March 2012 - Haskell-Cafe

IO() and other datatypes
by Kevin Clees 04 Mar '12

04 Mar '12

Dear Haskell programmers, I'm very confused, because I really don't know how to handle with IO's and other datatypes, such as Int or String. If I want to build a haskell program can I only use IO() method outputs ? How can I "give" a Int result from a different method back to the main? Would this mean that I have to create only IO() outputs? Is this correct? For example: -- A User has to choose something, so I need a IO() datatype main :: IO() main = do [...] let listtournementTime = [8,20,10,15] -- This is wrong: Couldn't match expected type `IO t0' with actual type `Int' -- The results of the method dauer is a Int. Do I have to transform the method to an IO() Output a <- Dauer listtournementTime [...] -- the methods dauer:: [Int] -> Int dauer (x:xs) | laenge(x:xs) == 1 = 0 | mod (laenge (x:xs)) 2 == 0 = (tmp x xs) + dauer xs | mod (laenge (x:xs)) 2 /= 0 = dauer xs | otherwise = 999999 -- failure tmp:: Int -> [Int] -> Int tmp y (x:xs) = x-y laenge :: [a] -> Integer laenge [] = 0 laenge (x:xs) = 1 + laenge xs The last three methods are working correct, if I directly put some data into the methods, like: dauer [10,15] ===Result===> 5 Thank you for any help Best greetings from Namibia

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ANNOUNCE: Haddock 2.10.0
by David Waern 04 Mar '12

04 Mar '12

-------------------------------------------- -- Haddock 2.10.0 -------------------------------------------- Version 2.10.0, which was released with GHC 7.4.1, is now uploaded on Hackage. Here are the release notes: -------------------------------------------- -- Changes in 2.10.0 -------------------------------------------- * Require GHC >= 7.4 * Safe Haskell indications on module pages * Type declarations on identifiers no longer necessary * Add flag --interface-version * Warn when comment refers to in-scope identifier without documentation * Bug fix: links to out-of-scope things (#78) * Bug fix: module references to other packages work again -------------------------------------------- -- Links -------------------------------------------- Hackage page: http://hackage.haskell.org/package/haddock-2.10.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://darcs.haskell.org/haddock.git -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Ian Lynagh Jose Pedro Magalhaes Simon Peyton-Jones Ryan Newton David Terei David Waern Also thanks to Simon Hengel for spotting a serious bug before the release. -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://darcs.haskell.org/haddock.git Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

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ANN: 11th GhentFPG Meeting Tuesday, 20th of March: Final Program
by Jeroen Janssen 04 Mar '12

04 Mar '12

(apologies if you receive multiple postings) Dear all, The program and *new location* for the 11th GhentFPG meeting has been determined. The meeting will take place on Tuesday, the 20th of March at 19:30 in auditorium V1 of building S9 at Campus De Sterre (Krijgslaan 281, 9000 Gent). We will post signs at building S9 to guide you towards the auditorium. The program is as follows: 1. Jasper Van der Jeugt: digestive-functors 0.3 Formlets are a way of building HTML forms that are type-safe, handle errors, abstract and are easy to combine into bigger forms. I present the digestive-functors library, a formlets implementation which solves a number of issues with the original formlets library, such as annotation of errors, labelling, and separation of the validation model from the actual HTML output. 2. Jeroen Janssen: the typeclassopedia. This is part of a new type of talks we are going to try with GhentFPG. Each meeting we would like to introduce you to some of the important papers in the world of FP. This meeting I am going to introduce you to the brilliant "Typeclassopedia" paper. This paper, which originally appeared in the monad reader issue 13, introduces many of the important typeclasses in the Haskell standard library and explains how they are interrelated, both in a formal fashion as in a practical fashion. Hope to see you all there! The GhentFPG Organizing Committee.

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Problems installing Data.Encoding package
by Pēteris Paikens 04 Mar '12

04 Mar '12

Hello, I'm stuck at trying to get Data.Encoding package functional. Cabal gives the following error message: cabal install encoding Resolving dependencies... /var/folders/xc/jnh69_cx1n32n1qwkgfrlc100000gq/T/encoding-0.6.620435/encoding-0.6.6/dist/setup/setup: /var/folders/xc/jnh69_cx1n32n1qwkgfrlc100000gq/T/encoding-0.6.620435/encoding-0.6.6/dist/setup/setup: cannot execute binary file cabal: Error: some packages failed to install: encoding-0.6.6 failed during the configure step. The exception was: ExitFailure 126 What could be done here? I'm on MacOS Lion, ghc v. 7.0.4 Regards, Peteris

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anyone know what happened to nlpwp?
by serialhex 04 Mar '12

04 Mar '12

i was going through nlpwp.org learning haskell & natural language processing at the same time, and a few days ago it just disappeared! does anybody know what happened to it? i can still access the google cache, and learn from there, but that'd be a pain for anybody new coming along who can't access this site... :( hex -- * my blog is cooler than yours: http://serialhex.github.com * The wise man said: "Never argue with an idiot. They bring you down to their level and beat you with experience." * As a programmer, it is your job to put yourself out of business. What you do today can be automated tomorrow. ~Doug McIlroy No snowflake in an avalanche ever feels responsible. --- CFO: “What happens if we train people and they leave?” CTO: “What if we don’t and they stay?”

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FreeSect -- generalised sections syntax extension
by Ras Far 03 Mar '12

03 Mar '12

Hello, I bit premature perhaps but I wanted to post it on a leap day... http://fremissant.net/freesect Thanks for eyebloom on #haskell for motivating me to finally implement an old idea. Thanks to the rest on #haskell for doing their best to talk me out of it. ;) I make no claims regarding the usefulness of the extension, but some folks might find it interesting, or may just appreciate additional examples of using HSE and SYB. I regret that I am not a better Haskell coder, but it is what it is! Kind Reg'ds, Andrew Seniuk (rasfar)

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External Interfaces
by Victor Miller 03 Mar '12

03 Mar '12

I'd like to write reasonable Haskell interfaces to a few external libraries. To be specific -- all of them do limited calculations (no I/O) but each has an internal state. I know that I can have results dump everything into the IO monad, but I'd like to avoid the "sin bin" (I think it was Phil Wadler who called it that). To be specific, I have in mind the library pari ( http://en.wikipedia.org/wiki/PARI/GP ) which does symbolic algebra calculations. The way that the pari state works is that state of works is that we can add definitions of new quantities, and bind them to a calculated value. As part of the state there might be certain cached calculations. If I avoid deleting bound objects, it seems to me that it would be perfectly valid to make the interface into a monad have a complicated state which is being managed by the pari library. As long as any queries from pari give the same value (which should be the case if I never delete anything) then it should be valid without dumping things into the IO monad. Does anyone see anything wrong with my reasoning? Victor

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ghc: could not execute: opt
by Graham Berks 03 Mar '12

03 Mar '12

Hi, trying out haskell. Have osx 10.7 and ghc 7.0.4 via homebrew When attempting to compile a program via cabal i get ghc: could not execute: opt I assume I have something missing / not configured with my env. Can someone point me in the direction ? Thanks

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Records in Haskell: Type-Indexed Records (another proposal)
by Matthew Farkas-Dyck 03 Mar '12

03 Mar '12

Hello all. I wrote a new proposal for the Haskell record system. It can be found at http://hackage.haskell.org/trac/ghc/wiki/Records/TypeIndexedRecords Records are indexed by arbitrary Haskell types. Scope is controlled as scope of key types. No fieldLabel declarations needed (as in DORF). Cheers, strake

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Does somebody know about these functions?
by Johan Holmquist 03 Mar '12

03 Mar '12

Two functions that I see useful are described here and I would like to know if they are defined in some more or less standard Haskell library. Hoogle (http://www.haskell.org/hoogle) did not reveal anything about that. Function 'inter' applies given function to each succeeding pair of elements of a list. inter :: (a -> a -> b) -> [a] -> [b] inter f [] = [] inter f l = map (uncurry f) $ zip l (tail l) Example usage: and $ inter (<=) l -- checks if 'l' is ordered inter (,) l -- gives succeeding pairs Function 'withPair' takes a pair and applies a function to it's first element, another function to it's second element and finally combines the results with yet another function. withPair :: (a' -> b' -> c) -> (a -> a') -> (b -> b') -> (a,b) -> c withPair f fa fb (a,b) = fa a `f` fb b Example usage: words [] = [] words s = withPair (:) id words (break isSpace $ dropWhile isSpace s) lines [] = [] lines s = withPair (:) id lines (break (== '\n') s) mapPair = withPair (,) This function can abstract away the (in my opinion ugly) pattern (as seen in the examples): foo list = let (a,b) = <somefun> list in <somefun2> a `<combinedWith>` foo b Anyone knows about these two functions or variants thereof? Cheers /Johan

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