October 2007 - Haskell-Cafe

Functional Programming Books
by Dan Piponi 17 Oct '07

17 Oct '07

I was just putting together my Amazon wish list and was wondering if there are any great books on Haskell and/or functional programming that people think are must-reads. Okasaki's "Purely Functional Programming", Pierce's "Types and Programming Languages" are frequent recommendations. Smullyan's "To Mock a Mockingbird" seems like it might be a good example from the lighter end. Are there any other classics I should know about?

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generic programming tutorial?
by Greg Meredith 17 Oct '07

17 Oct '07

Haskellians, i feel like i'm chasing a rabbit down the rabbit hole, but here goes. i'm currently redoing my implementation of an evaluator for a reflective process calculus, using Haskell instead of OCaml, this time. i thought i'd give a James Cheney's FreshLib a whirl to - test out the state of the nominal machinery in Haskell - see if the nominal stuff works, practically, with my reflective account of nominal machinery i've discovered that FreshLib makes essential use of a early version of generic Haskell features. i need to bone up on generic Haskell, quickly. i've got the papers. i need to see what's being released and supported in the mainstream Haskell codebase. Is there documentation for this, or should i grovel source? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell-cafe] Suspected stupid Haskell Question
by manu 17 Oct '07

17 Oct '07

> The problem there is that nub is O(n^2). Is there a place where one can look up the complexity of Standard Libraries functions ? E.D

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Performance problem with random numbers
by ntupel 17 Oct '07

17 Oct '07

Dear all, I have implemented a small module to generate random items with a given probability distribution using the alias approach [1] and unfortunately compared to similar implementations in C++ or Java it is about 10 times slower. I have to confess that I am still in the early stages of writing Haskell programs so there is hopefully something I can do about it and I hope some helpful soul on this list can give me a hint. I have attached my implementation and a small testing routine which runs in about 5 seconds on my machine (when compiled with -O2) whereas my Java-Implementation finishes in about 0.48 seconds. Profiling indicates that the time is mostly spend in System.Random.random and System.Random.randomR so I wonder if these are slow or what else might cause the relative slowness of my implementation. Many thanks for your responses, Thoralf PS: I would also appreciate any feedback about the module from a design perspective. I bet I miss lots of good Haskell idioms. ---------------- [1] The alias methods moves probability mass of a non-uniform probability distribution around to create a uniform distribution. Lets say you have three items "a", "b", and "c" with distribution [0.2, 0.1, 0.7] then a uniform distribution would assign 1/3 to each so "a" and "b" need to be "filled up" with exactly the same probability mass that "c" has too much. Then 2 uniform random numbers are generated. The first one to pick an item and the second one to pick either the item itself if the value is in the original part or the alias otherwise. A much better explanation can be found on the web somewhere. Anyway it should not matter with regards to the performance problem I have.

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OzHaskell?!?
by Manuel M T Chakravarty 16 Oct '07

16 Oct '07

There is AngloHaskell and now AmeroHaskell. Doesn't that call for OzHaskell? http://haskell.org/haskellwiki/OzHaskell Manuel

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job opportunity
by jeff p 16 Oct '07

16 Oct '07

Hello, Here are the essential details... Location: NYC Industry: finance Salary: enough to live comfortably in Manhattan Hard Requirements: Must be able to work in the USA (we can't get you a worker's visa). Must be an expert level Haskell user; must be comfortable with monads, monad transformers, type level programming (i.e. MPTC, overlapping and undecidable instances), and lazy evaluation (i.e. know how to find and eliminate space leaks). Must have a desire to use modern PL research in the real world. Soft Requirements: Experience with low-level Haskell programming (i.e. FFI). Experience with Linux and Cygwin/Windows. Knowledge/experience in dependently typed programming. Knowledge/experience in logic programming. If anyone is interested, please contact me to find out more. thanks, Jeff

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Re: [Haskell] IN Doubt...
by ajb＠spamcop.net 16 Oct '07

16 Oct '07

[Moved to haskell-cafe] G'day all. Quoting Don Stewart <dons(a)galois.com>: > You can't pattern match 'a' and 'a' like that -- there's no implicit > unification. Since we're being nostalgic in other threads, this is #1 on my list of things I miss from Miranda. It's also high on my "things that are damned useful in 1000-line programs but would wreak silent havoc in large applications" list. Cheers, Andrew Bromage

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Help parsing dates and times
by Justin Bailey 16 Oct '07

16 Oct '07

I am trying to parse various date and time formats using the parseTime function found in (GHC 6.6.1) Data.Time.Format. The one that is giving me trouble looks like this: 2008-06-26T11:00:00.000-07:00 Specifically, the time zone offset isn't covered by the format parameters given. I can almost parse it with this: %FT%X.000 But that doesn't take into account the "-07:00" bit. I'm sure this has been solved - can someone point me to the solution? Thanks in advance. Justin

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Equality Question
by PR Stanley 16 Oct '07

16 Oct '07

Hi is const = id? const 'x' 'y' 'x' id 'x' 'x' Cheers, Paul

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System.Posix
by Galchin Vasili 16 Oct '07

16 Oct '07

Hello, In a Hugs environment, I am able to import System.Directory but not to import System.Posix. Here is my environment ... .;{Hugs}\packages\*;C:\ftp\CatTheory\Haskell\SOE\graphics\lib\win32\*. I really want to use the Posix module. Help!!! Kind regards, Bill Halchin

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