- Haskell-Cafe - Haskell.org

RFC: only-read-or-write-vars
by Bas van Dijk 22 Mar '10

22 Mar '10

Hello, I've written a tiny package for restricting access to mutable variables to be read-only or write-only: http://code.haskell.org/~basvandijk/code/only-read-or-write-vars/ This is a Request For Comments, so any comments / patches / +1s / -1s about anything are more than welcome before I upload it to hackage (sometime this weekend). regards, Bas

2 3

ANN: Three package announcements
by Mario Blažević 22 Mar '10

22 Mar '10

There are three new packages on Hackage: - monad-parallel 0.5 (http://hackage.haskell.org/package/monad-parallel) - monad-coroutine 0.5 (http://hackage.haskell.org/package/monad-coroutine) - Streaming Component Combinators 0.5 (http://hackage.haskell.org/package/scc) The first two packages are completely new. Their functionality has been present in SCC 0.4, but I thought they might be useful on their own. I'm keeping all the version numbers in sync for now, and the source code for all three packages is in a single Darcs repository at http://code.haskell.org/SCC/. The monad-parallel library defines two Monad subclasses, MonadParallel and MonadFork, that enable monadic computations to be executed in parallel and their results combined. The library also exports a subset of the Control.Monad interface (ap, sequence, and related functions), adjusted to exploit the parallelism. The only currently defined MonadParallel instances are IO, Maybe, [], and Identity. More instances could be added, but I didn't want the package to depend on MTL or transformers. The library design was heavily influenced by the discussion in this Cafe thread: http://www.mail-archive.com/haskell-cafe@haskell.org/msg68581.html The monad-coroutine package exports a generic monad transformer Coroutine: Functor s => MonadTrans (Coroutine s). A Coroutine-transformed monad can suspend at any point, returning its resumption wrapped in the functor s. There are also some functions for manipulating and running coroutines, as well as a couple of useful suspension functors such as Yield and Await. Finally, version 0.5 of Streaming Component Combinators (a.k.a. SCC) comes with some significant code refactoring (as the two aforementioned packages prove), simplifications, and performance enhancements. No new features have been added since the 0.4 release.

1 0

ANNOUNCE: AusHac2010
by Ivan Lazar Miljenovic 22 Mar '10

22 Mar '10

Dreading the end of ZuriHac? Wish the Haskellian camaraderie could continue? Well, if you're able to make your way to Sydney, Australia between the 16th and 18th of July, then AusHac2010 is for _you_! It will be held at the School of Computer Science and Engineering at the University of New South Wales (thanks to Manuel Chakravarty and Ben Lippmeier for offering to organise the rooms!). If you are wanting to attend the inaugural Australian Hackathon, please register at http://axman6.wufoo.com/forms/aushac-2010-sign-up/ . For more information, see http://www.haskell.org/haskellwiki/AusHac2010 We will shortly be adding an extended version of the "Who" table; please also add your details there as well as which projects you would be interested in hacking in. Hoping to see you there! -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Occurs check error, help!
by boblettoj 22 Mar '10

22 Mar '10

Hi i am writing a shuffle function which takes in a random number i and a list and then produces a jumbled up version of the list at the end. Here is what i have at the moment (i'm not sure if it works yet as it won't compile! --function used to shuffle cards --list equals random member of array plus the rest of the array --i is randomly generated from range of length equal to that of cards. shuffle :: int -> [a] -> [a] shuffle i [] = [] shuffle i [cards] = (cards!!i) + (shuffle (randomR (0, ((length cards)-2))) (delete (cards!!i) cards)) I get this error message when i try to compile: Occurs check: cannot construct the infinite type: a = [[a]] I'm very new to haskell so you'll have to excuse any ignorance! thanks -- View this message in context: http://old.nabble.com/Occurs-check-error%2C-help%21-tp27966341p27966341.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

8 20

RE: No warning in GHC
by Simon Peyton-Jones 22 Mar '10

22 Mar '10

[Redirecting to haskell-cafe] Try http://www.haskell.org/ghc/docs/latest/html/users_guide/flag-reference.html… With -Wall I get bash-3.2$ ghc -c -Wall Foo.hs Foo.hs:3:0: Warning: Definition but no type signature for `func' Inferred type: func :: forall t t1. (Num t1) => t -> t1 Foo.hs:3:5: Warning: Defined but not used: `x' Foo.hs:4:10: Warning: This binding for `x' shadows the existing binding bound at Foo.hs:3:5 | -----Original Message----- | From: cvs-ghc-bounces(a)haskell.org [mailto:cvs-ghc-bounces@haskell.org] On Behalf Of | Grigory Sarnitskiy | Sent: 21 March 2010 19:37 | To: cvs-ghc(a)haskell.org | Subject: No warning in GHC | | Hello! I wonder, why GHC shows no warning in situations like | | func x = x | where x = 5 | | Sometimes I forget about argument names used, and use the same names in let or where. | And it can take me quite a lot time to understand what is wrong. I think it would be | nice to warn users if the argument is "redefined" in let/where clause. Shall I report | a bug/feature request? I'm using ghc 6.10.4. | | btw, I couldn't post to IRC: #ghc :Cannot send to channel | | _______________________________________________ | Cvs-ghc mailing list | Cvs-ghc(a)haskell.org | http://www.haskell.org/mailman/listinfo/cvs-ghc

1 0

Bubble sort algorithm implementations (Haskell vs. C)
by Yasir Arsanukaev 22 Mar '10

22 Mar '10

Hello. I have written 2 implementation of bubble sort algorithm in C and Haskell. Haskell implementation: module Main where main = do contents <- readFile "./data" print "Data loaded. Sorting.." let newcontents = bubblesort contents writeFile "./data_new_ghc" newcontents print "Sorting done" bubblesort list = sort list [] False rev = reverse -- separated. To see rev2 = reverse -- who calls the routine sort (x1:x2:xs) acc _ | x1 > x2 = sort (x1:xs) (x2:acc) True sort (x1:xs) acc flag = sort xs (x1:acc) flag sort [] acc True = sort (rev acc) [] False sort _ acc _ = rev2 acc I've compared these two implementations having run both on file with size of 20 KiB. C implementation took about a second, Haskell — about 1 min 10 sec. I have also profiled the Haskell application: Compile for profiling: C:\Temp> ghc -prof -auto-all -O --make Main Profile: C:\Temp> Main.exe +RTS -p and got these http://hpaste.org/fastcgi/hpaste.fcgi/view?id=24190#a24190 This is a pseudocode of the algorithm: procedure bubbleSort( A : list of sortable items ) defined as: do swapped := false for each i in 0 to length(A) - 2 inclusive do: if A[i] > A[i+1] then swap( A[i], A[i+1] ) swapped := true end if end for while swapped end procedure The performance may suffer from the memory allocation for the list. I wonder if it's possible to make Haskell implementation work faster without changing the algorithm (there's are actually a few tricks to make it work faster, but neither implementations have these optimizations). I'm interested not in particular algorithm performance but rather in performance of its implementations in various languages. I compiled the Haskell implementation in GHC (Haskell Platform 2009.2.0.2), which is the latest available from the site.

5 6

Google summer of code idea.
by Mujtaba Boori 22 Mar '10

22 Mar '10

Hello I would like to discuss my idea for google summer of code. I'm an active Haskell programmer . I have read the idea list . I think Improve Cabal's test support really suit my ability of haskell. I would like to give me advice about my idea . and how to improve my proposal . Thank you. -- Mujtaba Ali Alboori

2 1

Data.Number.BigFloat/Fixed
by H. 21 Mar '10

21 Mar '10

Hello, With ghc-6.10.4 the following results are produced with numbers 2009.8.9: (7e-3 :: BigFloat Prec50) < (6e-4 :: BigFloat Prec50) False (7e-3 :: BigFloat Prec50) < (8e-4 :: BigFloat Prec50) True where in the sourecode is written: data BigFloat e = BF (Fixed e) Integer deriving (Eq, Ord) and import Data.Ratio newtype Fixed e = F Rational deriving (Eq, Ord, Enum, Real, RealFrac) H.

5 6

How to set a breakpoint in GHCi
by Gracjan Polak 21 Mar '10

21 Mar '10

Hi all, Tried to use :break today, without success: guestbook-session-bug>ghci -DMIN_VERSION_template_haskell(a,b,c)=1 -isrc Main -i../happstack/happstack-ixset/src GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help ... ... [10 of 13] Compiling Happstack.Data.IxSet ( ..\happstack\happstack-ixset\src\Happstack\Data\IxSet.hs, interpreted ) ... ... *Main> :m +Happstack.Data.IxSet *Main Happstack.Data.IxSet> :bre getOrd No breakpoints found at that location. *Main Happstack.Data.IxSet> What now? -- Gracjan

1 0

Re: [Haskell-cafe] ANN: fixed-list -- A fixed length list library
by Casey McCann 21 Mar '10

21 Mar '10

Job Vranish <job.vranish(a)gmail.com> wrote: > Its main advantages are: > Very easy to use. > Almost entirely Haskell98 (the non Haskell98 pieces are not critical, just > nice) > The datatype is a member of Foldable, Traverable, Applicative, Monad, > etc... > Then length of the list is encoded in the type in a natural way. Unfortunately, it's very easy to get a context reduction stack overflow from GHC this way, which makes using such datatypes awkward for anything but very short lists. Explicit type annotations will often make things work, but at that point the type class isn't helping you anyway. For example, assuming the default stack size: import Data.FixedList fixed18 :: FixedList18 Int fixed18 = fromFoldable' [1..] -- this works fixed20 = 20 :. 20 :. fixed18 -- this works fixed22 :: FixedList22 Int fixed22 = 22 :. 22 :. fixed20 -- this only works with a type annotation show18 = show fixed18 -- this works -- this doesn't work: -- show20 = show fixed20 show20' :: FixedList20 Int -> String show20' list20 = show list20 show20 = show20' fixed20 -- this does work Using head and tail on longer lists fails likewise. I expect there's some way to make it work without simply increasing the stack depth, but I'm not sure how. Any thoughts? - C.

3 2