May 2008 - Haskell-Cafe

Short circuiting and the Maybe monad
by John Hamilton 18 May '08

18 May '08

I'm trying to understand how short circuiting works with the Maybe monad. Take the expression n >>= f >>= g >>= h, which can be written as (((n >>= f) >>= g) >>= h) because >>= is left associative. If n is Nothing, this implies that (n >>= f) is Nothing, and so on, each nested sub-expression easily evaluating to Nothing, but without there being a quick way to short circuit at the beginning. Now take the example do x <- xs y <- ys z <- zs return (x, y, z) which I believe desugars like xs >>= (\x -> ys >>= (\y -> zs >>= (\z -> return (x, y, z)))) Here the associativity of >>= no longer matters, and if xs is Nothing the whole expression can quickly be determined to be Nothing, because Nothing >>= _ = Nothing. Am I looking at this correctly? - John

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another Newbie performance question
by Philip Müller 18 May '08

18 May '08

Hi everybody, I was doing an assignment in Java for my university concerning a program that reads, modifies and writes CSV files, when I suddenly had the idea of implementing parts of this in Haskell for fun. When I finished the Haskell programm, I was disappointed by the performance: To parse a 200k lines CSV file, insert a line (yes I know i could insert a line without parsing the file, that's just an example) at pos. 199999 and write the file again, the Java program takes 1.1 seconds while the Haskell program takes 12.5 seconds. I have read Don's blog post but am unsure how to implement his tips into my program, as I am still kind of a Haskell beginner. The source code (40 lines incl. comments and empty lines) and the 200k CSV file I used for testing and a smaller CSV file demonstrating the special easy-to-parse CSV syntax are available on my ftp server, ftp://baah.servegame.org/public/haskell The call syntax is <program> <csv file> <line to insert> e.g. main lang.csv "test","this","line" I haven't posted the source code here directly because I thought it might be too long. If someone here finds the time to look at my code and give me some hints, that would really be nice. Regards Philip

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Mapping Haskell Concepts to C# 3
by Galchin, Vasili 18 May '08

18 May '08

Kaveh, Maybe I am misguessing why you are asking your question, but wouldn't it be better to ask how to map these Haskell concepts to CLR? If so, check out work on Mondrian. Vasili

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more runhaskell question
by Galchin, Vasili 18 May '08

18 May '08

vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs configure --prefix=$HOME Configuring mlock-1.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs build Setup.lhs: error reading dist/setup-config; run "setup configure" command? No dist/setup-config is being generated! what is the "setup configure" command?? Regards, Vasili

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Calling haddock in a portable way
by Misha Aizatulin 17 May '08

17 May '08

hello, the new version of haddock (2.0.0) needs a new option -B that tells it the GHC lib directory. How do I find out the correct value for this option in a makefile, so that the makefile stays portable? Cheers, Misha

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Mapping Haskell Concepts To C# 3
by Kaveh Shahbazian 17 May '08

17 May '08

I have question on mapping some Haskell concepts to C# 3 ones. Maybe there are not any strict equivalents; yet it helps: 1 - What is the equivalent of "Type Constructor" in C#? 2 - What is the equivalent of "Data Constructor" in C#? 3 - What is the logical implementation of pattern matching in C#? (For example using structures with indicator fields or using interfaces and inheritance and dynamically dispatch in calling overloaded methods. Also this question contain a hidden one...GADTs!) Best Regards -- Kaveh Shahbazian email: kaveh.shahbazian(a)gmail.com

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[ANN] Next Bay FP Meeting: Bryan O'Sullivan on Concurrent and multicore programming in Haskell
by Keith Fahlgren 17 May '08

17 May '08

Hi, Our next BayFP meeting will be this Thursday, May 8th, 2008 at 7:30pm. We'll feature Bryan O'Sullivan on Concurrent and multicore programming in Haskell. Bryan is a co-author of the upcoming O'Reilly book Real World Haskell [http://book.realworldhaskell.org/] (among all sorts of other snazzy endeavors) Talking about multicore is all the rage, so I expect folks to bring a lot of people! :-) Details here: http://www.bayfp.org/blog/2008/05/02/may-8th-meeting-bryan-osullivan-on-con… Many thanks to Alex Payne at Twitter for hosting this month's meeting. They've been very supportive of BayFP and we appreciate their continued hosting. Twitter's address is: 164 South Park St San Francisco, CA 94107 Alex says: It's a building with a dark green door. People can just come on in and walk to their right to a large conference room. We'll start at 7:30pm. As always, this is a free event. If you want pizza, please select which type here (and bring a few $$s): Pizza Selection Form [http://bayfp.wufoo.com/forms/bryan-osullivan-bay-fp/] Keith

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Haskell-Cafe Info Page
by D. Gregor 17 May '08

17 May '08

Hello, Common Lisp is a multiparadigm, general purpose programming language that supports imperative, functional, and object-oriented programming paradigms. Haskell is purely functional. Is this a reason why there is not macro feature in Haskell? I feel the object-oriented paradigm of CL and Scheme is the reason for the macro feature in these two languages. If it's not, then what does the macro feature provide, and why isn't it in Haskell? Douglas

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A point in favour of -XOverlappingInstances (and -XTypeSynonymInstances)
by Achim Schneider 17 May '08

17 May '08

Token.hs:103:15: Overlapping instances for Show (SourcePos, Tok) arising from a use of `anyToken' at Token.hs:103:15-22 Matching instances: instance (Show a, Show b) => Show (a, b) -- Defined in GHC.Show instance [overlap ok] Show Token -- Defined at Token.hs:(39,0)-(40,23) I was just trying _not_ to show the a of (a,b) -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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ghc on Ubuntu Linux?
by Galchin, Vasili 17 May '08

17 May '08

Hello, I am running ghc and tools on Ubuntu. I seem to be running into very strange problems between ghc-pkg and the Ubuntu package manager. Suddenly "runhaskell Setup.hs configure" just stops working ... Are any other ghc/Ubuntu users running into problems? If so, please tell me what problems. This is killing my Haskell work! Regards, Vasili

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