Dave", "Zoe"]) ] becomeFan :: Title -> fanName -> [Film] -> [Film] becomeFan _ _ [] = [] becomeFan Title fanName ((Film Title Director Year fan):xs) | filmName == title = (Film Title Director Year fanName:fan) : xs | otherwise = (Film Title Director Year fan) : becomeFan Title fanName xs main :: [Film] -> IO() main db = 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, 5 = Given fan, 6 = Stop : " input = getLine x = read input :: Int if x == 1 then do putStr "Enter film title: " filmTitle <- getLine putStr "Enter director name: " filmDirector <- getLine putStr "Enter release year: " filmYear <- getLine main insertFilm [Title Director Year [Film]] else if x == 2 then do putStr "Enter film title: " filmTitle <- getLine putStr "Enter fan name: " fanName <- getLine main becomeFan [Title fanName] else if x == 3 then do putStr "Enter film title: " filmTitle <- getLine main numberOfFans [Title] else if x == 4 then do putStr "Enter film release year: " filmYear <- getLine main filmsInGivenYear [Year [Film]] else if x == 5 then do putStr "Enter the fan name: " fanName <- getLine main givenUser [fanName [Film]] else if x = 6 then return () -- View this message in context: http://www.nabble.com/haskell---main-function-tp23459841p23459841.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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is value evaluated?
by Nikhil Patil 09 May '09

09 May '09

Hi, I am curious to know if there is a function in Haskell to find if a certain value has already been evaluated. The function I need would have the type: > (?!) :: a -> Bool And I expect it to be such that the following terminates after printing the first 101 fibonacci numbers. > fibs = 0 : 1 : zipWith (+) fibs (tail fibs) > > main = do print $ fibs !! 100 > print $ takeWhile (?!) fibs Although I guess I can imagine the following not terminating: > print $ filter (?!) fibs I would find such a function immensely useful in "printing out" my infinite lists. Thanks! nikhil

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Simulation and GHC Thread Scheduling
by Thomas DuBuisson 09 May '09

09 May '09

All, I have a simple Haskell P2P library that I've been playing with in simulations of 20 to 600 nodes. To run the simulation there is a Haskell thread (forkIO) for every node in the system, one that starts up all the nodes and prints the info (so prints aren't mangled), and one that acts as the router. Before its mentioned - I understand the best way forward would be to refactor the code into IO-less 'algorithm' sections and other sections that perform the needed IO when I'm not simulating. I know this would allow me to declare what order each node runs in and would free me from the scheduler. I'd like to do that if its practical... but! None-the-less, here I am saying that there are many interesting little simulations that could be done without refactoring and the correctness isn't altered by the order of operations (not if the nodes behave properly, the slight variation is actually a good test). What I would like to know is are there any plans for GHC to incorporate user-definable scheduler? It would be useful in numerous instance beyond this poor example; I know user scheduling was briefly mentioned in Li's paper but haven't seen or heard of any interest from others since then. Thomas

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runhaskell CLI parameters
by Vasili I. Galchin 09 May '09

09 May '09

Hello, I have forgotten the runhaskell CLI parameters ... sigh. In particular I want to a "local" build of a set of of package: runhaskell Setup.hs configure --user??? I just did a "runhaskell -?" which didn't tell me a lot! Kind regards, Vasili

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