March 2005 - Haskell-Cafe

Re: [Haskell] Arrows GUI Library Based on GTK+
by ross＠soi.city.ac.uk 22 Mar '05

22 Mar '05

On Sat, Mar 19, 2005 at 12:17:46PM -0700, Kevin Atkinson wrote: > On Sat, 19 Mar 2005 ross(a)soi.city.ac.uk wrote: > > I would also have expected loopFG to have been defined using fixIO. > > Could you be more specific. Ie How? For the type definitions newtype FG' a b = FG' (Control -> a -> IO (Control, b)) newtype FG a b = FG (FGState -> IO (FG' a b, FGState)) newtype Container a b = Container (FG ([WidgetP], a) b) the usual instances would be (give or take a ~): instance Arrow FG' where arr f = FG' $ \ c x -> return (c, f x) FG' f >>> FG' g = FG' $ \ c x -> do (c', x') <- f c x g c' x' first (FG' f) = FG' $ \ c (x, y) -> do (c', x') <- f c x return (c', (x', y)) instance ArrowLoop FG' where loop (FG' f) = FG' $ \ c x -> do (c', x', _) <- mfix $ \ ~(_, _, y) -> do ~(c', ~(x', y')) <- f c (x, y) return (c', x', y') return (c', x') instance Arrow FG where arr f = FG $ \ s -> return (arr f, s) FG f >>> FG g = FG $ \ s -> do (f', s') <- f s (g', s'') <- g s return (f' >>> g', s'') first (FG f) = FG $ \ s -> do (f', s') <- f s return (first f', s') instance ArrowLoop FG where loop (FG f) = FG $ \ s -> do (f', s') <- f s return (loop f', s') instance Arrow Container where arr f = Container (arr (f . snd)) Container f >>> Container g = Container $ arr (\ (ws, a) -> (ws, (ws, a))) >>> second f >>> g first (Container f) = Container $ arr (\ ~(ws,~(x,y)) -> ((ws,x),y)) >>> first f instance ArrowLoop Container where loop (Container f) = Container $ loop (arr (\ ((ws,x),y) -> (ws,(x,y))) >>> f) The FG instances seem to match yours, except for ArrowLoop.

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Re: [Haskell-cafe] Linux device drivers
by Keean Schupke 22 Mar '05

22 Mar '05

I don't think I said anything controversial. I guess I was just over-simplifying things by only considering PC IDE hardware - but then again that must get you running on 90% of the systems people are likely to have lying around to play with a developmental OS on. On the other hand the average network driver seems to be about 2,000 lines of code, whereas if you add all the parts of the generic ide driver together you get about 20,000 lines of code. I guess that answers my question - storage is an order of magnitude harder than networking, even before including SCSI. Regards, Keean. Simon Marlow wrote: >Keean, you should be aware that Lennart is something of a device driver >guru. He knows what he's talking about :-) Go grep for Augustsson in >the NetBSD kernel sometime. > >Cheers, > Simon > >

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Re: Arrows GUI Library Based on GTK+
by Kevin Atkinson 22 Mar '05

22 Mar '05

[From haskell@ to haskell-cafe@] On Sat, 19 Mar 2005 ross(a)soi.city.ac.uk wrote: > Some more minor suggestions: > - I'd suggest making Container a newtype, so it could have an Arrow > instance (it would be a reader arrow). Then hbox and vbox could be > used as arrow operators/combinators. What will that give me? Could you give me an example of how I would use it in such a manner? -- http://kevin.atkinson.dhs.org

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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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