July 2010 - Haskell-Cafe

understanding Peano
by Patrick Browne 04 Jul '10

04 Jul '10

Hi, I would like to understand the Peano module below. I do wish to implement or improve the code. I am unsure of the semantics of (suc = Suc) and then the subsequent use of (Suc n) Is this an example of type-level programming? module Peano where data Nat = Zero | Suc Nat deriving Show class Peano n where suc :: n -> n eq :: n -> n -> Bool plus :: n -> n -> n instance Peano Nat where suc = Suc eq Zero Zero = True eq (Suc m) (Suc n) = eq m n eq _ _ = False plus m Zero = m plus m (Suc n) = Suc (plus m n) -- Some evaluations let t1 = plus (Suc (Zero)) (Suc ( Suc (Zero))) let t2 = plus (Suc (Suc(Zero))) ( Suc (Zero)) is 1+2=2+1? eq t1 t2 -- true *Peano> :t plus (Suc (Suc(Zero))) ( Suc (Zero)) regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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What is the point of many and some functions in Control.Applicative (for Alternative?)
by Thomas Hartman 03 Jul '10

03 Jul '10

When I load up Control.Applicative in ghci and try, eg many [1,2] or many (Just 1) or some [1,2] or some (Just 1) this never returns. What are the practical uses of these combinators, or for using the Alternative class in general?

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Read Instance code.
by Andy Stewart 03 Jul '10

03 Jul '10

Hi all, I have some incorrect "Read instance" make i got error "Prelude.read: no parse", and i don't know how to fix it. ------------------------------> code start <------------------------------ newtype SerializedWindow = SerializedWindow (Maybe DrawWindow) instance Show SerializedWindow where show _ = "SerializedWindow Nothing" instance Read SerializedWindow where readsPrec _ str = [(SerializedWindow Nothing, idStr) | (val :: String, idStr) <- reads str] ------------------------------> code end <------------------------------ Any help? Thanks! -- Andy

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Spiking neural networks in Haskell
by Dmitry V'yal 03 Jul '10

03 Jul '10

Greetings anyone, Some time ago I with a friend of mine implemented a toy interactive simulator of neural network activity. It has a totally imperative design and uses Gtk2hs for user interface parts. The darcs repository is available here: http://vyal.ru/code/spiking_neuro Now we decided to develop it further to try out some nice biologically inspired ideas. At first I wanted to rewrite the simulator in C++, since that language seems to match the current design more closely. But recently I've read some FRP related papers and have decided to rethink the overall approach. I'd like to hear some comments from the community on the current code and advises on possible improvements and other designs and their pros and conses. The performance is not critical for me, I'm looking for maximal flexibility and clearness so the model can easily be modified and extended. Best wishes, Dmitry

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Parsec combinator like Prolog's cut operator?
by Erik de Castro Lopo 03 Jul '10

03 Jul '10

Hi all, I'm reading John Hughes' paper "Generalizing Monads to Arrows" and found the statement regarding parser combinators: "... depend on the programmer using an additional combinator similar to Prolog's 'cut' operator do declare that a parser need never backtrack beyond a certain point." Does something like this already exist in Parsec? If not is there a way to write it? Having this would really help with a parsing problem I have. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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a very pedestrian question about maillists
by Vasili I. Galchin 03 Jul '10

03 Jul '10

Hello, Suppose I have the following fragment of a posting: Message: 3 Date: Fri, 2 Jul 2010 12:32:43 -0500 From: aditya siram <aditya.siram(a)gmail.com> Subject: Re: [Haskell-cafe] How easy is it to hire Haskell programmers To: Andrew Coppin <andrewcoppin(a)btinternet.com> Cc: haskell-cafe(a)haskell.org Message-ID: <AANLkTil-p8qMNkPdqEwDSciqw2RjASzbRL2-sPyf079u(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 On another posting I tried counter-posting using the "Message-ID" but the counter-post bounced back to me as non-deliverable. How do I post on a "thread" like "How easy is it to hire Haskell programmers"? Thanks, Vasili

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Status of status on freenode
by Walt Rorie-Baety 03 Jul '10

03 Jul '10

My work environment is what I'd call typical US corporate - IRC nodes are blocked, but I can use a web-based client to access it. A couple of days ago, someone had the poor taste to throw a pack of rabid spam bots into the #haskell channel, forcing the short-term solution of shutting down all web-based users. I noticed today that I can now access #haskell again, but have no voice. I'm wondering: is there any way to modify access to allow registered users to speak? I've tried using other web-based clients, but none of then connect to the freenode server. At home, I can (and do) use mIRC, so I'm not totally cut off - and now I can read, but not communicate back to, the #haskell channel. BTW, please direct me to another list, if this issue does not belong on it. Walt "BMeph" Rorie-Baety "A mountain that eats people? I want one!." - Richard, of LFGComic.com

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ANNOUNCE: hs-cryptohash 0.4
by Vincent Hanquez 03 Jul '10

03 Jul '10

Hi Haskell-cafe, I'ld like to announce the existence of hs-cryptohash [1] which provides most common digests (sha1, sha2 family, md[245], ripemd160) in a incremental and one-pass api. It's also much faster than pure haskell implementation i've played with, since the underlaying algorithms are all fairly optimized in C. The API remains pure with the use of unsafePerformIO, which after reading everything i could about it and some testing, seems to be safe in this context. The main reason for this library is the lack of incremental api exposed by current digest libraries, and filling the void about some missing digest algorithms; Also the speed comes as a nice bonus. I'm looking forward to hear any comments, [1] http://github.com/vincenthz/hs-cryptohash -- Vincent Hanquez

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MState: A consistent State monad for concurrent applications
by Nils Schweinsberg 03 Jul '10

03 Jul '10

Hi, I have been trying to use the State monad for concurrent applications and came up with a little library.[1] My "MState" uses an IORef to maintain the state between different threads. The library also offers a simple way to fork off new threads using its own "forkM" function. This function will asure that every thread in an "evalMState"/"execMState" call will be finished before the final result is returned. The library is also on github.com.[2] Please let me know what you think of it and whether or not I should put it on hackage. Cheers, Nils Schweinsberg [1] http://n-sch.de/hdocs/mstate/Control-Concurrent-MState.html [2] http://github.com/mcmaniac/mstate

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Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Ivan Lazar Miljenovic 02 Jul '10

02 Jul '10

Deniz Dogan <deniz.a.m.dogan(a)gmail.com> writes: > Doing so would only mean that haskell-mode will most likely never be a > part of Emacs, meaning that anyone looking for programming Haskell in > Emacs would have to download an external library, be it through > package.el or not. And what's so bad about that? I suppose it might be possible to have a cut-down haskell-mode in Emacs for casual hacking, the way there's python-mode and pymacs. > To put it short, there are no pros in making the new Haskell indenter > a strict part of haskell-mode. There are if it brings in a lot of cool features. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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