September 2006 - Haskell-Cafe

Re: [Haskell-cafe] does the compiler optimize repeated calls?
by John Hughes 06 Sep '06

06 Sep '06

On 9/6/06, Tamas K Papp <tpapp(a)princeton.edu> wrote: >> or does the compiler perform this optimization? More generally, if a >> function is invoked with the same parameters again (and it doesn't >> involve anything like monads), does does it makes sense >> (performancewise) to "store" the result somewhere? > > I was wondering something like this too, and then I found this email: http://www.haskell.org/pipermail/glasgow-haskell-bugs/2004-December/004530.… So I guess it is as Stephane said: theoretically possible but not actually done? --eric the perpetual newbie The trouble is that this isn't always an optimisation. Try these two programs: powerset [] = [[]] powerset (x:xs) = powerset xs++map (x:) (powerset xs) and powerset [] = [[]] powerset (x:xs) = pxs++map (x:) pxs where pxs = powerset xs Try computing length (powerset [1..n]) with each definition. For small n, the second is faster. As n gets larger, the second gets slower and slower, but the first keeps chugging along. The problem is that the second has exponentially higher peak memory requirements than the first. Round about n=25, on my machine, all other programs stop responding while the second one runs. You don't really want a compiler to make that kind of "pessimisation" to your program, which is why it's a deliberate decision to leave most CSE to the programmer. You can still write the second version, and suffer the consequences, but at least you know it's your own fault! John

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getContents and lazy evaluation
by Tamas K Papp 06 Sep '06

06 Sep '06

Hi, I am newbie, reading the Gentle Introduction. Chapter 7 (Input/Output) says Pragmatically, it may seem that getContents must immediately read an entire file or channel, resulting in poor space and time performance under certain conditions. However, this is not the case. The key point is that getContents returns a "lazy" (i.e. non-strict) list of characters (recall that strings are just lists of characters in Haskell), whose elements are read "by demand" just like any other list. An implementation can be expected to implement this demand-driven behavior by reading one character at a time from the file as they are required by the computation. So what happens if I do contents <- getContents handle putStr (take 5 contents) -- assume that the implementation -- only reads a few chars -- delete the file in some way putStr (take 500 contents) -- but the file is not there now If an IO function is lazy, doesn't that break sequentiality? Sorry if the question is stupid. Thanks, Tamas

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Re: how do you debug programs?
by Johan Tibell 06 Sep '06

06 Sep '06

Note: I meant to send this to the whole list a couple of messages ago but accidentally I only sent it to Lennart, sorry Lennart! I know that Linus Torvalds doesn't find debuggers all that useful either and he hacks C [1]. 1. http://linuxmafia.com/faq/Kernel/linus-im-a-bastard-speech.html On 9/6/06, Lennart Augustsson <lennart(a)augustsson.net> wrote: > I've also used Visual Studio, and I wouldn't mind having something > like that for Haskell. But I have to agree with Jon, I think the > best way of debugging is to understand your code. I think people who > come from imperative programming come with a mind set that you > understand your code by stepping through it in the debugger. But I > find this paradigm much less useful for functional code. > > -- Lennart > > On Sep 6, 2006, at 06:22 , Neil Mitchell wrote: > > > Hi > > > >> But why should c and d exist at runtime? They're only used > >> once each, so the compiler is free to replace f with > >> > >> \a b -> (a+b)+ a*b > > > > Yes, and if the compiler is this clever, it should also be free to > > replace them back at debug time. > > > > > >> I've said this before, but I think it's worth repeating: in > >> most cases, if you need to debug your programme it's because > >> it's too complicated for you to understand, so the correct > >> thing to do is to simplify it and try again. > > > > That makes it impossible to ever improve - initially all programs are > > beyond you, and only by pushing the boundary do you get anywhere. As > > for me, I always write programs I can't understand, and that no one > > else can. Perhaps I understood it when I originally wrote it, perhaps > > not, but that was maybe over a year ago. > > > > It's been my experience that debugging is a serious weakness of > > Haskell - where even the poor mans printf debugging changes the > > semantics! And everyone comes up with arguments why there is no need > > to debug a functional language - that sounds more like excuses about > > why we can't build a good lazy debugger :) > > > > [Sorry for the slight rant, but I've used Visual Studio C++ so I know > > what a good debugger looks like, and how indispensable they are] > > > > Thanks > > > > Neil > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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does the compiler optimize repeated calls?
by Tamas K Papp 06 Sep '06

06 Sep '06

Hi, I have a question about coding and compilers. Suppose that a function is invoked with the same parameters inside another function declaration, eg -- this example does nothing particularly meaningless g a b c = let something1 = f a b something2 = externalsomething (f a b) 42 something3 = externalsomething2 (137 * (f a b)) in ... Does it help (performancewise) to have g a b c = let resultoff = f a b something2 = externalsomething resultoff 42 something3 = externalsomething2 (137 * resultoff) in ... or does the compiler perform this optimization? More generally, if a function is invoked with the same parameters again (and it doesn't involve anything like monads), does does it makes sense (performancewise) to "store" the result somewhere? Thank you, Tamas PS: I realize that I am asking a lot of newbie questions, and I would like to thank everybody for their patience and elucidating replies.

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installing modules
by Tamas K Papp 06 Sep '06

06 Sep '06

Hi, This is a newbie question. I found some modules on the internet that I would like to try (eg the Tim Docker's chart library [1], etc). Most of the code I see is in darcs repositories, I figured out how to get them, but I don't know how to install it _cleanly_. Cleanly means that it ends up in the "right" directory. How do people usually do that? I am using ghc6 on debian testing. Am I supposed to use the thing called "cabal"? Is it perhaps possible to create a debian package from sources that were packaged with this tool? Thank you, Tamas [1] http://dockerz.net/software/chart.html

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[Parsec] No identEnd in ParsecToken?
by Stephane Bortzmeyer 06 Sep '06

06 Sep '06

I'm trying to use Parsec for a language which have identifiers where the '-' character is allowed only inside identifiers, not at the start or the end. ParsecToken has identStart to tell that the '-' is not allowed at the start but I find no equivalent identEnd? I tried also to express the same rule with ordinary combinators, without ParsecToken but this fails: identifier = do start <- letter rest <- many (alphaNum <|> char '-') end <- letter return ([start] ++ rest ++ [end]) <?> "characters authorized for identifiers" because the parser created by "many" is greedy: it consumes everything, including the final letter. Any idea?

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Exercise in point free-style
by Julien Oster 05 Sep '06

05 Sep '06

Hello, I was just doing Exercise 7.1 of Hal Daumé's very good "Yet Another Haskell Tutorial". It consists of 5 short functions which are to be converted into point-free style (if possible). It's insightful and after some thinking I've been able to come up with solutions that make me understand things better. But I'm having problems with one of the functions: func3 f l = l ++ map f l Looks pretty clear and simple. However, I can't come up with a solution. Is it even possible to remove one of the variables, f or l? If so, how? Thanks, Julien

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User data type with operator contructors only
by Dušan Kolář 05 Sep '06

05 Sep '06

Hello all, my question probably comes from not reading manual properly. But, why is it not possible to have something like: infixr 5 :> data Stack a = a :> (Stack a) | :|| And if yes, how can I do that? I know that lists are a hack in Haskell, but anyway. Having: infixr 5 :> data Stack a = a :> (Stack a) | Bottom is not what I would like to have. :-( Thanks for any reference to/or explanation, Dusan

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practice problems?
by Tamas K Papp 05 Sep '06

05 Sep '06

Hi, I am a Haskell newbie. Having read some tutorials (Yet Another, Gentle Introduction) and some papers/tutorials on monads, I would like to spend some time practicing what I have learned before embarking on more abstract/obscure things and/or using Haskell for everyday tasks. I am looking for small to medium sized practice problems, preferably with solutions. Hal Daume's tutorial had some good one-liners (eg rewrite something point-free) but I am looking for something which would take 1-3 hours for a newbie, and then I could look at a solution by a Haskell expert and discover what I could have done better/more elegantly. Online references (or even book suggestions) would be appreciated. If you are teaching Haskell and have problems with confidential solution sets (some instructors like it that way), I can promise not to disclose the latter ;-) Thanks, Tamas

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BSPHlib-0.1 Release
by Frederico Franzosi 04 Sep '06

04 Sep '06

Is with happiness that I announce my first project release. I announced it as an idea to the summer of code and as Paolo Martini said to me that it was a "good idea" I decided to publish it. The link to the summer of code proposal follows: http://hackage.haskell.org/trac/summer-of-code/ticket/80 That link briefly explains what BSPHlib is. To download the source: http://www.inf.ufes.br/~ffranzosi/BSPHlib-0.1.tar.gz I know that many mistakes are likely to appear and I am counting on the community to help me as I am trying to help the community with my still poor coding hability. I have tested that code and it has done some good job, hope I can hear the same from you. In a few weeks the code will be in a darcs repository in order to make code management more organized. Any questions, corrections, patches or suggestions please mail me. Cheers, Frederico Franzosi

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