- Haskell-Cafe - Haskell.org

"Prime" monads?
by Greg Meredith 12 Sep '07

12 Sep '07

Haskellians, Is there a characterization of "prime" monads? Here the notion of factorization i'm thinking about is decomposition into adjoint situations. For example, are there monads for which there are only the Kleisli and Eilenberg-Moore decompositions into adjoint situations? Would this be a characterization of quintessentially "free" or "generative"? 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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Licenses and Libraries
by H. 12 Sep '07

12 Sep '07

Hello, I have some questions connected more with the licenses and libraries as the language itself: Is it possible to: - publish Haskell source code under the BSD3 license - provide an executable binary together with the code "including" (compiled e.g. with 'ghc --make') "standard" (in the ghc compiler download) libraries of the base and/or other packages - publish Haskell source code under the BSD3 license which will work with GTK2HS - provide an executable binary together with the code compiled with 'ghc -- make' - does there have to be any special flags set to do so - does there have to be included further notes about the libraries and/or there license -- Thanks in advance, H.

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Tiny documentation request
by Andrew Coppin 12 Sep '07

12 Sep '07

I have the following page bookmarked: http://haskell.org/ghc/docs/latest/html/libraries/ I'd like to ask 2 things. 1. Would it be possible to make the *huge* list of package names at the top collapsable? (That way I don't have to scroll through several pages of uninteresting text to get to the bit I actually want.) 2. Could we make is so all items are collapsed initially? (Currently they're all expended initially - which makes it take rather a long time to find anything.) Just thought I'd ask...

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Can somebody give any advice for beginners?
by clisper 12 Sep '07

12 Sep '07

haskell is greate but i don't know how to start. 2007-09-11 clisper

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Re: [Haskell-cafe] haskell and reflection
by Reinier Lamers 12 Sep '07

12 Sep '07

Op 11-sep-2007, om 18:43 heeft Greg Meredith het volgende geschreven: > Thanks for these comments. i wouldn't judge Haskell solely on the > basis of whether it embraced reflection as an organizing > computational principle or as a toolbox for programmers. Clearly, > you can get very far without it. And, it may be that higher-order > functional gives you enough of the 'programs that build programs' > capability that 80% of the practical benefits of reflection are > covered -- without having to take on the extra level of complexity > that reflection adds to typing. i was really just seeking information. Template Haskell [1] is a system that lets you write programs that get executed at *compile time*, and that produce parts of the Haskell program to be compiled by manipulating a representation of the program as structured data. It's a form of reflection restricted to compile time, if you'd ask me. Regards, Reinier [1] http://www.haskell.org/haskellwiki/Template_Haskell

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Comments and/or Criticisms
by PR Stanley 11 Sep '07

11 Sep '07

Hi Any comments and/or criticisms would be most appreciated: --count the occurrences of char in string countC :: Char -> [Char] -> Int countC x xs = sum [1 | c <- xs, c == x] --count occurrences of chars in string countCS :: [Char] -> [(Char, Int)] countCS xs = [(x, (countC x xs)) | x <- [' '..'z'], (countC x xs) > 0] Can anyone come up with a better alternative? Thanks, Paul

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Custom unary operator extension?
by Peter Verswyvelen 11 Sep '07

11 Sep '07

I find it unfortunate that one can't (I guess) define custom unary operators in Haskell. Is this correct? If so, is this just because eg (* 100) declares a function that partially applies the * operator, so this syntax disallows unary operators? Could this be fixed by introducing a different syntax for partial operator application? E.g. (? * 100) instead of just (*100)? This would also fix the ambiguity between (-10) as being either the number minus 10 or the "subtract by 10' function. Cheers, Peter

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turning an imperative loop to Haskell
by Axel Gerstenberger 11 Sep '07

11 Sep '07

Hi all, I am completely stuck with a problem involving a loop construct from imperative programming, that I want to translate to Haskell code. The problem goes as follows: Based on a value u_n, I can calculate a new value u_{n+1} with a function f(u). u_n was calculated from u_{n-1} and so on down to some initial value u_0. So far it looks like a standard recursion to me. The main goal is to write each result u_{n+1} to a file or screen. The problem arises (for me), when u is an array of doubles or a complex data object instead of a simple Double/Integer value, so that I can store only maybe the last or the two last steps in memory. I never need older values. In C, I would write a for loop, calculate the new u and write it to the file. Then I update the old values to the new ones and do the next step in the for loop. Here is what I did in Haskell: I create an infinite list and tried to print the n-th value to the screen/file. But it always calculates all values in the list "all_results", before it starts printing values to screen. On the other side, the function f is called exactly 50 times as the loop suggests. The result is correct, however, it would prohibitive much memory for more complex data and more steps. Can anyone help in explaining me, how I can print to screen and still keep only the last needed values in memory? I can only find imperative solutions, but maybe it is an imperative problem anyway.? Thanks for your time. Best Axel My approach: module Main where import System.IO import Text.Printf main :: IO () main = do let all_results1 = take 20000 $ step [1] --print $ length all_results1 -- BTW: if not commented out, -- all values of all_results -- are already -- calculated here loop [1..50] $ \i -> do let x = all_results1!!i putStrLn $ show i ++ " " ++ show x -- create an infinite list with values u_{n+1} ++ [u_n,u_{n-1},...,u_1] -- where u_{n+1} = f (u_n) step history = case history of [] -> error "no start values" xs -> xs ++ (step [ f (head $ reverse (xs) )]) f u = u + 1 + (sqrt u) -- some arbitrary complex function -- copied from some blog, not sure if this is a good way loop ns stuff = mapM_ stuff ns

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Where would I find fromInt?
by PR Stanley 11 Sep '07

11 Sep '07

Hi Where would I find the fromInt function, please? Better still, would anyone know how to write a func for converting int to float and vice versa? Thanks, Paul

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simple parsing (parsec) intro
by Tim Newsham 11 Sep '07

11 Sep '07

I wrote a small intro about how to write a parser in haskell. Its basically about parsec and how it works, but its written without directly referencing parsec and aimed towards beginners (basically wrote it for some friends). Any feedback is appreciated. http://www.thenewsh.com/%7Enewsham/formal/parse/parser.lhs Tim Newsham http://www.thenewsh.com/~newsham/

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