March 2005 - Haskell-Cafe

Re: [Haskell] State, StateT and lifting
by Juan Carlos Arevalo Baeza 19 Mar '05

19 Mar '05

Andrew Pimlott wrote: >[I think it is preferred to post this on haskell-cafe.] > > Oops! I guess you're right. >On Fri, Mar 18, 2005 at 02:00:37PM -0800, Juan Carlos Arevalo Baeza wrote: > > >>matchRuleST :: String -> State RuleSet (Maybe Rule) >>makeST :: String -> StateT RuleSet IO () >> >> matchRuleST doesn't really need IO for anything (it just works on the >>current state "RuleSet"). makeST needs IO (it does file date >>comparisons, actions on the files, etc... the usual "make" stuff). The >>problem is how to properly use matchRuleST from makeST. >> >> > >You might solve this by changing the type of matchRuleST: > > matchRuleST :: MonadState RuleSet m => String -> m (maybe Rule) > > I don't know... The original using IO somehow offended me because it was not an operation that required IO. This one leaves the inner monad unspecified, but still looks like baggage to me. >> Then, I decided to try again, and came up with this function: >> >>liftState :: State s a -> StateT s m a >> >> > >(I think you left out the constraint (Monad m).) > > Yes, I did, thanx. I wrote the message a tad little bit too early :-P >>liftState s = do >> state1 <- get >> ( >> let (result, state) = evalState (do {result <- s; state <- get; >>return (result, state)}) state1 in do >> put state >> return result >> ) >> >> > >You can turn this into a one-liner if you work on it a bit. But I would >go with the above. > > Yes. I prefer clarity, too. And I did it ugly (still groping with the syntax). This is the final version: liftState :: Monad m => State s a -> StateT s m a liftState s = do state1 <- get let (result, state) = evalState (do {result <- s; state <- get; return (result, state)}) state1 put state return result >Aside: It bugs me that this is not defined by Control.Monad.State >(alongside modify and gets): > > state :: MonadState s m => (s -> (a, s)) -> m a > >I almost always end up defining it myself and use it to implement other >state transformers. I would do the same for other monad classes >(Writer, etc): provide a function that captures the general operation. > > Cute, thanx! It's good to know I wasn't just missing something obvious. So, this is my final implementation (works!): state :: MonadState s m => (s -> (a, s)) -> m a state sm = do s <- get let (result, newState) = sm s put newState return result liftState :: Monad m => State s a -> StateT s m a liftState (State f) = state f JCAB

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Re: [Haskell] Arrows GUI Library Based on GTK+
by ross＠soi.city.ac.uk 19 Mar '05

19 Mar '05

[moving to haskell-cafe, as this thread is getting long] On Fri, Mar 18, 2005 at 07:18:29PM -0700, Kevin Atkinson wrote: > What follows is my first attempt of using Arrows to create a GUI Library > based on GTK+. It uses many ideas from Fruit (http://haskell.org/fruit/) > However it is based on discrete events rather than a continuous signal. > The interface is only updated during an Event. It also ideas from Fudgets > (http://www.md.chalmers.se/Cs/Research/Functional/Fudgets/) some of which > were also used by Fruit. I would also have expected loopFG to have been defined using fixIO.

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RE: [Haskell-cafe] Typeclasses and instances
by Simon Peyton-Jones 18 Mar '05

18 Mar '05

Strange. This is a bug in GHC that crept in when I implemented GADTs. It turns out to be slightly structural; fixing it will change the interface-file format. So it won't get fixed in 6.4; but will in the HEAD. That doesn't bother me; the context on a data/newtype decl is a strange thing anyway; only useful for generating extra errors... but yes, it's a bug thanks for pointing it out. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Mark | Carroll | Sent: 17 March 2005 19:58 | To: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] Typeclasses and instances | | Another note, with more help from friends: | | It turns out that GHC 6.4 will let me do, | | newtype Floating a => Test a = Test [a] deriving Show | | x = Test [False, True] | | but, if I change "newtype" to "data", it then says, | | No instance for (Floating Bool) | | I'm not sure I quite understand what's going on. | | -- Mark | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: [Haskell-cafe] Best practices for modular programming in Haskell
by Simon Marlow 17 Mar '05

17 Mar '05

On 17 March 2005 06:11, Sean Perry wrote: > Benjamin Pierce wrote: >> >> Other people seem to rely on Haddock to generate interfaces as >> documentation. This is nicer in many ways (e.g., it solves the above >> problem because Haddock elides the right-hand side of a "data" or >> "newtype" declaration if the constructors are not exported by the >> module), but it means that you are stuck browsing interfaces in a >> web browser can't just have a look with emacs. This is especially >> annoying if you want to read the interface in tandem with the >> implementation (because you are trying to understand how the thing >> works internally but want to use the interface to get a high-level >> picture of the functionality it presents to the world). >> > > haddock can also output docbook and you can make info pages from > docbook. Actually the DocBook backend isn't really there yet. It wouldn't be too hard to add a plain ASCII backend to Haddock that generates the interfaces for modules without the implementation - that would address Benjamin's concern to some extent. Cheers, Simon

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Re: Best practices for modular programming in Haskell
by oleg＠pobox.com 17 Mar '05

17 Mar '05

Benjamin Pierce wrote: > For someone coming to Haskell from an OCaml background, one of the hardest > things to get used to is the somewhat more bare bones module system that > Haskell provides. > ... > This works fine as long as what you're exporting is just values, but it's > awkward for types, since there is no way to *declare* a type (e.g., giving > just its kind) without *defining* it. Actually Haskell fully matches the module system of OCaml -- and then adds some. Haskell provides both generative and applicative (recursive) functors. The following two messages elucidate the correspondence http://www.haskell.org/pipermail/haskell/2004-August/014463.html http://www.haskell.org/pipermail/haskell/2004-September/014515.html The end of the first message gives the translation dictionary: from OCaml/ML module terminology to Haskell terminology. I have recently found out that the translation works the other way around, too. Both messages are doubly-literate code: messages can be loaded into OCaml or into a Haskell system. Regarding the specific question of hiding a type: oftentimes declaring a class and exporting it suffices. (forall a.C a => a) is an `abstract' type in Haskell. If the type to abstract has a kind *->*, we can write (forall a.C a => a b), etc. If we really want to prevent the user from ever knowing the details of the implementation, existential quantification (along with the typeclass) does the trick. That trick incidentally permits `self-tuned' data structures, the simple example of which is http://www.haskell.org/pipermail/haskell/2005-February/015356.html

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interesting IO oriented code
by robert dockins 17 Mar '05

17 Mar '05

I while back following the most recent discussion about filepaths and IO generally, I decided to pick up the torch and try my hand at a solution that delt with all the issues. My conception of the problem can be found here: http://www.haskell.org//pipermail/haskell-cafe/2005-January/008955.html However, I quickly got sidetracked by the character encoding issue, and sequential IO operations generally. I have come up with some code that I think is interesting, not least of which because it is somewhat performant. In the spirit of 'release early, release often' I am now making it available. I ended up developing a sequential IO API and a couple of test cases to drive it. I currently have a simple word count and a md5 implementation. There is a darcs repo at: http://www.eecs.tufts.edu/~rdocki01/filepath/ The code relies on multi-parameter typeclasses, fundeps and unboxed tuples, so its GHC only. The good news: 1) The API is semi-manageable. It is based around Producer, Transformer and Consumer functors (at least I think they can be called functors; I'm not real knowledgeable here). I think that with a little more work (and the input of people more experienced at this stuff than I) it could be sugared into a pretty usable API. 2) It is performant (mostly). At least it outperforms other Haskell IO methods I have tried. My 'wc' is about twice as fast as the current shootout version in informal tests (the shootout code is included in the repo). My md5 can sum somewhere between 2-4Mb/Sec on my hardware. 3) Very importantly, the code appears to have reliable constant-space behavior. 4) We are not artificially forced into the IO or ST monads for performance reasons. Explicit state passing is used where necessary, which seems to have the added benefit of helping the compiler to find good optimizations (pure speculation). The bad news: 1) Mostly performant is still not great. My wc takes about 6 times as long as the C version on my machine, and md5 takes about about 80 times as long. (interestingly, the C md5sum takes about 1/5 of the time as C wc!). However, it is within an order of magnitude of a java implementation of md5 (using the standard digest classes). 2) The performance is pretty fragile. Small changes can cause large performance hits for no easily discernible reason. This probably relates to the way particular optimizations do or don't get applied. The situation is hugely complicated by the typeclass-heavy API, I am sure. 3) The stream-observer paradigm is a somewhat difficult programming environment. My next step is to try some actual character encoding implementations (the original purpose after all) and see how that goes. I'd also like to try gzip and gunzip transformer layers. Any ideas for improvements (including patches!) are welcome. Robert Dockins PS the code currently includes a number of vestigial remnants of false starts, and is generally kind of ugly; you are warned.

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FFI export questions
by John Goerzen 17 Mar '05

17 Mar '05

Hi, I'm trying to make a Haskell binding for the FUSE library, http://fuse.sourceforge.net/. The FUSE concept is centered around a struct whose members are pointers to callback functions. Here's the problem: the functions I want to callback to have types like this: vGetLine :: HVFS a -> a -> IO String Now, I can't just pass some sort of opaque pointer to C, because then the exact type of a would be lost. Is there any other method I might try? Thanks, John

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Parse error
by Dmitri Pissarenko 17 Mar '05

17 Mar '05

Hello! In the attachment you will find a file, in which I try to access Java from Haskell using the Java bridge for functional languages. For details see http://sourceforge.net/projects/jvm-bridge/ and http://dapissarenko.com/resources/2005_02_17_eigenvaluesJava/2005_02_17_eig… luesJava.pdf When I try to compile the attached file using ghc +RTS -c -RTS -package javavm -c EigenvalueCalculatorHaskell.hs -o EigenvalueCalculatorHaskell.o I get the error EigenvalueCalculatorHaskell.hs:28: parse error on input `putStrLn' Unfortunately, I have not the slightest idea about how to fix/isolate it (I already commented out almost the whole code). Please tell me what I could try to correct the error. I appreciate ANY hint. Many thanks in advance Dmitri Pissarenko PS: The source code of the files related to EigenvalueCalculatorHaskell.hs is located at http://dapissarenko.com/resources/2005_02_17_eigenvaluesJava/2005_02_17_lik… -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Re: [Haskell-cafe] Typeclasses and instances
by Mark Carroll 17 Mar '05

17 Mar '05

Another note, with more help from friends: It turns out that GHC 6.4 will let me do, newtype Floating a => Test a = Test [a] deriving Show x = Test [False, True] but, if I change "newtype" to "data", it then says, No instance for (Floating Bool) I'm not sure I quite understand what's going on. -- Mark

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Re: [Haskell-cafe] Typeclasses and instances
by Mark Carroll 17 Mar '05

17 Mar '05

> newtype Floating a => Vector a = Vector [a] Okay, I now know a little more about this, with help from friends. The obvious Functor instance seems not to work with GHC 6.2.2 but does work with GHC 6.4. With 6.2.2 I can still use GHC's newtype-deriving extension to derive an instance for Functor, I'm just not sure how I can get an explicit instance past 6.2.2 without it complaining about my syntax or not being able to deduce Floating a or b. This isn't a show stopper, I'm just intrigued. -- Mark

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