December 2005 - Haskell-Cafe

RE: Haskell-Cafe Digest, Vol 28, Issue 66
by Scherrer, Chad 20 Dec '05

20 Dec '05

> From: Daniel Carrera <daniel.carrera(a)zmsl.com> > Robert Dockins wrote: > > -divides a b = (mod a b == 0) > > +divides a b = (mod b a == 0) > > Oh, thanks. My program assumed one way to define 'divides' > and the example assumed the other. > > When I wrote it I was thinking of (divides a) being a > function that tells me if the input divides 'a'. > > Thanks! > > Cheers, > Daniel. Daniel, Have you used Haskell's infix notation? It can help keep the order straight for operators like these. You can write a `mod` b -- instead of mod a b a `divides` b -- instead of divides a b. This can help with readability, too. Chad Scherrer

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RE: [Haskell-cafe] First steps in Haskell
by Simon Peyton-Jones 20 Dec '05

20 Dec '05

| Actually, I have sometimes wished that the various interactive Haskell | interfaces had the possibility to enter also declarations interactively GHCi does. ghci> let f x = "hello" ghci> f True True But there's no editor -- it's strictly a one-line definition Simon | -----Original Message----- | From: Bjorn Lisper [mailto:lisper@it.kth.se] | Sent: 19 December 2005 13:06 | To: Simon Peyton-Jones | Cc: wolfgang(a)jeltsch.net; haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] First steps in Haskell | | Simon P-J: | >Daniel is right, by definition. He is a new user. He had difficulty. | >That much is incontrovertible. | > | >While he may seem unusual, perhaps he is only unusual in that he's told | >us about his experience rather than trying Perl instead. For which, | >much thanks, Daniel! | | Actually, I have sometimes wished that the various interactive Haskell | interfaces had the possibility to enter also declarations interactively, as | Daniel originally asked for. Lisp interpreters often support this to some | extent, so it is not out of line for a newcomer to expect it also for | Haskell. I often use hugs as a calculator, and sometimes it would be very | convenient to be able to make one or a few short declarations while making | some interactive calculations. It can be quite tedious to create and edit a | source code file on the side and then load it, when all you want is a short | declaration or two. Would it be that complex to have an interactive | interface enter a "declarations mode" when it sees a declaration coming, and | then let it create, compile and load a temporary module when the declaration | is finished? | | Björn Lisper

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Re: [Haskell-cafe] Prime numbers
by Daniel Carrera 20 Dec '05

20 Dec '05

Henning Thielemann wrote: >>factors :: Integer -> Integer -> Bool >>factors m n | m == n = False >> | m < n = divides m n || factors (m+1) n > > > Btw. I find the recursion harder to understand than the explicit > definition: > > factors n = any (divides n) [2..(n-1)] For what it's worth, I also found this function un-intuitive. What I'd expect a function called "factors" to do is exactly what yours does, and not what the one in the example does. Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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RE: [Haskell-cafe] +RTS -M800M
by Simon Marlow 20 Dec '05

20 Dec '05

On 17 December 2005 21:57, Ketil Malde wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: > >> On 16 December 2005 10:05, Joel Reymont wrote: >> >>> I'm trying to restrict GHC to 800Mb of heap at runtime by passing in >>> +RTS -M800M, the machine has 1Gb of memory and top shows free >>> physical memory dropping below 175Mb. > >> -M800m should do more or less the right thing, but it is possible for >> GHC to exceed this figure by a small percentage. > > Is there any chance there is a bug in recent GHCs (or perhaps Linux?) > affecting this? I'm fairly sure I've lately seen Haskell programs > consuming significantly larger amounts of memory than they should be > allowed to, and I can try to isolate a test case if it is of > interest. Please do isolate it if you can. I'd be interested in investigating. Cheers, Simon

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Re: [Haskell-cafe] Prime numbers
by Daniel Carrera 20 Dec '05

20 Dec '05

John Peterson wrote: > Add a type signature: > > prime :: Integer -> Bool > > It's defaulting to Int and you're getting overflows Thanks. Hmm... it's still not working. Btw, I mis-reported the problem. The offending number is 38466629, which is /not/ prime but the sample program reports as prime. 38466629 = 31 * 1240859 The offending program is: --//-- prime :: Integer -> Bool prime n = not (factors 2 n) factors :: Integer -> Integer -> Bool factors m n | m == n = False | m < n = divides m n || factors (m+1) n divides :: Integer -> Integer -> Bool divides a b = (mod a b == 0) --//-- The math behind the program seems correct... :( Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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Prime numbers
by Daniel Carrera 20 Dec '05

20 Dec '05

Hello all, Trying to learn Haskell here... In a Haskell tutorial I found a function deciding if a number is prime: --//-- prime n = not (factors 2 n) factors m n | m == n = False | m < n = divides m n || factors (m+1) n divides a b = (mod a b == 0) --//-- Reference: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/Function%20Definiti… This function seems to produce a wrong result on the number 38466629 (warning, slow computation). This number is prime but the function says that it's not. How do I know that 38466629 is prime? Beause: * http://www.rsok.com/~jrm/printprimes.html says that it is. * I wrote another prime function that says that it is. This is my function: --//-- prime 1 = False prime 2 = True prime n = filter (divides n) primes == [] where numbers = [x | x <- [2..n-1], x*x <= n] primes = filter prime numbers divides a b = (mod a b == 0) --//-- I can't figure out what's wrong with the first one. Can anyone spot the problem? (I'm just curious - trying to learn Haskell). Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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RE: [Haskell-cafe] Tutorial uploaded
by Bayley, Alistair 20 Dec '05

20 Dec '05

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Daniel Carrera > > Hi all, > > I've finished a first draft of what I call "First steps in Haskell". > It's intended to be the very first thing a new user sees when they > decide to try out Haskell. > > http://www.haskell.org/hawiki/FirstSteps?action=show > > Thoughts and comments? There's not much wrong with doing a Hello World: main = putStrLn "Hello World" Or: fac n = if n <= 0 then 1 else n * fac (n-1) main = print (fac 12) Compiling, or at least running a haskell script (via runhaskell) should also get a mention, so you show how to achieve things outside of the interactive prompt. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] Proposal for a first tutorial.
by Bayley, Alistair 20 Dec '05

20 Dec '05

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Daniel Carrera > > Hal Daume III wrote: > > Daniel -- can you tell me what was missing from YAHT that wasn't > > sufficient for starting to use Haskell? It was really > intended to solve > > these problems, at least partially, so if it's missing out, > I'd like to > > fix it! > > I haven't read it. I refuse to give out personal information > to read a > tutorial. >From this page http://haskell.org/hawiki/LearningHaskell there's a link to the tutorial http://www.isi.edu/~hdaume/htut/tutorial.pdf which seems to make no informational demands. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: First steps in Haskell
by Bayley, Alistair 20 Dec '05

20 Dec '05

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Paul Moore > > I suspect that the reference documentation is fine, and the tutorials > are great, given what they are trying to do. But there is scope for a > very prominent "Getting going" document. Daniel's "40-second intro to > Haskell" looks good. Expand it a little with a few lines showing tow > to build and compile a "Hello, world" program in GHC and Hugs (with a > disclaimer that the code itself is magic for now, you'll understand > when you read about monads and the IO monad) to help people who need > to get over the "but how do I write a *program*" hump, and you're > pretty much there. For someone getting started, in IMO the Learning Haskell page has too much stuff on it. The total beginner doesn't have the knowledge they need to decide which implementation to install, which tutorial to read, or which textbook to buy. At the risk of alienating some tutorials and implementations, we ought to pick one of each and say "here, start with these". There's a balance to be struck between being fair to all resource producers (be it implementation, textbook, tutorial, or reference), and being as easy as possible for someone to get started, and I think the needle is currently too far over to the "fair" side of the scale. There should be a "getting started" page which says: - download and install this interpreter (Hugs or GHCi) - run it, type these expressions, and see the results - create a HelloWorld program, compile and execute. - now start the following tutorial... (BTW, the link to Hal Daume's tutorial from the Learning page needs fixing) As an example (checking out the competition), the Ruby community have a good free book. The book is fairly easy to find from the ruby-lang.org homepage, although it could be more prominent. The preface contains a brief tutorial for working with the interpreter (how to build & install, the REPL, and a Hello World program): http://www.rubycentral.com/book/preface.html And there's this interesting site, which demos a interpreter running in your browser (although it really seems to be showcasing some AJAX-ish javascript libraries): http://tryruby.hobix.com/ The Ruby community have put quite a bit of effort into easing beginners into the langauge and tools. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] First steps in Haskell
by Simon Peyton-Jones 20 Dec '05

20 Dec '05

| (mild) culture shock here. It is typical for people in the Haskell | community to view things in a rather principled way. A language | tutorial is supposed to introduce /the language/. If you want to know | how to compile or execute a Haskell program, well then, look at the | appropriate tutorial on the /implementation/. At first this may appear | like deliberately creating hurdles, but it isn't, it's merely the way | many (though not all) Haskell people tend to think. They take it for | granted that a new user is at least educated enough to be aware of the | difference between the language itself, and its concrete implementation | in the form of an interpretation or a compilation system. I, for one, don't take it for granted! Furthermore, I think the Haskell community is pretty friendly; for example, a great deal of entirely non-condescending advice is given to newcomers on Haskell Café. I'm certain there are hurdles, but I think on the whole they are there by accident rather than design. (Apart from functional programming itself, which often is a bit of a hurdle, but that's the reason we are here!) It turns out that there are a couple of introductory Wiki pages already: http://www.haskell.org/hawiki/HaskellNewbie http://www.haskell.org/hawiki/HaskellDemo It'd be great if someone would like to improve them in the light of this thread. (Anyone can do this.) Ideally there should be a single Wiki page that the Haskell.org web site can point to; perhaps the editing could bear that in mind? Simon

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