September 2008 - Haskell-Cafe

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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Re: [Haskell-cafe] Class Quantification
by Sean Leather 30 Sep '08

30 Sep '08

> Now I would like to generalise 'foo' and 'bar' to 'bla' so that I can > write: > > testBla1 = bla fromInteger 1 :: (Int, Float) > testBla2 = bla read "1" :: (Int, Float) > > My question is how to define 'bla'. > > I can write: > > > bla :: (forall b. a -> b) -> a -> (c, d) > > bla f x = (f x, f x) > > But then 'testBla1' gives the following expected error: > > Could not deduce (Num b) from the context () > arising from a use of `fromInteger' > > [...] > > So, somehow I need to quantify over the type class. > > bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a -> > (c, d) > > But this isn't legal. > > Is there another way of defining 'bla'? > The simple, from-the-gut answer involves the enumeration of all the classes that you need in the context. > bla :: (Read c, Read d, Num c, Num d) => (forall b. (Read b, Num b) => a -> b) -> a -> (c, d) > bla f x = (f x, f x) But perhaps you're looking for potentially unknown classes? Sean

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Distributing Haskell binaries as OSX App Bundles
by Stephen 30 Sep '08

30 Sep '08

I wrote a command-line program recently for a friend in haskell. However, he's far away and not particularly computer literate. I sent him the raw binaries, but they came up with errors about not being able to find libgmp stuff. So then I thought I should probably be able to somehow distribute all the stuff together in a .app bundle. However, the only experience I have of doing this sort of stuff before is via xcode. What's the best way to go about this? It just occurred to me now that maybe cmake is the way to go? Any advice that might save me some time would be really appreciated. Cheers, Stephen

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Haskell versus F#, OCaml, et. al. ...
by Galchin, Vasili 30 Sep '08

30 Sep '08

Hello, Frank mode on ... ;^) In terms of functionality, where is Haskell superior vs inferior to ML, Caml, OCaml, F#, Erlang, etc.? E.g. in terms of library functionality? Regards, Vasili

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GHC, names of inferred type variables
by Shiqi Cao 30 Sep '08

30 Sep '08

Hi, Error messages from GHC contain inferred type variables, is there anyway to find out which term an inferred type variable is for(if the term exists)? Thanks, Shiqi

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Class Quantification
by Bas van Dijk 30 Sep '08

30 Sep '08

Hello, I was writing some Haskell when I stumbled on the following problem: With the following language extension... > {-# LANGUAGE RankNTypes #-} ... it's possible to define 'foo' and 'bar' like so: > foo :: (Num c, Num d) => (forall b. Num b => a -> b) -> a -> (c, d) > foo f x = (f x, f x) > bar :: (Read c, Read d) => (forall b. Read b => a -> b) -> a -> (c, d) > bar f x = (f x, f x) Which allows us to write: > testFoo = foo fromInteger 1 :: (Int, Float) > testBar = bar read "1" :: (Int, Float) Now I would like to generalise 'foo' and 'bar' to 'bla' so that I can write: testBla1 = bla fromInteger 1 :: (Int, Float) testBla2 = bla read "1" :: (Int, Float) My question is how to define 'bla'. I can write: > bla :: (forall b. a -> b) -> a -> (c, d) > bla f x = (f x, f x) But then 'testBla1' gives the following expected error: Could not deduce (Num b) from the context () arising from a use of `fromInteger' Possible fix: add (Num b) to the context of the polymorphic type `forall b. a -> b' In the first argument of `bla', namely `fromInteger' In the expression: bla fromInteger 1 :: (Int, Float) In the definition of `testBla1': testBla1 = bla fromInteger 1 :: (Int, Float) And 'testBla2' gives a similar error complaining that it can't deduce (Read b) from the context. So, somehow I need to quantify over the type class. bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a -> (c, d) But this isn't legal. Is there another way of defining 'bla'? Thanks, Bas

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Re: [Haskell-cafe] not able to compile Happy GLR examples
by brianalexander1＠cableone.net 30 Sep '08

30 Sep '08

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