- Haskell-Cafe - Haskell.org

Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

Hello everyone, Due to the scarcity of monad tutorials available (:^), I have written one of my own. I hope that this one is both more gentle and more comprehensive than many of the other tutorials out on the 'net, and I am looking for feedback from experienced Haskellers and novices alike. You can find the tutorial at http://www.nomaware.com/monads/html/ The tutorial begins from first principles and introduces the monad concept, details the implementation and usage of all of the standard monads in Haskell and the monad template library and explains the use of monad transformers. The tutorial contains 23 individual sections and 18 files of example code illustrating different monad concepts. I hope that this tutorial is useful to many people, and I would love to hear your experiences and suggestions for improving it. Thanks, Jeff Newbern jnewbern(a)yahoo.com __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com

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12 Aug '03

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Some humor
by Josef Svenningsson 11 Aug '03

11 Aug '03

Hi! All you people nourishing a dream of being part of the Haskell committee. Forget it. You might be taken for a serial killer. Check out: http://www.malevole.com/mv/misc/killerquiz/ :) /Josef

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Editor in Linux for Hint/Helium
by Tim Stitt 11 Aug '03

11 Aug '03

Hi all, I'm using the Helium interpreter/compiler with the Hint interface within Linux. Unfortunately, I want an editor that allows me to jump to the row/column of the source when I receive an error. Does anyone know of a good editor in Linux to do this or the commands for Emacs etc. that can be used from within Hint to achieve it? Context works fine in Windows but I want something similar in Linux. There isn't much help from the Helium website. Thanks in advance, Tim Stitt

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[newbie] UTF-8
by danon＠epita.fr 11 Aug '03

11 Aug '03

Hi, We try to make a program which write on stdout the UTF-8 character corresponding to an input unicode value. And this under MacOS X, Debian linux, and Windows. It seems to be easy, but we can't have found anything valuable, on the net. Would you mind to help us? Niko. Ps: excuse my poor english.

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[Newbie] Streams and IO
by Christian Hofer 10 Aug '03

10 Aug '03

Hello, I am completely new to functional programming, am just reading through SOE (great book so far!) and try to understand streams in client-server-interactions. Therefore I wrote the following program (which has nothing to do with the book exercises), which is intended to read integers from stdin and to write the sum of the inputs so far to stdout. When I start it (in Hugs), I can enter numbers, but don't get back the sum, but every second time a zero, every forth time the value I just entered and every forth time the sum of the two values I entered the latest. Could s.o. explain me, what is going wrong? I can't figure out, what is happening here. (I know that there are easier solutions to the problem than using streams, but the latter was my intention for learning.) Thanks, Christian module Main where main = sequence reqs type Request = IO Int type Response = IO Int reqs :: [Request] reqs = client resps resps :: [Response] resps = server reqs client :: [Response] -> [Request] client (y:ys) = (do toPrint <- y putStr ("So far: " ++ show toPrint ++ "\nEnter number: ") line <- getLine return (parseToInt line)) : client ys server :: [Request] -> [Response] server = scanl helper (return 0) where helper :: IO Int -> IO Int -> IO Int helper sofar x = do a <- x sf <- sofar return (a + sf) parseToInt :: String -> Int parseToInt str = foldl nextDigit 0 (intList str) where intList :: String -> [Int] -- only for positive numbers so far intList = map (\x -> fromEnum x - 48) nextDigit :: Int -> Int -> Int nextDigit a b = a * 10 + b

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by a_seidenschnur2＠yahoo.de 10 Aug '03

10 Aug '03

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Outlook
by birene＠erols.com 09 Aug '03

09 Aug '03

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idiom for producing comma-seperated lists?
by Antony Courtney 08 Aug '03

08 Aug '03

I often need to format a list of strings using some character as a *seperator* rather than a terminator for the items. Is there some simple combinator or idiom from the Prelude or standard libraries that could be used for this purpose? I ended up defining my own variation on foldr1 to solve this (code below), but I'd be much happier using some more standard solution: -- A variation on foldr1 that takes an extra argument to be returned if -- the list is empty. foldrs :: (a -> a -> a) -> a -> [a] -> a foldrs f z [] = z foldrs f _ xs = foldr1 f xs -- Example: format a list of strings, using a comma as a seperator: mkSepStr :: [String] -> String mkSepStr xs = foldrs (\x s -> x ++ ", " ++ s) "" xs t0 = mkSepStr [] -- ==> "" t1 = mkSepStr ["hello"] -- ==> "hello" t2 = mkSepStr ["10","20","30"] -- ==> "10, 20, 30" What do the rest of you do to solve this particular problem? If there isn't a standard solution, perhaps something like foldrs would be a useful addition to the Prelude. Thanks, -antony -- Antony Courtney Grad. Student, Dept. of Computer Science, Yale University antony(a)apocalypse.org http://www.apocalypse.org/pub/u/antony

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Re: Another typing question
by oleg＠pobox.com 08 Aug '03

08 Aug '03

> Is there any way to parametrize a type by a value, rather than another > type? I believe the following web page answers your question: http://pobox.com/~oleg/ftp/Haskell/number-parameterized-types.html It uses parameterization by decimal numbers -- which seem more natural to read. > It would be interesting to know if/how you could, e.g., define a > concatenation function with a type like Yes, you can -- and the page referenced above does that.

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