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RE: [Haskell-cafe] Instances of constrained datatypes
by Simon Peyton-Jones 07 Apr '05

07 Apr '05

I believe that you may find these papers relevant: "Bulk types with class" http://research.microsoft.com/%7Esimonpj/Papers/collections.ps.gz "Restricted data types" http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps John's paper mentions that it would be great if the constraint simplifier could generate recursive dictionaries --- and indeed GHC's constraint simplifier now does exactly that. So you can apply his technique. I'm not 100% certain this addresses your problem, but I think it does. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Keean Schupke | Sent: 07 April 2005 09:30 | To: Keean Schupke | Cc: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] Instances of constrained datatypes | | One way to do roughly what you want is to pass the dictionary yourself: | | >data EqDict a = EqDict { | > leq :: a -> a -> Bool } | > | >data EqList a = EqList (EqDict a) [a] | > | >test :: EqList a -> EqList a -> Bool | >test (EqList dict (a0:as)) (EqList _ (b0:bs)) = (leq dict) a0 b0 | | In this way the definition of equality on elements of type 'a' is passed | with the list type, so it can be used wherever the list type is used, | without requiring extra constraints. | | Keean. | | Keean Schupke wrote: | | > I think it is more a problem of imlpementation than one of what is | > desirable. A Constrained data type: | > | > data (Eq v) => EqList v = EqList [v] | > | > The problem is how to get the dictionary for the class Eq to the | > application site: | > | > f :: EqList v -> EqList v | > f (EqList (u0:us)) (EqList (v0:vs)) | v0 == u0 = ... | > | > Which of course does not work... the constraint needs to be in the | > function | > type signature: | > | > f :: Eq v => EqList v -> EqList v | > | > Things are worse though, as even functions that use no methods of Eq will | > require the constraint. | > | > The constraint on the data type does not stop you construction EqLists | > from | > non Eq members... of course this gets detected the moment you try and | > use it | > in a constrained function. | > | > | > In other words using the constraint in the data type does nothing... | > you may as well just do: | > | > f :: Eq v => [v] -> [v] | > | > | > Infact I believe it was decided to remove the feature from Haskell98 | > entirely, but there was apparently some use for the 'syntax' although | > with a different effect. | > | > Keean. | > | > Cale Gibbard wrote: | > | >> I don't believe you can, but it would be nice. There are certain | >> types, such as Set, where it's not really possible to just remove the | >> constraint from the data declaration, and yet it would be nice if sets | >> could be instances of Monad and Functor. Currently, to be an instance | >> of Functor or Monad, your type has to be a functor defined on the | >> whole category of types. | >> | >> Could this issue be fixed somehow? Constrained instances would make | >> various typeclass-based libraries more applicable. What would it break | >> to allow instances where the types of functions defined by the | >> typeclass are further restricted? I suppose that checking that types | >> are correct becomes more difficult and non-local, because functions | >> which are defined using the typeclass won't already have that | >> constraint for obvious reasons. Still, the constraint is in the | >> instance, which must be around when the functions actually get | >> applied. There are probably bad interactions with the module system, | >> but I'm not certain. | >> | >> People must have talked about this before... was a consensus reached | >> that I'm not aware of? | >> | >> - Cale | >> | >> On Apr 6, 2005 2:10 AM, Arjun Guha <guhaarju(a)grinnell.edu> wrote: | >> | >> | >>> This is a contrived example, but contains the essence of what I'd like | >>> to do. Suppose I have this datatype: | >>> | >>> > data (Eq v) => EqList v = EqList [v] | >>> | >>> I'd like to make it an instance of Functor. However, fmap takes an | >>> arbitrary function of type a -> b. I need an Eq constraint on a and | >>> b. Is there any way to do this without creating my own `EqFunctor' | >>> class with explicitly-kinded quantification: | >>> | >>> > class (Eq a) => EqFunctor (f :: * -> *) a where | >>> > eqfmap:: (Eq b) => (a -> b) -> f a -> f b | >>> | >>> Thanks. | >>> | >>> -Arjun | >>> | >> | > | > _______________________________________________ | > Haskell-Cafe mailing list | > Haskell-Cafe(a)haskell.org | > http://www.haskell.org/mailman/listinfo/haskell-cafe | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Strange breakage after Hugs upgrade
by John Goerzen 05 Apr '05

05 Apr '05

Hi, I have this code: class (Show a) => HVFSStat a where ... vModificationTime :: a -> EpochTime ... class (Show a) => HVFS a where ... vGetModificationTime :: a -> FilePath -> IO ClockTime ... vGetModificationTime fs fp = do s <- vGetFileStatus fs fp return $ TOD (fromIntegral (withStat s vModificationTime)) 0 This works fine in Hugs 200311, but in Hugs 20050308, I get: ERROR "/home/jgoerzen/tree/projects/missingh/MissingH/IO/HVFS.hs":198 - Implementation of vGetModificationTime requires extra context *** Expected type : HVFS a => a -> FilePath -> IO ClockTime *** Missing context : Integral CTime I can't figure out what's going on; the type of the expression (withStat s vModificationTime) is EpochTime, and I don't understand why Hugs would have decided to break this code. Thanks, John

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Hugs CPP macros?
by John Goerzen 05 Apr '05

05 Apr '05

Hi, I need to use some conditional compilation in MissingH so that it can work with GHC 6.2, 6.4, and the old/new Hugs libraries. I found the appropriate macros to test for with ghc, but I can't for Hugs. The cpphs-hugs doesn't even define __HUGS__. It seems that only hugs-hc and ffihugs define that, but I don't need them. And where __HUGS__ is defined, it isn't defined with a version number, so it doesn't help me differentiate the Hugs versions. Any suggstions? Thanks, John

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Data.Map
by Gracjan Polak 03 Apr '05

03 Apr '05

Hi all, As I tried to convert some of my code to newer libraries comming with GHC 6.4, I would like to share two things with you: I noticed that there are two functions missing: deleteList and insertList. The first one is easy with foldl: deleteList list map = foldl (flip Data.Map.delete) map list Second one is even shorter: insertList asclist map = union map (Data.Map.fromList asclist) Still I find both of them useful also execution speed seems to be of some concern here. Why generate temporary map just to join it with second in a moment? Isn't it slower? Also why Data.Map.lookup and Data.Map.findWithDefault? Why not lookupWithDefault? Besides, I really like new library :) -- Gracjan

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Ord type on lists
by Lloyd Smith 03 Apr '05

03 Apr '05

Hi everyone, I defined my own list datatype and then tried to declare it as an instance of type class Ord. However when I test it with Nil > Cons 1(Nil) I get an "ERROR - Control stack overflow" I am under the impression that the ord class defines default implementations of (<=), (>),(>=) so that I only have to supply the implementation of (<) shown below. Can some one tell me why this does not work the way I expect it to? My very own list data type > data List a = Nil > | Cons a (List a) > deriving (Show) Derive the equality type for list. > instance (Eq a) => Eq (List a) where > Nil == Nil = True > Nil == Cons y ys = False > Cons x xs == Nil = False > Cons x xs == Cons y ys = (x == y) && (xs == ys) Derive the ordered type for list > instance (Ord a) => Ord (List a) where > Nil < Nil = False > Nil < Cons y ys = True > Cons x xs < Nil = False > Cons x xs < Cons y ys = (x < y) && (xs < ys) -- Lloyd G Smith

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Existential problem
by Niklas Broberg 01 Apr '05

01 Apr '05

I just can't see why the following code will not type check. I have scaled it down as much as possible: ------------------------------------- -- IO with an extra level annotation newtype IO' t a = MkIO' (IO a) -- levels data H data L -- Explicitly recursive continuation type data C t a = forall t2 . C (C t2 a -> IO t a) transition :: Bool -> IO tx () -> IO t () -> IO t () transition b c d = undefined s1 :: C t () -> IO H () s1 (C k) = trans (True) (k (C s1)) -- :: IO t () (s1 (C k)) -- :: IO H () -------------------------------------- The error message is: Inferred type is less polymorphic than expected Quantified type variable `t2' is unified with `H' When checking an existential match that binds k :: C t2 a -> IO t a The pattern(s) have type(s): C t1 () The body has type: IO H () In the definition of `s1': s1 (C k) = trans (True) (k (C s1)) (s1 (C k)) The thing is, I'm perfectly happy with t2 being unified with H for this particular definition of s1, but I want to use it in other cases as well. The code that follows the one above is s2 :: C t () -> IO L () s2 (C k) = trans (True) (k (C s2)) -- :: IO t () (s2 (C k)) -- :: IO L () main = s1 (C s2) where t2 instead gets unified with L for the definition of s2. Could someone tell me why this does not work? I'm using GHC 6.2.2. Thanks, /Niklas

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New user, pardon what may be a dumb question
by Mark Goldman 31 Mar '05

31 Mar '05

I am using ghci to learn Haskell. My question is it possible to assign an object that contains state (for lack of knowing the proper terminology) so a variable or some such from the interactive prompt. More concretely I would like to do something like this non-working code snippet: System.Random> x <- return (mkStdGen 45) System.Random> random x :: (int, StdGen) Thanks

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A few questions on using Cabal
by Niklas Broberg 31 Mar '05

31 Mar '05

I've just started experimenting with the new Cabal system, and I must say it's really sweet. Thanks a lot to all involved! After trying it on some simple tasks I have collected a few questions: * What about 'setup uninstall'? Surely there should be an automatic way of uninstalling packages and executables? * Is there some way to direct the installation of a package to an auxiliary package-conf file, i.e. separate from the 'global' and 'user' package databases? E.g. 'runhaskell Setup.hs register --package-conf=foo.conf' (doesn't work) * Is there some way to bundle several packages into a single installation unit? Similar to how you can specify several executables, or executables and a library package, I would want a way to say that a bundle will install several (possibly interrelated) packages at the same time. * If I specify a library package and an executable in the same cabal bundle, where the executable uses the library, the files that make up the library get compiled twice. Once when setting up the package, and then again when compiling the executable. How come? That's all for now, possibly more to come later. =) /Niklas

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assistance with using haskell to calculate the determinant
by Carter Tazio Schonwald 29 Mar '05

29 Mar '05

I've written a simple program to calculate determinants of matrices, and while it executes to completion it fails to yield a correct answer. (and yes, I know I'm using lists to represent matrices) I would greatly appreciate any assistance in correcting this algorithm --Carter Schonwald import Prelude first [] = [] first (a:b) = a nth::Int->[a]->a nth _ [] = error "list is too small" nth 1 (a:b) = a nth n (a:b) = nth (n-1) b takeAfter::Int->[a]->[a] takeAfter _ [] = error "list too small" takeAfter 0 a= a takeAfter 1 (a:b) = b takeAfter n (a:b) = takeAfter ( n-1) b type Matrix = [[Rational]] pad a = [x++x| x<- a] time2 [] _ = [] time2 _ [] = [] time2 (a:b) (c:d) = (a * c):(time2 b d) tupleProd (a,b) = a * b altSign a = [b* (-1^num) | b<-a, num <- [2..]] index::Int->Int->Matrix->Rational index a b c = nth b (nth a c) --- ath row, bth column slice::(Int,Int)->(Int,Int)->Matrix->Matrix slice (a,b) (c,d) list = let rowSliceFront = takeAfter (a-1) list --- a and c are rows, b and c are columns in let rowSlice = take (c-a+1) list in let columnSliceFront = map (takeAfter (b-1)) rowSlice in map (take (d-b+1)) columnSliceFront determinant::Matrix->Rational determinant a = det (pad a) (length a) -- only called from determinant det::Matrix->Int->Rational det a 1 = index 1 1 a det a size =let coeffs = altSign (first (take 1 a)) newsize = size - 1 leastToMax = newsize - 1 slices = [ slice (2, i) (size, i + leastToMax) a | i <- [2..(size+1)] ] in sum (time2 coeffs (map (\l-> det l newsize) (map (pad) slices)) )

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Implementation of Non-Deterministic State Machine
by Pierre Barbier de Reuille 28 Mar '05

28 Mar '05

Hello, As an exercice to learn monadic programming, I programmed a Non-Deterministic State Machine. I extended the exercice to include the deterministic "cut" function, exactly as the one existing in Prolog. That means I defined an evalutaion frame, and if the machine evaluates a "cut", that means no other alternatives are to be evaluated in the inner-most frame. In Prolog, each predicates defines also a frame. To further extend my experiment, I wanted to implement my State Machine as a Monad Transformer. But there, even if it's working, I cannot anymore work with infinite choice points :( So I was looking for some existing implementation, to try understand what I did and even if it is possible. Thanks, Pierre -- Pierre Barbier de Reuille INRA - UMR Cirad/Inra/Cnrs/Univ.MontpellierII AMAP Botanique et Bio-informatique de l'Architecture des Plantes TA40/PSII, Boulevard de la Lironde 34398 MONTPELLIER CEDEX 5, France tel : (33) 4 67 61 65 77 fax : (33) 4 67 61 56 68

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Re: [Haskell-cafe] assistance with using haskell to calculate the determinant
by Carter Tazio Schonwald 27 Mar '05

27 Mar '05

Thanks for the feeback regarding good programming style and pointing out the logical error (which I just realized this morning). --Carter Schonwald

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Question on leazy evaluation
by Pierre Barbier de Reuille 26 Mar '05

26 Mar '05

Hello, I'm very new to Haskell and I'm learning it in my spare time (ie. just for fun ;) ). I already had a quick look through tutorials, some articles, the Wiki, ... My question is: is there some (simple?) rule/criteria to ensure leazy evaluation will be efficient ? Typically, I process a list and I know each list item can be processed as it come. But I'm not sure my code uses correctly this property ! Is there some way to be sure leazy evaluation is correctly used by my code ? And are there coding rules that will "ensure" correct leazy evaluation ? Thanks, Pierre -- Pierre Barbier de Reuille INRA - UMR Cirad/Inra/Cnrs/Univ.MontpellierII AMAP Botanique et Bio-informatique de l'Architecture des Plantes TA40/PSII, Boulevard de la Lironde 34398 MONTPELLIER CEDEX 5, France tel : (33) 4 67 61 65 77 fax : (33) 4 67 61 56 68

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Re: [Haskell] URLs in haskell module namespace
by Dimitry Golubovsky 24 Mar '05

24 Mar '05

Dear list members, I'd like to share some sketchy ideas I have on the subject to address some of the issues raised. At 12:14 22/03/05 +0000, Malcolm Wallace wrote: >I cannot see any of the Haskell compilers ever implementing this idea >as presented. It would introduce an enormous raft of requirements >(networking client, database mapping, caching, etc) that do not belong >in a compiler - they belong in separate (preprocessing/packaging) >tools. Furthermore, these tools already exist, albeit they are young >and have a long maturation process still ahead of them. An external program acting as an "URI streamer" might be the solution. Such a program (identified via an environment variable or a compiler's command line option, just like Hugs external editor) would take an URI as its command line argument, and respond with a streamed contents of that URI on its stdout. Like e. g. curl/wget -O -. All the compiler has to do is popen() that program if an import statement contains an URI. Using curl/wget helps get around various issues with proxies/encryption/etc. as those programs are specifically designed for that. I do not believe this would result in significant overhead comparing to regular fopen() used by the compiler for opening source files. On a non-networked system, such a program would be a not so complicated shell script pretending it downloads from an URI, but reading from local disk (flash, any other kind of storage) instead. To address the problem of module/package URI changes over time, the following may be suggested. Either purl.org is used (and then it is responsibility of a package maintainer to keep its URL pointer valid). Or, some kind of a purl server may be set up somewhere (at haskell.org for example) which also supports mirroring. This means that for each package/module registered with this server, multiple locations are known (well, probably willing people might allocate their computer resources for that, at least I would not object as long as I have my own web server). The hypothetical purl server serves redirects as usual, but shifting randomly to another mirroring location for each new request for a module/package. So, if an attempt to retrieve a module/package fails, it may be repeated, and other mirror location will be tried. Mirrors will be synchronized behind the scenes. Will such a centralized purl server a bottleneck or a single point of failure? Probably not more than a centralized Hackage database (or is it planned to be distributed?) Also, some resolver might be part of the URI streamer which maps module names to URIs. For example, the Prelude will most likely be stored locally, but some other module will not. This means that the resolver consults the local package database (cabal), and its own cache, and either streams a local file or popens curl with some URI constructed specifically based on the desired module name. Once downloaded, a module may be cached with some TTL, so further recompilations do not result in curl involvement (until the TTL expires). PS This all is written in the assumption that Haskell source files are served. Binary distributions, of course would require different techniques. -- Dimitry Golubovsky Anywhere on the Web

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QuickChecking classes
by Benjamin Pierce 24 Mar '05

24 Mar '05

Today I've been learning about the wonders of QuickCheck. :-) One (obvious?) question that is not addressed in at least the original paper is how one might use QC to automatically verify the algebraic laws that are often associated with type classes. E.g., could we add some QC properties as members of the Functor class and somehow arrange (probably by some combination of automation and idiom) that they would be checked every time somebody creates an instance of Functor? Regards, - Benjamin

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Re: [Haskell] URLs in haskell module namespace
by Graham Klyne 23 Mar '05

23 Mar '05

Switching to -cafe... At 12:14 22/03/05 +0000, Malcolm Wallace wrote: > > import http://domain.org/package-1.0.cabal#Network.HTTP as HTTP > > import http://hage.org/package-2.1.cabal#Network.HTTP as HTTP2 > > --note use of HTTP fragment identifier for module name > >I cannot see any of the Haskell compilers ever implementing this idea >as presented. It would introduce an enormous raft of requirements >(networking client, database mapping, caching, etc) that do not belong >in a compiler - they belong in separate (preprocessing/packaging) >tools. Furthermore, these tools already exist, albeit they are young >and have a long maturation process still ahead of them. Well, I'd agree the networking stuff don't belong in the compiler. In the operating system, I'd suggest, as part of the I/O and file system. In the long, I believe the distinction between local and networked resources will fade away (other than for practical purposes, as today with specific networked file systems), and URIs are an effective and standardized way to identify all kinds of resources. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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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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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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Functional Reactive Programming
by Andrew Cheadle 17 Mar '05

17 Mar '05

Hi Please excuse the broadcast! Does anyone know where I can get hold of the FRP software bundle from Yale that pre-dates AFRP and Yampa, i.e with behaviours and clocked events. We're trying to resurrect some software and want to start with a working code base before migrating it to AFRP. Many thanks Andy ********************************************************************* * Andrew Cheadle email: a.cheadle(a)doc.ic.ac.uk * * Department of Computing http://www.doc.ic.ac.uk/~amc4/ * * Imperial College * * University of London * *********************************************************************

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Best practices for modular programming in Haskell
by Benjamin Pierce 17 Mar '05

17 Mar '05

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. Naturally, as I've gotten into the Haskell groove I've discovered (or learned from reading other people's code) nice ways of doing *some* of the things that I was used to doing with OCaml modules, but there are still things that I miss. This leads me to wonder how expert Haskell programmers address these issues. For example, here's my biggest current puzzle: What is the best way to expose the *interface* of a module (what would go in the .sig file in ML) to programmers that need to know how to use it? My impression is that there is no universally accepted "best way" to do this. Some people write their modules in the form: module Foo (<exported names>) where <type declarations for exported names> ------------------------------------------------------------- line of dashes <definitions of exported names (and other stuff)> 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. I suppose one could do something like module Foo (T,x) where type T = T_internal x :: T ------------------------------------------------------------- type T_internal = Int x = 5 but it seems clunky. 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). Are there other (perhaps better) ways that I'm not aware of? Thanks, Benjamin

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Re: (Newbie) Dynamic Programming, Memoizing Etc.
by oleg＠pobox.com 17 Mar '05

17 Mar '05

Bryce Bockman wrote: > How would you guys memoize the following code. > > simpleCalc :: (Int,Int) -> (Int,Int) > simpleCalc (1,l) = (1,l+1) > simpleCalc (x,l) | (odd x) = simpleCalc (((3*x) + 1), 1 + l) > | otherwise = simpleCalc ((x `div` 2), 1 + l) > > sCalc x = simpleCalc (x,0) > > sCalcListRange a b = map sCalc [a..b] > > sCalcListLengthRange a b = map snd (sCalcListRange a b) > > The key is I need to calculate maximum (sCalcListLengthRange 1 1000000). One observation is that `l' is merely `an iteration counter'. Therefore, the code can be simplified. The other observation is that memoizing all intermediate results may be expensive: the memoization table (a list, for example) will be big and sparsely populated. The cost of indexing in such a list can be significant. Here's a solution that seems to be balancing both costs. One can change the balance by modifying sc_upb. > module Foo where > > import Data.Array > > sc_upb = 100 > sc = listArray (1,sc_upb) $ map sc_calc [1..] > > sc_get i | i <= sc_upb = sc ! i > sc_get i = sc_calc i > > sc_calc :: Int -> (Int,Int) > sc_calc 0 = (0,0) > sc_calc 1 = (1,1) > sc_calc x | odd x = let (r,c) = sc_get (3*x + 1) in (r, c+1) > sc_calc x = let (r,c) = sc_get (x `div` 2) in (r, c+1) > > sCalcListLengthRange' a b = map (snd . sc_calc) [a..b] *Foo> maximum $ sCalcListLengthRange 1 10000 262 (1.57 secs, 82200088 bytes) *Foo> maximum $ sCalcListLengthRange' 1 10000 262 (1.18 secs, 61043084 bytes) there seems to be a benefit.

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Typeclasses and instances
by Mark Carroll 17 Mar '05

17 Mar '05

If I have, newtype Floating a => Vector a = Vector [a] if I want to make it an instance of Functor (with the obvious meaning), how do I write that? Thanks, Mark -- Haskell vacancies in Columbus, Ohio, USA: see http://www.aetion.com/jobs.html

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(Newbie) Dynamic Programming, Memoizing Etc.
by Bryce Bockman 16 Mar '05

16 Mar '05

How would you guys memoize the following code. simpleCalc :: (Int,Int) -> (Int,Int) simpleCalc (1,l) = (1,l+1) simpleCalc (x,l) | (odd x) = simpleCalc (((3*x) + 1), 1 + l) | otherwise = simpleCalc ((x `div` 2), 1 + l) sCalc x = simpleCalc (x,0) sCalcListRange a b = map sCalc [a..b] sCalcListLengthRange a b = map snd (sCalcListRange a b) The key is I need to calculate maximum (sCalcListLengthRange 1 1000000). This is a common programming contest problem that I'm playing with for fun. My first attempt fails miserably. It begins to thrash much more quickly then simple version above: calcListLengthRange a b = map snd (calcListRange a b) calcListRange a b = map calcList [a .. b] where calcList = ((map calc' [0 ..]) !!) where calc' :: Int -> (Int,Int) calc' i = calcList' (i,0) calcList' :: (Int,Int) -> (Int,Int) calcList' (1,l) = (1,l+1) calcList' (x,l) | (odd x) = ((fst((calcList ((3*x) + 1))), snd(calcLi\ st((3*x) + 1)) + 1 + l)) | otherwise = ((fst((calcList (x `div` 2))), snd(calcLi\ st((x `div` 2))) + 1 + l)) I tried to associate a table entry with a particular solution to the problem, so taht calcList would just be a lookup for those we had finished calculating. Sorry this code is ugly I know, but I'm just learning. Cheers, Bryce

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What is MonadPlus good for?
by Benjamin Pierce 16 Mar '05

16 Mar '05

I have seen lots of examples that show how it's useful to make some type constructor into an instance of Monad. Where can I find examples showing why it's good to take the trouble to show that something is also a MonadPlus? (I know there are many examples of things that *are* MonadPluses; what I want to know is why this is interesting. :-) Thanks, - Benjamin

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RE: [Haskell-cafe] invalid character encoding
by Simon Marlow 16 Mar '05

16 Mar '05

On 16 March 2005 03:54, Ian Lynagh wrote: > On Tue, Mar 15, 2005 at 10:44:28AM +0000, Ross Paterson wrote: >> On Mon, Mar 14, 2005 at 07:38:09PM -0600, John Goerzen wrote: >>> 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? >> >> A Haskell 98 Handle is a character stream, and doesn't support binary >> I/O. This would have bitten you sooner or later on systems that do >> CRLF conversion, but Hugs is now much stricter, because character >> streams now use the encoding determined by the current locale (for >> the C locale, that means ASCII only). > > Do you have a list of functions which behave differently in the new > release to how they did in the previous release? > (I'm not interested in changes that will affect only whether something > compiles, not how it behaves given it compiles both before and after). > > Simons, Malcolm, are there any such functions in the new ghc/nhc98? > > Also, are you all agreed that the hugs interpretation of the report is > correct, and thus ghc at least is buggy in this respect? (I'm afraid I > haven't been able to test nhc98 yet). GHC (and nhc98) assumes a locale of ISO8859-1 for I/O. You could consider that to be a bug, I suppose. We don't plan to do anything about it in the context of the current IO library, at least. Cheers, Simon

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re parser question
by Carter Tazio Schonwald 16 Mar '05

16 Mar '05

term :: Parser Int term = do f <- factor do symbol "*" e <- expr return (f * t) <--------------- replace t with e +++ return f I hope that helps, Carter Schonwald

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wxHaskell htmlWindow usage
by David Owen 15 Mar '05

15 Mar '05

Good day all, I am attempting to display html pages using wxHaskell's HtmlWindow functions. I am hoping that it is indeed possible for me to do just that and that I haven't misunderstood what HtmlWindow is used for. My code so far will create a frame, a button and an HtmlWindow and other assorted things. I would like an html file to be displayed in the window when the button is clicked. At the moment, only the path (a string) of the html file that I would like to display is printed in the window when the button is clicked. Here is some of the code i've been trying to get to do this: do -- the main frame f <- frame [text := "Browser", fullRepaintOnResize := False] -- create html window hw <- htmlWindowCreate f 1 (Rect 50 150 50 150) 5 "theWindow" -- 'load page' button go <- button f [text := "Load Page", on command := htmlWindowSetPage hw "c:/example.html"] (I've left out the layout part here) htmlWindowSetPage might be the wrong function to use for this or I might just be using it incorrectly. This may well be more complicated than I'm hoping it is too! Thankyou to whoever has a look at this. bye all David

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RE: [Haskell-cafe] Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

Thanks folks! Writing it in a lispy manner seems to work. I see what Arthur means about the layout - I think I'm still thinking too much in C. :) Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

Curiouser and curiouser... expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t + e) +++ return t solves the undefined variable problem but introduces a new 'Last operator in do {...} must be an expression' error, which then disappears if I explicitly return e expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return e return (t + e) +++ return t to give me the original undefined variable t error at the line expr = do t <- term . It looks in scope to me... :( Thanks, Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: [Haskell-cafe] Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

perhaps like this: > expr = do t <- term > (do symbol "+" > e <- expr > return (t+e) > ) > +++ > (return t) > > >although I think you may also want a 'try' before the first alternative. No, that still gives the same undefined variable error. :( Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: [Haskell-cafe] wxHaskell htmlWindow usage
by Bayley, Alistair 15 Mar '05

15 Mar '05

> From: David Owen [mailto:d_w_o@hotmail.com] > > htmlWindowSetPage might be the wrong function to use for this > or I might > just be using it incorrectly. This may well be more > complicated than I'm > hoping it is too! I-Am-Not-A-WxHaskell-Expert, but... a quick google finds this page: http://www.wxwidgets.org/manuals/2.4.2/wx499.htm#wxhtml ... where is is explained that wxHtmlWindow.LoadPage is used to load a page from a file, while wxHtmlWindow.SetPage is used to display a literal HTML string in the widget. And in the wxHaskell docs there are two matching functions: - htmlWindowSetPage - htmlWindowLoadPage which probably correspond. So, I guess you want to use htmlWindowLoadPage. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Parity of the number of inversions of a permutation
by Henning Thielemann 15 Mar '05

15 Mar '05

I think it is a quite popular problem. I have a permutation and I want to count how often a big number is left from a smaller one. More precisely I'm interested in whether this number is even or odd. That's for instance the criterion to decide whether Lloyd's shuffle puzzle is solvable or not. Example: 1 4 3 2 I can choose six pairs (respecting the order) of numbers out of it, namely (1,4) (1,3) (1,2) (4,3) (4,2) (3,2), where in the last three pairs the first member is greater than the second one. Thus I have 3 inversions and an odd parity. I' searching for a function which sorts the numbers and determines the parity of the number of inversions. I assume that there are elegant and fast algorithms for this problem (n * log n time steps), e.g. a merge sort algorithm. A brute force solution with quadratic time consumption is countInversions :: (Ord a) => [a] -> Int countInversions = sum . map (\(x:xs) -> length (filter (x>) xs)) . init . tails

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Parser question
by Nicola Whitehead 15 Mar '05

15 Mar '05

Hi folks, I have a parser problem. I have a basic calculator program (Graham Hutton's from Nottingham) which contains the following code: -- Define a parser to handle the input expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t + e) +++ return t term :: Parser Int term = do f <- factor do symbol "*" e <- expr return (f * t) +++ return f factor :: Parser Int factor = do symbol "(" e <- expr symbol ")" return e +++ natural symbol and natural are defined elsewhere and work fine, but when I compile it I get the error ERROR "C:/HUGS/Calculator.hs":66 - Undefined variable "t" I suspect I'm missing something obvious, but for the life of me I can't see it. Any suggestions? Thanks, Nik (Trying to keep a couple of weeks ahead of her students) Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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