- Haskell-Cafe - Haskell.org

Problem with Infinite Lists
by Wamberto Vasconcelos 03 Sep '03

03 Sep '03

Folks I am new in this forum, so apologies if this has been asked before. If this is the case, please point me to the direction of the answer and I won't bother you again! Also, could you make sure you CC your answers to me? I am not sure how long it takes for my subscription to be activated and I don't want to miss out on the action :-) The Problem I have defined this function to calculate the Fibonacci numbers: all_fib :: [Float] all_fib = 1:(1:(add_fib all_fib)) where add_fib (x:y:rs) = (x + y):(add_fib (y:rs)) Which seems to work: Main> take 20 all_fib [1.0,1.0,2.0,3.0,5.0,8.0,13.0,21.0,34.0,55.0,89.0,144.0,233.0,377.0, 610.0,987.0,1597.0,2584.0,4181.0,6765.0] However, when I tried Main> filter even (take 20 all_fib) ERROR - Illegal Haskell 98 class constraint in inferred type *** Expression : filter even (take 20 all_fib) *** Type : Integral Float => [Float] What is going on here? Thanks in advance for any help/hint. -- Wamberto Vasconcelos, PhD wvasconcelos(a)acm.org Department of Computing Science http://www.csd.abdn.ac.uk/~wvasconc University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK Phone +44 (0)1224 272283 Fax +44 (0)1224 273422

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Histograms
by Konrad Hinsen 03 Sep '03

03 Sep '03

How would one best (read: most efficiently) create a histogram in Haskell? More precisely, given a list of integers in a known range, I want a count of how often any given number occurs. In imperative languages this would be done by iterating over the list and updating the count for the element under consideration. Without updatable data structures, the most straightforward solution I can think of would start by sorting the list, but for a long list that is very inefficient. Any suggestions? Konrad.

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OT - Book on Programming
by Matthew Donadio 03 Sep '03

03 Sep '03

Hi all, This is a little off-topic, but I know there are a lot of people here with teaching experience. I am looking for a good book on programming. I recently picked up a gig doing ActionScript programming for a multimedia company. They have very good designers, but not very good programmers. So, they decided to hire a professional programmer to help them out with some of their projects. They have several people in the company who understand the mechanics of programming, but don't really understand the concepts. I am looking for a book to recommend to them. I would really like to find a book that just discusses programming, and avoids any one particular language. It would have to cover the common imperative controls, as well as basic data structures. Thanks. -- Matthew Donadio (m.p.donadio(a)ieee.org)

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RE: Help on Homework help
by Simon Peyton-Jones 03 Sep '03

03 Sep '03

Good write-up. One suggestion: it'd be good to suggest *strongly* that people only send their homework-style questions to Haskell-café, not to the main Haskell list. Given the existence of the fomer, using the latter is likely to irritate, which isn't likely to elicit helpful responses. I know I could make the change myself, but you'll do it better :-) Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Andrew | J Bromage | Sent: 03 September 2003 06:52 | To: haskell-cafe(a)haskell.org | Subject: Help on Homework help | | G'day all. | | In response to the recent poll, and because I promised to, here is a | first attempt at a page to direct people to, should they ask for | homework help in the wrong way: | | http://haskell.org/hawiki/HomeworkHelp | | Comments, criticisms and contributions are most welcome. | | Cheers, | Andrew Bromage | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Help on Homework help
by Andrew J Bromage 03 Sep '03

03 Sep '03

G'day all. In response to the recent poll, and because I promised to, here is a first attempt at a page to direct people to, should they ask for homework help in the wrong way: http://haskell.org/hawiki/HomeworkHelp Comments, criticisms and contributions are most welcome. Cheers, Andrew Bromage

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Poll: How to respond to homework questions
by Tom Pledger 03 Sep '03

03 Sep '03

Hi. I'm curious about what the people on this list consider appropriate, as responses to homework questions. Even if there isn't a consensus, it may be interesting to see how opinion is divided. Please consider the following. (A) Give a perfect answer. (B) Give a subtly flawed answer. (C) Give an obfuscated answer. (D) Give a critique of what the questioner has tried so far. (E) Give relevant general advice without answering the specific question. I mean them to include giving references, e.g. a Wiki URL to relevant general advice would count as (E). Which of those do you approve of? Which do you disapprove of? Please email me a concise vote, e.g. OK: B D E Not OK: A and I'll post a collated result next week. - Tom

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IO StateTransformer with an escape clause
by Thomas L. Bevan 01 Sep '03

01 Sep '03

Hi, I'd like some help building an IO StateTransformer which can be escaped midway through without losing the state accummulated up to that point. I tried constructing a StateT s MaybeIO a monad but the state is lost when the rest of the monad collapses. Tom

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Haskell 6.0 install
by Mark Espinoza 01 Sep '03

01 Sep '03

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Re: Haskell for non-Haskell's sake
by Nick Name 30 Aug '03

30 Aug '03

Alle 13:33, sabato 30 agosto 2003, Alastair Reid ha scritto: > - Knit (http://www.cs.utah.edu/flux/knit/) > > A component language for C Are there component languages wich work on multilanguage projects? Something like "my project has a module written in haskell, one in C and one in Ocaml, and I don't want an haskell 'list a' to pass through C nor corba to get into Ocaml". Is there any hope in this sense? V.

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Type classes question
by Eugene Nonko 29 Aug '03

29 Aug '03

Hello, I am trying to define class Dual and few instances as follows: === class Dual a where dual :: a -> a instance Dual Bool where dual = not instance (Dual a, Dual b) => Dual (a -> b) where dual f = dual . f . dual instance Dual a => Dual [a] where dual = reverse . map dual instance Num a => Dual a where dual = negate === For some reason Hugs does not lake the last definition saying 'ERROR dual.hs:13 - syntax error in instance head (constructor expected)'. What am I doing wrong? -- Eugene

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