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Symbolic differentation using GHC's simplification rules
by Henning Thielemann 22 Mar '05

22 Mar '05

I tried to implement symbolic differentation using GHC's simplification rules. I assume that someone has already thought about this, right? I have also heard that the rules system is not strong enough for implementing a full computer algebra system. But here is a simple trial and some questions and problems which arise: 1. How to specify a type class context to which the rules have to be restricted? (see the first comment) 2. How to handle lambdas? 3. The module can only be used for compiled programs, since in GHCi optimisation is not available. {-# OPTIONS -O -fglasgow-exts #-} module SymbolicDifferentation where {- These rules need Num context. -- How to write that? "derive/const" forall y. derive (const y) = const 0 ; "derive/id" derive id = const 1 ; "derive/compos" forall f g. derive (f . g) = derive f . g .* derive g ; -} {-# RULES "derive/plus" forall f g. derive (f .+ g) = derive f .+ derive g ; "derive/minus" forall f g. derive (f .- g) = derive f .- derive g ; "derive/times" forall f g. derive (f .* g) = derive f .* g .+ f .* derive g ; "derive/divide" forall f g. derive (f ./ g) = derive f .* g .- f .* derive g ./ ((^2).g) ; "derive/power" forall n. derive ((^) n) = (n*) . (^(n-1)) ; "derive/sin" derive sin = cos ; "derive/cos" derive cos = negate . sin ; "derive/exp" derive exp = exp ; "derive/log" derive log = recip ; #-} -- lift a binary operation to the function values fop2 :: (c -> d -> e) -> (a -> c) -> (a -> d) -> (a -> e) fop2 op f g x = op (f x) (g x) infixl 6 .+, .- infixl 7 .*, ./ (.+), (.-), (.*) :: Num a => (t -> a) -> (t -> a) -> (t -> a) (./) :: Fractional a => (t -> a) -> (t -> a) -> (t -> a) (.+) = fop2 (+) (.-) = fop2 (-) (.*) = fop2 (*) (./) = fop2 (/) derive :: (t -> a) -> (t -> a) derive = error "Could not derive expression symbolically." test :: IO () test = do print (derive log 2) print (derive sin 0) print (derive cos 0.1) print (derive (cos .+ sin) (pi/4)) print (derive (\x -> exp x) 0) print (derive (\x -> x^2 + x) 0) print (derive (exp . sin) 0)

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Linux device drivers
by Mark Carroll 22 Mar '05

22 Mar '05

I was wondering about the possibility of using Haskell for developing device drivers that would be kernel modules for Linux. If nothing else, it would be quite an educational experience for me, as I've not yet experimented with either the Linux kernel or Haskell FFI, nor have I had to learn how to squeeze much performance out of my Haskell code. Clearly, this application demands special things from the compiler and the runtime. But, I'm not exactly sure what, nor how to achieve such given current compilers. Does anyone have any thoughts? Thanks, Mark -- Haskell vacancies in Columbus, Ohio, USA: see http://www.aetion.com/jobs.html

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tuple and HList
by Frederik Eaton 22 Mar '05

22 Mar '05

HList seems just like a tuple, but more powerful because one can access the type structure directly, and more cumbersome because one has to use lengthier constructors a 'nil' terminator. So why not just make tuples synonyms for HLists, so one can use HLists with the shorter notation, and have the added benefit of being able to access the internals of a tuple (e.g. make instances which concatenate generic tuples, project elements, etc.)...? Frederik -- http://ofb.net/~frederik/

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implicit parameters THANK YOU!
by S. Alexander Jacobson 22 Mar '05

22 Mar '05

I just discovered implicit parameters. To everyone involved with making them, THANK YOU. They are blazingly useful/powerful for server handler style libraries where you want to make a veriety of local environment information available to the handlers without burdening the handlers with a big dictionary object to carry around. FANTASTIC. That being said, they so powerful they are proabably easy to abuse. Could those experienced with this feature provide warnings about possible problems with overuse? I see so many places I think I could make HAppS code nicer using this feature that I am concerned about making mistakes. -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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invalid character encoding
by John Goerzen 21 Mar '05

21 Mar '05

I've got some gzip (and Ian Lynagh's Inflate) code that breaks under the new hugs with: <handle>: IO.getContents: protocol error (invalid character encoding) What is going on, and how can I fix it? Thanks, John

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Parsec question: how to access parser state
by Dimitry Golubovsky 21 Mar '05

21 Mar '05

Hi, I am trying to develop a parser with the Parsec library. At some point I need to do something with parser state, say, convert it to a string. I declared the type for the parser: type TParser a = GenParser Token (FiniteMap String Declaration) a The FiniteMap (which is the user state) is expected to be updated during parsing whus building some internal lookup table. and in one of the parsing functions I want to call show getState assuming it will apply show to the FiniteMap and return some string of characters. During compliation, I get: No instance for (Show (GenParser tok st st)) arising from use of `show' at (file location) In the first argument of `xxxx', namely `(show getState)' But GenParser is a newtype, not a data constructor, so attempt to write instance Show (GenParser tok st t) results in error message about missing constructor GenParser. How could I extract actual user state from the result of getState? The parser itself works as needed, but dealing with user state gets me in trouble. Dimitry Golubovsky Middletown, CT

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Re: [Haskell-cafe] invalid character encoding
by Wolfgang Thaller 20 Mar '05

20 Mar '05

> If you try to pretend that I18N comes down to shoe-horning everything > into Unicode, you will turn the language into a joke. How common will those problems you are describing be by the time this has been implemented? How common are they even now? I haven't yet encountered a unix box where the file names were not in the system locale encoding. On all reasonably up-to-date Linux boxes that I've seen recently, they were in UTF-8 (and the system locale agreed). On both Windows and Mac OS X, filenames are stored in Unicode, so it is always possible to convert them to unicode. So we can't do Unicode-based I18N because there exist a few unix systems with messed-up file systems? > Haskell's Unicode support is a joke because the API designers tried to > avoid the issues related to encoding with wishful thinking (i.e. you > open a file and you magically get Unicode characters out of it). OK, that part is purely wishful thinking, but assuming that filenames are text that can be represented in Unicode is wishful thinking that corresponds to 99% of reality. So why can't the remaining 1 percent of reality be fixed instead? Cheers, Wolfgang

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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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