August 2002 - Haskell

can't write instance Storable
by Hal Daume III 02 Aug '02

02 Aug '02

Okay, I'm dumb and can't figure this out. I'm migrating hMPI from GHC 4.08 to GHC 5.04 (fun fun) and am having trouble writing an instance of Storable. The relevant information is: > import Foreign.Storable > type MPI_Rank_Type = Int > newtype MPI_Rank = MPI_Rank MPI_Rank_Type deriving (Eq) > > instance Storable MPI_Rank where > sizeOf (MPI_Rank r) = sizeOf r > alignment (MPI_Rank r) = alignment r > peek addr = do r <- peek addr > return (MPI_Rank r) > poke addr (MPI_Rank r) = poke addr r which doesn't work; ghc complains: /nfs/isd/hdaume/download/hmpi-0.9.3/src/Foo.hs:12: Couldn't match `MPI_Rank_Type' against `MPI_Rank' Expected type: MPI_Rank_Type Inferred type: MPI_Rank In the first argument of `MPI_Rank', namely `r' In the first argument of `return', namely `(MPI_Rank r)' Failed, modules loaded: none. I have no idea what's wrong. if i make it "...do (r :: Int) <- ..." it complains: /nfs/isd/hdaume/download/hmpi-0.9.3/src/Foo.hs:12: Couldn't match `Int' against `MPI_Rank' Expected type: Int Inferred type: MPI_Rank When checking the pattern: r :: Int In a 'do' expression pattern binding: (r :: Int) <- peek addr Failed, modules loaded: none. it's like it doesn't realize that "peek" is part of the class and can be overloaded. someone please clear this up and tell me what obvious thing i'm missing... Thanks! - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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slightly ot: data structure question
by Hal Daume III 02 Aug '02

02 Aug '02

Not really Haskell-specific, but... I need a data structure which is a map from Ints to Doubles; the distribution of the Ints is in the range say 0-20000 and a map will contain somewhere around 100-200 elements. I need to be able to query *very* quickly, insert *quite* quickly, and iterate through the elements very quickly. I'm thinking hash tables, but I'm not sure. Does anyone have any thoughts on this? - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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monads, modules, sandboxes
by Daniel Crealer 01 Aug '02

01 Aug '02

Warning: pseudo-caml below since I don't know Haskell too well. But in trying to learn I came across interesting concept of monad, and that is what inspired the following idea. I would just like to know if anyone thinks it's interesting and if someone has already developed it. Inspired by monads, I had the following idea for controlled sandboxing of an application. I'll give you a very concrete scenario, but maybe it isn't the most interesting use of idea. I download a program off the Internet that claims to do some interesting transformation on my data. But I don't trust the application not to send my secret data back to the web site. I could achieve this by running the program in a sandbox that doesn't allow it to send data over the Internet. But maybe the program has legitimate reasons to send data, e.g., the company charges based on how much data I transform, so the program wants to periodically send messages to the web site saying it performed some units of work. The idea is just that if my data has type t, instead of a program of type t -> t, the website provides a program of type m -> m written in a monadic style. Actually m = t, and the bind operation (>>=) is trivial. But it is run in the sandbox. Here is pseudo-code, where MYDATA is thought of as a well-known signature of a well-known standard, like JPEG or something, with standard implementation MyData (though I could change MyData as long as I think my changes won't break the code I got off the web site) and DataTransformer is the untrusted code from the web site. In some applications, probably you could get away with just bind and return, but for the example I give here, the untrusted code needs a way to find out if the data is done being transformed. I implement that by providing an is_done function, but note that this could be a security hole. A safer way might be to have really_do_100_steps throw an exception when it finishes that would be caught by bind. module MyData = struct type t = ... type m = t let (>>=) mm f = try run_in_sandbox (fun _ -> f mm) with _ -> failwith "gotcha" let return x = x (* functions that only run in the sandbox *) ... things that take t's ... (* functions that can run outside the sandbox *) let is_done = ... ... other things that take m's ... end module type MYDATA = sig type t = ... type m val (>>=) : m -> (t -> m) -> m val return : t -> m is_done : m -> bool ... other stuff ... end module DataTransformer = struct let really_do_100_steps x = ... let do_100_steps x = x >>= (function y -> return (really_do_100_steps y)) let rec doit x = bill_customer_for_100_steps (); let x' = do_100_steps x in if is_done x' then x' else doit x' end _________________________________________________________________ Join the worlds largest e-mail service with MSN Hotmail. http://www.hotmail.com

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Re: good advanced fp (haskell) book
by Fritz K Ruehr 01 Aug '02

01 Aug '02

Chris Okasaki's book "Purely Functional Data Structures" is not on Haskell per se, but it addresses data structures (and efficiency issues) from a higher level than the typical introductory FP book. All the data structures and code are available in Haskell in an appendix (and on-line). Amazon has 15 sample pages (hmm, must be a big seller). You should also look at the book "Algebra of Programming" by Richard Bird and Ooge de Moor: you will find much to satisfy your mathematical tastes, as well as motivations which include efficiency. Again the language is not exactly Haskell, but if past history is any guide, then Haskell will probably look more like the language of this book after a few years anyway :) . -- Fritz Ruehr PS: I found "A of P" on Barnes and Noble's site more easily than on Amazon.

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good advanced fp (haskell) book
by Hal Daume III 01 Aug '02

01 Aug '02

Hi all, I'm looking to purchase one (perhaps two) books for my collection. I'm looking for two things in particular: - good algorithmic reference - good explanation of the meat of haskell (advanced stuff) I don't care about introductory stuff; I've written tens of thousands of lines of Haskell code and don't want to waste my time reading what a tuple is. I was looking at the "Algorithms: A Purely Functional Approach" online which, based on the contents, seems to satisfy well my first desire, but I fear it doesn't get advanced quickly enough and am left with only a little on advanced topics. All the other books I looked at on the bookshelf seemed too introductory. I'm not afraid of math (it was my undergraduate degree) and rather enjoy theorems, but I'm also insanely practical and am interested in a book which has a large section on *efficiency*. I fear what I'm looking for doesn't exist, and in the absense of a book, perhaps people could point me to good extended (perhaps journal?) papers -- though papers tend to largely ignore the efficiency stuff and serve as very poor references. Thanks! - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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