January 2005 - Haskell-Cafe

RE: [Haskell-cafe] I/O interface
by Simon Marlow 19 Jan '05

19 Jan '05

On 18 January 2005 00:27, Ben Rudiak-Gould wrote: > Marcin 'Qrczak' Kowalczyk wrote: > > >Convenience. I'm worried that it uses separate types for various > >kinds of streams: files, pipes, arrays (private memory), and > sockets. >Haskell is statically typed and lacks subsumption. This > means that >even though streams are unified by using a class, code > which uses >a stream of an unknown kind must be either polymorphic > or use >existential quantification. > > Yes, this is a problem. In my original proposal InputStream and > OutputStream were types, but I enthusiastically embraced Simon M's > idea of turning them into classes. As you say, it's not without its > disadvantages. I recognised this problem, which is why TextInputStream and TextOutputStream are existential wrappers around streams. Most clients will be using the Text streams, so they won't suffer from the polymorphism problem. However, we could also provide a non-overloaded version using one of Ben's solutions: > I see several possibilities here. > > * We could adopt Avery Lee's suggestion (from the discussion in > 2003) to use field labels instead of methods. Advantages: InputStream > and OutputStream behave more like their OOP equivalents, with no loss > of extensibility. Disadvantages: potentially less efficient (no > specialization possible); loses some static type information. Unfortunately GHC isn't nearly as good at optimising records as it is at optimising type class overloading. Of course, type class overloading is just a special case of polymorphic records, but there you go. > * We could use a single type for all input and output streams in > the standard library, but retain the type classes also. > > * We could provide existential wrappers: > > data IStream = (InputStream a) => MkIStream !a > instance InputStream IStream where ... This one gets my vote. BTW, Marcin: the library in the prototype implementation differs somewhat from the version of the interface in the Haddock docs. The docs were written before I started hacking on the prototype. Cheers, Simon

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Re: [Haskell] Re: Why is getArgs in the IO monad?
by Marcin 'Qrczak' Kowalczyk 19 Jan '05

19 Jan '05

Keean Schupke <k.schupke(a)imperial.ac.uk> writes: > Surely both requirements can be satisfied if the programs arguments are made > parameters of main: > > main :: [String] -> IO () >From info '(libc)Error Messages', about program_invocation_name and program_invocation_short_name: *Portability Note:* These two variables are GNU extensions. If you want your program to work with non-GNU libraries, you must save the value of `argv[0]' in `main', and then strip off the directory names yourself. We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from `main'. This suggests that making global parameters available only as arguments of main would be a bad idea. But they should be settable, to account for the rare case of wanting to substitute something else to a library which reads them itself. -- __("< Marcin Kowalczyk \__/ qrczak(a)knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/

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Re: performance question (John Meacham)
by Stijn De Saeger 19 Jan '05

19 Jan '05

thanks Ben and John, your ideas looked like they may speed up things indeed, so i went along with them and reimplemented the whole thing.... to my surprise, the program got considerably slower even ! here is the new code. > data ILtype = II | IE | EI | EE | NII | NIE | NEI | NEE deriving (Eq, Ord) (... I found no increase in performance by using the UNPACK pragma as you suggested, so I just ommitted it in this version...) > data Interval = Range ILtype !Double !Double deriving (Eq, Ord) > instance Show Interval where > show (Range iltype x y) = case iltype of > II -> if x == y then (show x) > else "[" ++ (show x) ++ ".." ++ (show y) ++ "]" > IE -> "[" ++ (show x) ++ ".." ++ (show y) ++ ")" > EI -> "(" ++ (show x) ++ ".." ++ (show y) ++ "]" > EE -> "(" ++ (show x) ++ ".." ++ (show y) ++ ")" > NII -> "Not " ++ show (Range II x y) > NIE -> "Not " ++ show (Range IE x y) > NEI -> "Not " ++ show (Range EI x y) > NEE -> "Not " ++ show (Range EE x y) the flip of interval types: > ilFlip :: ILtype -> ILtype > ilFlip ilt = case ilt of > II -> NII > IE -> NIE > EI -> NEI > EE -> NEE > NII -> II > NIE -> IE > NEI -> EI > NEE -> EE The complement of Intervals : > intComplement :: Interval -> Interval > intComplement (Range iltype x y) = (Range (ilFlip iltype) x y) membership function for Intervals: > isIn :: Double -> Interval -> Bool > isIn r (Range iltype x y) = case iltype of > II -> r >= x && r <= y > IE -> r >= x && r < y > EI -> r > x && r <= y > EE -> r > x && r < y > NII -> r > y || r < x > NIE -> r > y || r <= x > NEI -> r >= y || r < x > NEE -> r >= y || r <= x As always, any further comments or insights greatly appreciated. stijn.

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styles
by Matthew Roberts 18 Jan '05

18 Jan '05

Haskell seems to be a language that allows for lots of different programming styles. The most obvious being pointed v.s. point free. As a programmer born and raised on OO - I have found the pointed style to suit me better. However, there are other distinctions. I heavily use "let in" and "where" to break up each function into smaller parts while I see other programmers putting everything in one line. I have seen the evolution of a Haskell programmer joke and most of it goes over my head. My question is "Is there standard terminology for the different programming styles in Haskell (besides pointed and point-free)?" Matt

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Re: Books on Haskell
by David Owen 18 Jan '05

18 Jan '05

Hi, I have 3 Haskell books, The Craft of Functional Programming (Thompson), Introduction to Functional Programming Using Haskell (Bird) and The Haskell School of Expression (Hudak). I recommend Thompson's book because it contains good explanations and lots of exercises, although the book is quite big and takes some time to work through. Bird's book is a bit shorter and is good for explanations too but it isn't quite so hands on as it contains fewer exercises. Hudak's I feel is a bit too advanced for a first read, it focusses on using the Hugs graphics library and I think that the graphics examples can sometimes cloud the simple ideas that are trying to be explained. I'll probably try reading it again in the future. All the best Dave O

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About instance
by Ulises Juarez Martinez 18 Jan '05

18 Jan '05

Hi. I have the next definitions: type Ocurrence = (String, Int) type Inside = [Ocurrence] data Pattern = Return Inside | Abort | Filter (Ocurrence -> Bool) Pattern beyond :: Pattern -> Pattern beyond (Filter pred Abort) = Abort -- more definitions of beyond optimous :: Pattern -> Pattern optimous p = fixPoint beyond p where fixPoint f x = if(f x == x) then x else fixPoint f (f x) When I load the program an error is produced: ERROR - An instance of Eq (Event -> Bool) is required to derive Eq Pattern I put: data Pattern = Return Inside | Abort | Filter (Ocurrence -> Bool) Pattern deriving (Eq) and a new error was produced: ERROR - An instance of Eq (Ocurrence -> Bool) is required to derive Eq Pattern Who can I do an instance of Eq (Ocurrence -> Bool)? Is there another option to avoid the error? Regards Ulises _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

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Re: [Haskell-cafe] Unit testing in Haskell
by Graham Klyne 18 Jan '05

18 Jan '05

I use hunit, and find it works very well. #g -- At 20:05 11/01/05 +0100, Dmitri Pissarenko wrote: >Hello! > >When programming in an imperative language like Java, unit tests are a very >important development tool IMHO. > >I want to try out unit testing in Haskell and wonder what experienced >Haskellers >think about unit testing in Haskell in general and the hUnit testing framework >(see URL below) in particular? > >http://hunit.sourceforge.net/ > >What other unit testing frameworks for Haskell do you use? > >TIA > >dap >-- >Dmitri Pissarenko >Software Engineer >http://dapissarenko.com >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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performance question
by Stijn De Saeger 18 Jan '05

18 Jan '05

Hello all, A question on that most elusive of subjects.... performance in haskell (GHC 6.2) Using the GHC profiler, I obtained the following analysis results (i hope the code doesn't come out too ugly by mail): total time = 0.92 secs (46 ticks @ 20 ms) total alloc = 83,373,032 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc isIn Main 50.0 22.8 getCon Main 13.0 16.7 o' Main 8.7 6.6 satisfies Main 6.5 0.0 powerList Main 6.5 46.9 CAF Main 6.5 0.1 showCon Main 4.3 0.3 MAIN MAIN 4.3 0.0 a' Main 0.0 6.7 The problem child, that isIn function, has got about 78000 entries in the profile log. I should probably mention that this is an incredibly dumbed down version of the program, the dimensions of the data it is supposed to handle are such that, on a trial run I killed the process after about 15 minutes, when I found out it hadn't even completed 3% of its task. sad stuff, really. Anyways, 'isIn' is a predicate that checks whether a given Double lies within an interval, where intervals are defined as ... define an interval bound, either inclusive (I) or exclusive (E) > data Bound = I Double | E Double deriving (Eq, Show, Ord) > data Interval = Il Bound Bound | Nil Bound Bound deriving (Eq,Ord) where Nil acts as the complement of an interval, this is reflected in the isIn function. .... > isIn :: Double -> Interval -> Bool > isIn r (Nil x y) = not (isIn r (Il x y)) > isIn r (Il (I x) (I y)) = r >= x && r <= y > isIn r (Il (I x) (E y)) = r >= x && r < y > isIn r (Il (E x) (I y)) = r > x && r <= y > isIn r (Il (E x) (E y)) = r > x && r < y I tried rewriting it to something that intuitively 'feels' like it should be faster, but i have no real idea about the cost of the respective haskell expressions: ... version 2 > isIn :: Double -> Interval -> Bool > isIn r (Nil x y) = not (isIn r (Il x y)) > isIn r (Il x y) = case x of > (I x') -> if r >= x' then case y of > (I y') -> r <= y' > (E y') -> r < y' > else False > (E x') -> if r > x' then case y of > (I y') -> r <= y' > (E y') -> r < y' > else False ... which indeed turns out to be a tad bit faster, according to the new profile log. total time = 0.80 secs (40 ticks @ 20 ms) total alloc = 64,404,104 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc isIn Main 30.0 0.0 getCon Main 25.0 21.6 powerList Main 15.0 60.7 showCon Main 7.5 0.3 o' Main 7.5 8.6 CAF Main 7.5 0.1 MAIN MAIN 5.0 0.0 a' Main 2.5 8.6 But it can hardly be called impressive. Can anyone see another obvious optimization, or have I just hit the ceiling because of the sheer number of function calls to isIn? I am still pretty new to haskell, and I find it hard to wrap my head around the way the compiler deals with my code. If someone has a few leads on general performance heuristics in haskell/GHC, I would be happy to read them too... thanks for your time. stijn

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Books on Haskell
by Dmitri Pissarenko 18 Jan '05

18 Jan '05

Hello! I've completed reading of "Yet another Haskell tutorial" and now want to learn Haskell more thoroughly. I'm searching for a book, in which the features of Haskell are explained in the form of examples and exercises (like in the book "Clause and Effect" on PROLOG). My purpose in exploring Haskell is to determine whether I can program much more productively by using Haskell instead of Java/C#. In order to do that, I have to learn Haskell quite thoroughly. What book can you recommend? TIA Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Re: [Haskell-cafe] Matroids in Haskell
by Michael Matsko 17 Jan '05

17 Jan '05

Dimitri Matriods are generalization of vector spaces. Basically, they are defined by a set of linear dependence axioms and basis exchange properties. Oxley's "Matriod Theory" is the standard reference. There are a multitude of equivalent formulations. Mike Matsko ----- Original Message ----- From: Dmitri Pissarenko <mailing-lists(a)dapissarenko.com> Date: Friday, January 14, 2005 2:00 pm Subject: Re: [Haskell-cafe] Matroids in Haskell > Hello! > > Gerhard Navratil wrote: > > Recently I had a course on matroids and would like to investigate > the> topic a little further. Did anybody write (or start writing) a > > Haskell-implementation for matroids? > > What is a matroid? > > Thanks > > Dmitri Pissarenko > -- > Dmitri Pissarenko > Software Engineer > http://dapissarenko.com > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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