- Haskell-Cafe - Haskell.org

The Haskell Platform
by Galchin, Vasili 01 Oct '08

01 Oct '08

Hello, I probably missed some details for which I apologize. My feeling is that "periodically" the "haskell platform server" should attempt to rebuild the Haskell library. Any library that fails to rebuild then the "maintainer" of that library should be notified, e.g. email, pager(;^)), .... Kind regards, Vasili

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Haskell, GHC, and ABI
by Sean Leather 01 Oct '08

01 Oct '08

> Dynamic linking doesn't solve all the problems, we still have the problem > that GHC does a lot of cross-module inlining, regardless of whether dynamic > linking is used. However, I really would like to have a way to have > complete control over what is exposed across a package boundary. We need > this not just for licensing reasons, but also for making a dynamic library > with a fixed ABI, so it can be upgraded later. > > Incedentally the lack of this feature is one reason I've not being rushing > to get shared libraries into GHC. They're just not that useful unless you > can upgrade a library independently of the things it depends on. > I completely agree. Having a fixed ABI that survives library changes and compiler upgrades is an important factor in achieving a useful system. Incidentally, I perused http://hackage.haskell.org/trac/ghc/wiki/DynamicLinking and didn't see much about this here. Is there some notes or a plan for reaching this goal with GHC (or any other Haskell compiler)? Sean

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Re: [Haskell-cafe] Version 0.4.3 of happs-tutorial is a HAppS job board, done in HAppS.
by Thomas Hartman 01 Oct '08

01 Oct '08

Oh, the whole point of this was to put preview of the latest chapters I've been working on. The new stuff is at http://happstutorial.com:5002/ So far there's an introduction to macid, macid data safety and backups, and using utf8 data with happs. I'm particularly intereted in hearing what the happs devs and others have to say on the topic of keeping data safe: http://happstutorial.com:5002/tutorial/macid-data-safety Thomas. 2008/9/30 John Melesky <john(a)phaedrusdeinus.org>: > Thomas, > > The tutorial site seems to be unreachable or otherwise down. > > I know the modern programming landscape is fast-moving, HAppS is new and > constantly changing, so i probably should have grabbed the content when you > posted it, but perhaps keeping it up for more than a day and a half might be > good? :-) > > -john melesky > > > On Sep 28, 2008, at 6:36 PM, Thomas Hartman wrote: > >> Hello, world. >> >> In Version 4 of the ongoing self-demoing HAppS Tutorial, we implement >> a HAppS job board using HAppS. >> >> demo: http://happstutorial.com:5001 >> >> install: sudo cabal install happs-tutorial >> >> darcs head: darcs get http://code.haskell.org/happs-tutorial >> >> There aren't any new lessons compared to the last release, but the >> code is much cleaner, and... well... the web site actually does >> something. >> >> Coming soon, lessons on form processing and HAppS State. >> >> Enjoy! >> >> Thomas. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Re: [Haskell-cafe] Class Quantification
by Bas van Dijk 01 Oct '08

01 Oct '08

On Wed, Oct 1, 2008 at 3:01 AM, Reiner Pope <reiner.pope(a)gmail.com> wrote: > I believe there is no way to simply express this "abstraction over classes", > but the Scrap your boilerplate with class[1] paper discusses this same > problem and present a workaround by defining the class's dictionary of > methods as an explicit type. > > What follows is the code to implement their workaround for your example. > > First some fairly standard extensions: > >> {-# LANGUAGE Rank2Types, EmptyDataDecls, FlexibleInstances, KindSignatures >> #-} > > And some more controversial, but necessary ones: > >> {-# LANGUAGE UndecidableInstances, OverlappingInstances #-} >> >> module Cls where >> > > The working bla function. Unfortunately, the "pseudoclass" cls needs > to have its type explicitly named, hence the (uninhabited) Proxy type. > >> bla :: forall cls a c d. (Sat (cls c), Sat (cls d)) => Proxy cls -> >> (forall b. Sat (cls b) => a -> b) -> a -> (c,d) >> bla _ f x = (f x, f x) > > Again, testFoo and testBar unfortunately have to name which dictionary type > to use, via the Proxy. > >> testFoo = bla (undefined :: Proxy NumD) fromInteger 1 :: (Int,Float) >> testBar = bla (undefined :: Proxy ReadD) read "1" :: (Int,Float) > > The Sat class, straight from SYB: > >> class Sat a where >> dict :: a >> data Proxy (cxt :: * -> *) > > The explicit dictionary construction for the Read class: > >> data ReadD a = ReadD { readsPrecD :: Int -> ReadS a } >> instance (Read a) => Sat (ReadD a) where >> dict = ReadD readsPrec >> instance (Sat (ReadD a)) => Read a where >> readsPrec = readsPrecD dict > > The explicit dictionary construction for the Num class: > >> data NumD a = NumD { plusD :: a -> a -> a, >> timesD :: a -> a -> a, >> negateD :: a -> a, >> absD :: a -> a, >> signumD :: a -> a, >> fromIntegerD :: Integer -> a } >> instance (Num a) => Sat (NumD a) where >> dict = NumD (+) (*) negate abs signum fromInteger > > We define these fake Eq,Show instances just to make the > Num instance valid. It would be longer, but not more > difficult, to genuinely encode the (Eq,Show)=>Num hierarchy. > >> instance (Sat (NumD a)) => Show a where {} >> instance (Sat (NumD a)) => Eq a where {} >> instance (Sat (NumD a)) => Num a where >> (+) = plusD dict >> (*) = timesD dict >> negate = negateD dict >> abs = absD dict >> signum = signumD dict >> fromInteger = fromIntegerD dict > > [1] http://homepages.cwi.nl/~ralf/syb3/ Sections 3.2 and 4.1 > > On Wed, Oct 1, 2008 at 9:01 AM, Bas van Dijk <v.dijk.bas(a)gmail.com> wrote: >> >> On Tue, Sep 30, 2008 at 11:25 PM, Sean Leather <sean.leather(a)gmail.com> >> wrote: >> > But perhaps you're looking for potentially unknown classes? >> >> Yes indeed. >> >> Thanks, >> >> Bas >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > Nice! The explicit passing of the proxy type is indeed unfortunate but it's nice that the type of your 'bla' is almost identical to mine: > bla :: forall cls a c d. (Sat (cls c), Sat (cls d)) => Proxy cls -> (forall b. Sat (cls b) => a -> b) -> a -> (c,d) > bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a -> (c, d) Thanks, Bas

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tutorial site back up Re: [Haskell-cafe] Version 0.4.3 of happs-tutorial is a HAppS job board, done in HAppS.
by Thomas Hartman 01 Oct '08

01 Oct '08

Well, at least I know people are trying to read the tutorial :) I'll say a few words about what happened. The underlying problem is that HAppS dies unpredictably because of problems in the haskell network library, a but I reported at http://code.google.com/p/happs/issues/detail?id=40 I wrote a cron job to restart my server when it dies, but I recently tweaked the job in order to serve the "head" version as well as what is on hackage, and the tweak broke the cron job's functionality. (fixed now -- at least, I hope it's fixed.) To be honest I don't really understand the underlying problem in the haskell network library, but it would be great if somebody who does could fix this. Thomas. 2008/9/30 John Melesky <john(a)phaedrusdeinus.org>: > Thomas, > > The tutorial site seems to be unreachable or otherwise down. > > I know the modern programming landscape is fast-moving, HAppS is new and > constantly changing, so i probably should have grabbed the content when you > posted it, but perhaps keeping it up for more than a day and a half might be > good? :-) > > -john melesky > > > On Sep 28, 2008, at 6:36 PM, Thomas Hartman wrote: > >> Hello, world. >> >> In Version 4 of the ongoing self-demoing HAppS Tutorial, we implement >> a HAppS job board using HAppS. >> >> demo: http://happstutorial.com:5001 >> >> install: sudo cabal install happs-tutorial >> >> darcs head: darcs get http://code.haskell.org/happs-tutorial >> >> There aren't any new lessons compared to the last release, but the >> code is much cleaner, and... well... the web site actually does >> something. >> >> Coming soon, lessons on form processing and HAppS State. >> >> Enjoy! >> >> Thomas. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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postmortem question about xmonad
by minh thu 01 Oct '08

01 Oct '08

Hi, I'd like to know, now that time got by a bit, what the writers of the X monad think about the use of the ReaderT/WriterT/IO brought to them (to isolate Configuration data and dynamic data and glue them together with IO). Are you happy of it, did it make things easier or not, would you do it again ? Thanks a lot, Thu

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state monad and continuation monads ...
by Galchin, Vasili 01 Oct '08

01 Oct '08

Hello, I would like to read 1) pedagogical examples of State monad and the Continuation monad 2) library usage of these monads .... Regards, Vasili

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ANNOUNCE: graphviz-2008.9.20
by Ivan Miljenovic 01 Oct '08

01 Oct '08

The latest version of Matthew Sackman's Haskell bindings to Graphviz [1] are now available on Hackage [2]. The reason there's a new release only two weeks after the previous one is that I've made some extensions to it (hence why I'm writing the announcement) that Matthew has kindly included. Since Matthew doesn't recall writing an announcement for graphviz before, here is a brief synopsis of what it does. The Graphviz program is probably _the_ way of drawing graphs (note: this is graph-theory graphs, not function plotting). As it stands, there are currently at least four different Haskell bindings available for Graphviz that I've managed: * The inbuilt Graphviz module in FGL [3] * graphviz (which this announcement is for) * dotgen [4] * A really simple generator by Duncan Coutts [5] In addition to this, the following utilities on Hackage output graphs in Graphviz's .dot format (either using one of the above libraries or their own parser): * prof2dot : converts profiling information to .dot [custom, I think] [6] * flow2Dot : convert textual descriptions to .dot [also custom, I think] [7] * graphmod : draw the dependencies between Haskell modules [uses dotgen] [8] As it can be seen, there's a plethora of possible ways of creating .dot graphs in Haskell. What seperates the graphviz package from the others is: * It uses FGL graphs, rather than passing through lists of nodes and edges manually (of course, if you're not doing any other graph-related activity you might not want to use an FGL graph), whilst providing more control than the default FGL module. * A large list of attributes that can be used are available: http://hackage.haskell.org/packages/archive/graphviz/2008.9.20/doc/html/Dat… * A "sane" interface that provides a large degree of customizability (don't specify the attributes manually for each node/edge/etc., just pass through a function that will create the attribute you want). * Limited parsing of .dot format (note that as yet it can't convert a parsed DotGraph into an FGL graph). * The graphToGraph function allows you to pass a graph through Graphviz and then extract out positional information and combine it with the original graph. There are some things things that graphviz can't do, such as drawing undirected graphs and clusters (which dotgen can)... at least until now. The changes that I have made provide the additional functionality to graphviz: * Differentiate between undirected and directed graphs. Whilst FGL represents undirected graphs with a directed graph by duplicating all edges (an undirected edge {1,2} is represented by the two edges (1,2) and (2,1)), you don't really want to draw a graph this way. Furthermore, many reports say that graphviz's "neato" command is better at drawing undirected graphs than the normal "dot" command is. Thus, graphviz will now draw only one edge out of every directed pair. To do so, however, requires that the edge labels are an instance of Ord: if this is a problem for you, please contact either Matthew or myself and we'll see what else we can do (this is required because it's assumed that if two edges (1,2) and (2,1) are meant to represent the undirected edge {1,2}, then their labels must be the same). * Add clustering support. Graphs can be drawn (but as yet not parsed) to provide nested clustering support to arbitrary depth. To do so, you need to provide a function (LNode a -> NodeCluster c a), where NodeCluster is the following recursive data type that indicates in which subcluster a node in the graph belongs: data NodeCluster c a = N (LNode a) | C c (NodeCluster c a) The c type represents the cluster label, and is the parameter by which cluster attributes are assigned. Note that c has to be an instance of Ord. Clusters are not parseable, because there's no clear way of how to convert a cluster to an FGL graph. The interface for graphviz remains unchanged, so you can safely upgrade to this version. Here is an example function of how the library can be used to draw an FGL graph to a (very plain) Graphviz graph (well, it will convert it to the DotGraph datatype, which when shown produces the .dot graph code): graphviz :: (Graph g, Show a, Ord b, Show b) => String -> g a b -> DotGraph graphviz title g = graphToDot g attrs nattrs eattrs where attrs = [Label title] nattrs (_,a) = [Label (show a)] eattrs (_,_,b) = [Label (show b)] Enjoy! [1] http://graphviz.org/ [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphviz [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fgl [4] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dotgen [5] http://haskell.org/~duncan/WriteDotGraph.hs [6] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/prof2dot [7] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/flow2dot [8] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphmod -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] Class Quantification
by Bas van Dijk 30 Sep '08

30 Sep '08

On Tue, Sep 30, 2008 at 11:25 PM, Sean Leather <sean.leather(a)gmail.com> wrote: > But perhaps you're looking for potentially unknown classes? Yes indeed. Thanks, Bas

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Re: [Haskell-cafe] System.Process
by Donn Cave 30 Sep '08

30 Sep '08

Quoth David Roundy <droundy(a)darcs.net>: ... | My point was that which of these is correct entirely depends on what | your input is. It's often a huge (and security-bug-prone) chore to | escape all those shell characters, and so in many cases it's much | nicer to execute an external program direcly without invoking a | shell. That's true, and in the other cases - when you want the shell to process the command line - you should be able to invoke it. I'm not looking at the source code (nor even running my example, since I don't have ghc handy), but the basic difference is not "runCommand parses the arguments", but "runCommand [only] invokes the shell". If you want runProcess to invoke the shell: runProcess "/bin/sh" ["-c", commandLine] ... (... ought to be ["sh", "-c" commandLine], but the way I remember it ghc gratuitously prevents you from specifying argv 0.) Donn Cave, donn(a)avvanta.com [I just sifted this thread out of ca. 300 spams and stuff, so please pardon me if this has already been mentioned in another followup.]

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