February 2010 - Haskell-Cafe

[Fwd: Re: [Haskell-cafe] Category Theory woes]
by Mike Pentney 18 Feb '10

18 Feb '10

As well as books and reading material online, nowadays you can also find video lectures...for example, the following was at the top of Googling "category theory video": http://golem.ph.utexas.edu/category/2007/09/the_catsters_on_youtube.html Cheers, Mike. Nick Rudnick wrote: > I haven't seen anybody mentioning «Joy of Cats» by Adámek, Herrlich & > Strecker: > > It is available online, and is very well-equipped with thorough > explanations, examples, exercises & funny illustrations, I would say > best of university lecture style: > http://katmat.math.uni-bremen.de/acc/. (Actually, the name of the book > is a joke on the set theorists' book «Joy of Set», which again is a > joke on «Joy of Sex», which I once found in my parents' bookshelf... ;-)) > > Another alternative: Personally, I had difficulties with the somewhat > arbitrary terminology, at times a hindrance to intuitive understanding > - and found intuitive access by programming examples, and the book was > «Computational Category Theory» by Rydeheart & Burstall, also now > available online at http://www.cs.man.ac.uk/~david/categories/book/, > done with the functional language ML. Later I translated parts of it > to Haskell which was great fun, and the books content is more beginner > level than any other book I've seen yet. > > The is also a programming language project dedicated to category > theory, «Charity», at the university of Calgary: > http://pll.cpsc.ucalgary.ca/charity1/www/home.html. > > Any volunteers in doing a RENAME REFACTORING of category theory > together with me?? ;-)) > > Cheers, > > Nick >

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Re: Pointfree composition for higher arity
by oleg＠okmij.org 18 Feb '10

18 Feb '10

Sean Leather wrote: > (...) :: (c -> d) -> (a -> b -> c) -> a -> b -> d > (...) f g x y = f (g x y) > Does anybody else care about this? What are some alternative > solutions? Here is a different solution: http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp f:: a1->a2-> .... ->cp (where cp is not a function type) g:: cp->d f `mcomp` g:: a1->a2-> .... ->d Now that we know how to generically decide if a type is not a functional type, mcomp can be defined fully generically, for any type cp that is not a function. If there is interest, I can write that code. One particular application of the mcomp combinator is the prod combinator: Given two functions, f:: a1->a2->...->an->c |c,d non-exponential types g:: b1->b2->...->bn->d their product f `prod` g:: a1->a2->...->an->b1->b2->...->bn->(c,d) The number of as and bs is arbitrary. The definition of prod is very simple: > prod = (. ((. (,)) . mcomp)) . mcomp The web page http://okmij.org/ftp/Haskell/polyvariadic.html#categorical-max3 gives a few explanations and further examples.

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What is the meaning of tilde ("~") symbol
by kg 17 Feb '10

17 Feb '10

Hi, I'm reading the following subject : http://www.haskell.org/pipermail/haskell-cafe/2007-July/028227.html In the sample code, we can see : instance ReadAsAnyOf () ex where readAsAnyOf ~() = mzero And, I've search the meaning of the symbol "~", but I've found nothing about this (note that's not easy to search "~" on google ...) Could you give me a link about this, or explain me it ? Thx in advance.

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ANN: concurrent-extra-0.1
by Roel van Dijk 17 Feb '10

17 Feb '10

Hello, We would like to announce the release of concurrent-extra [1]. A library which offers a few extra synchronization primitives. These primitives are found in the standard libraries of languages like Java and Python, but not in Haskell. Quick overview: * Lock: Enforce exclusive access to a resource. Also known as a mutex or a binary semaphore. * RLock: A lock which can be acquired multiple times by the same thread. Also known as a reentrant mutex. * Event: Wake multiple threads by signaling an event. Includes both pessimistic and optimistic versions. * ReadWriteLock: Multiple-reader, single-writer locks. Used to protect shared resources which may be concurrently read, but only sequentially written. * ReadWriteVar: Concurrent read, sequential write variables. Plug & Play: cabal install concurrent-extra Darcs: darcs get http://code.haskell.org/~roelvandijk/code/concurrent-extra/ Thanks to Neil Brown and Simon Marlow for an initial review. Comments are still more than welcome! Regards, Roel & Bas van Dijk [1] http://hackage.haskell.org/package/concurrent-extra

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Lambda's
by Henk-Jan van Tuyl 17 Feb '10

17 Feb '10

I love lambda's: http://hawtness.com/2009/12/30/wtf-girl-photo-more-reasons-why-half-life-is… -- Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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RFC: concurrent-extra
by Roel van Dijk 17 Feb '10

17 Feb '10

Hello, We wrote a small library (1) which offers a few extra synchronization primitives. These primitives are found in the standard libraries of languages like Java and Python, but not in Haskell. Before releasing concurrent-extra on hackage, we would like some comments on its name, design, implementation, documentation (2) and general usefulness. All primitives are implemented with MVars and IORefs, except for Control.Concurrent.STM.Event. There is a small test suite which tests some properties: cabal configure -ftest cabal build cabal test The test suite still needs to be expanded a great deal. We found that testing concurrent algorithms is really hard. Now we test things like "this shouldn't deadlock" by checking if it doesn't take too long. But that doesn't feel very formal. Is there a more principled approach to testing concurrent algorithms in Haskell? After writing our version of a read-write lock we noticed the package rwlock (3) on hackage. But there is a difference: rwlock is implemented with STM while our version is implemented entirely with MVars. Regards, Roel & Bas van Dijk 1 - http://code.haskell.org/~roelvandijk/code/concurrent-extra/ 2 - http://code.haskell.org/~roelvandijk/code/concurrent-extra/doc/html/concurr… 3 - http://hackage.haskell.org/package/rwlock

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Example for RVarT?
by Kemps-Benedix, Torsten 17 Feb '10

17 Feb '10

Hello folks, is there an example (simpe or not) for the use of Data.Random.RVarT with an underlying monad other than Identity, e.g. StateT, ReaderT etc.? Thx Torsten

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Notes on migrating from uvector to vector
by Don Stewart 16 Feb '10

16 Feb '10

I've summarised my notes on how to migrate array code from the uvector package to Roman's vector package: http://donsbot.wordpress.com/2010/02/15/migrating-from-uvector-to-vector/ And I'll take this opportunity to declare that uvector is now in official maintainance-only mode. Enjoy the new decade of flexible, fusible, fast arrays for Haskell! -- Don

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Efficient M to N conversion on fusion?
by iquiw 16 Feb '10

16 Feb '10

Hi, Is there any efficient way to convert M-length ByteString, Text or Vector into N-length one? For example, (a) converting byte array into array of their hex representations. ("abc" => "616263") (b) escaping some specific characters in string. ("a\bc" => "a\\bc") The only way I found is to use concatMap... Thanks, iquiw

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pandoc code bocks not working
by Walter De Jonge 16 Feb '10

16 Feb '10

http://johnmacfarlane.net/pandoc/README.html#delimited-code-blocks "Like regular code blocks, delimited code blocks must be separated from surrounding text by blank lines." goodluck walter

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