- Haskell-Cafe - Haskell.org

RE: Yet Another Monad Tutorial
by Bayley, Alistair 12 Aug '03

12 Aug '03

Thanks. Quite comprehensive. Peter Van Roy (the Mozart/Oz guy) said this on the PragProg list, and I didn't have the knowledge to respond. It's a point you might want to address, perhaps in this section?: http://www.nomaware.com/monads/html/laws.html#nowayout I can't decide for myself if he's right or not. It seems to me that monadic IO functions aren't pure, but that doesn't mean that *all* monadic functions are impure, does it? Or could you argue that monadic IO functions are pure: it's just that their input includes the outside world, which means that you can never invoke an IO function more than once with the same arguments? ----------------------------------------------------------------------- > Care to explain why you think Haskell is not pure? Purely functional = you are always defining and calling pure functions. Calling a pure function with the same arguments always gives the same results. In Haskell, if monadic state is used, then this is not true (there is support from the syntax + type system to hide the state argument). So Haskell's monadic state is an implementation of state *on top of* a purely functional language. Saying that Haskell is 'purely functional' is reasonable if you want to present it to non-functional programmers. But the reality is not that simple. It's perhaps better to say that Haskell allows a clear separation of the purely functional core from the rest, which is a Good Thing. It's like saying that Erlang is a functional language. It's true that Erlang has a functional core. But the concurrent message passing built on top is very important and definitely not functional! Peter > -----Original Message----- > From: Jeff Newbern [mailto:jnewbern@yahoo.com] > Sent: 12 August 2003 10:41 > To: haskell-cafe(a)haskell.org > Subject: Yet Another Monad Tutorial > > > 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 ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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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

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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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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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