January 2009 - Haskell-Cafe

Monades - I've got it! (hopefully)
by Tobias Kräntzer 28 Jan '09

28 Jan '09

Hi, I think I now understand all the stuff about monades (not the mathematics). I have just one further question. Beside of the syntactic sugar of the "do" construct and the fact that the IO monade is an "internal type of haskell": Is haskell aware of the concept of monades? As I now understand it, it is "just" a programming model rather than a concept of the language. If not, I have to start new. One problem understanding monades could be, that everywhere is written, that it is so complicated and therefore I was a bit prestressed. . . . Tobias By the way: Merry Christmas! -- Tobias Kräntzer info(a)tobias-kraentzer.de

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ANN: data-spacepart - space partitioning data structure[s] (initial release)
by Corey O'Connor 28 Jan '09

28 Jan '09

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-spacepart The goal of this package is to be a collection of space partitioning data structures. Currently, however, there is only a simple quadtree implementation. The package neither has real documentation or tests I'd like for a quality release. However, if you need a quadtree implementation then hopefully this will help. Cheers, Corey O'Connor

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Text.Xhtml & output encoding
by akillah 27 Jan '09

27 Jan '09

Can Text.Xhtml output some specific encoding ? Only thing I'm able to get out of it is text literally filled with trigraphs, which strikes me as ugly and is longer than ie. UTF-8 formatted output. I've found that it had been recommended to sb. to use latin-1 as output encoding (question was concerning diacritical characters) so i'm guessing that the answer is no ... Am I mistaken ? Is there another lib (preferrably XHTML compliant) that can acomplish it ? Would using primHtml to incorporate properly encoded Strings be frowned upon ? Thanks, akillah.

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Type families are awesome
by John Ky 27 Jan '09

27 Jan '09

Hi Haskell Cafe, I'm finding that I really like type families. For instance, the GHC.List.lookup and Data.Map.lookup functions annoy me because their names clash, yet their type are so similar. With type families, I could define a more generic lookup function like this: import Data.Map as MAP import GHC.List as LIST class MapType ma where type Key ma type Item ma lookup :: Key ma -> ma -> Maybe (Item ma) instance (Ord ka) => MapType (MAP.Map ka a) where type Key (MAP.Map ka a) = ka type Item (MAP.Map ka a) = a lookup ka ma = MAP.lookup ka ma instance (Eq ka) => MapType [(ka, a)] where type Key [(ka, a)] = ka type Item [(ka, a)] = a lookup ka ma = LIST.lookup ka ma This lookup function works on both "Map ka a" and "[(ka, a)]" types and I no longer need to qualify my lookup function with the module name. The downside I suppose is that lookup is no longer a function that can be manipulated freely: *Main> let x = lookup *Main> let y = Fx.Data.Map.lookup <interactive>:1:8: Ambiguous type variable `ma' in the constraint: `Fx.Data.Map.MapType ma' arising from a use of `Fx.Data.Map.lookup' at <interactive>:1:8-25 Probable fix: add a type signature that fixes these type variable(s) A shame that. I had been hoping it would be possible to have a generic lookup function that could be used in every way the current collection of various lookup functions can be used. So much nicer if 'y' above could be bound to the Fx.Data.Map.lookup with the same type: *Main> :t Fx.Data.Map.lookup Fx.Data.Map.lookup :: forall ma. (Fx.Data.Map.MapType ma) => Fx.Data.Map.Key ma -> ma -> Maybe (Fx.Data.Map.Item ma) And then have the ambiguity resolve later when 'y' is actually used. -John

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Fwd: [Haskell-cafe] Haskell Web Framework
by Alberto G. Corona 27 Jan '09

27 Jan '09

Sorry, I forgot to forward it to the list: ---------- Forwarded message ---------- From: Alberto G. Corona <agocorona(a)gmail.com> Date: 2009/1/25 Subject: Re: [Haskell-cafe] Haskell Web Framework To: Donnie Jones <donnie(a)darthik.com> hi, I´m working in a web application rather than a web framework. But I sometimes think about how a complete web application server should be. For my case, I don´t care about the presentation, because HSP is more than enough. However, I need active-active clustering, distributed transactions and workflows. I have created the module Workflow for the latter. For the former, I developped TCache, that is a transactional cache using STM, with configurable persistence. It´s like hibernate for Java and (the data part of) Rails for Rubi. Now I´m working in a extension of TCache with distributed transactions and remote clustering with some additonal interesting characteristics. My own view about haskell and Web applications is that something like " a web framework" is not in the philosophy of Haskell. What is in the philosophy of Haskell is the creation of modules that the people can combine to create their own web frameworks. 2009/1/25 Donnie Jones <donnie(a)darthik.com> Hello Michael, > > I am interested in contributing to a Haskell web framework. :) > > I have started an attempt to create my own Haskell web framework, but I got > busy working on my Master's thesis research, so I had to stop working on > it... I was a web developer at a start-up company for 1.5yrs with Ruby on > Rails, and I really really like the MVC design; however, I prefer a stricter > MVC implementation than Rails provides. More specifically, I don't think > code should appear in the view at all. I believe that using html tags, the > web framework can provide a function to "render" the view which would parse > the html tags and insert the content (whatever it may be). Functions can be > easily composed together to build more complex content in the views. For > example, > > <person id="name"></person> > <person id="age"></person> > > person_name = > renderView "person" "name" "John Doe" > > person_age = > renderView "person" "age" "40" > > I can show you my code which implements this parsing in a more structured > manner, but I think this stricter MVC implementation leads to more > maintenable projects and especially facilitates a graphic designer to work > independent of the web developer since the graphic designer won't be > confused by the code in the views. > > What do you think? > -- > Donnie Jones > > 2009/1/25 Michael Snoyman <michael(a)snoyman.com> > >> I'm interested on starting a project with others to create a powerful >> Haskell web framework in the same league as Rails or Django. I've enumerated >> (perhaps ad nauseum) my ideas for it in this blog post: >> http://blog.snoyman.com/2009/01/25/haskell-web-framework/. If people are >> interested in this, please respond to me either directly or on this mailing >> list. >> >> >> Just as a quick summary of the post, I would say the most salient points >> are that the framework should work in a shared hosting environment and >> should automatically abstract away most of the issues of writing an Ajax web >> application. It should also leverage the strengths of Haskell, eg type >> safety and speed. >> >> I welcome all comments and critiques as well. >> >> Michael >> >> _______________________________________________ >> 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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Gentle introduction questions / comments
by Matthijs Kooijman 27 Jan '09

27 Jan '09

Hi all, I've been reading the gentle introduction to Haskell a bit more closely today and there a few things which I can't quite understand (presumably because they are typo's). I've found two issues with the "Using monads" section [1]. Not sure if this is the right place to report this, but there's probably somewhere here who can fix it :-) The first one is in the state monad data type. It reads: data SM a = SM (S -> (a,S)) -- The monadic type I can't seem to find out what "S" is exactly here. It seems to be a universally quantified type variable, but then it should be lowercase. Considering that it is referred to later on as "s", I suspect this is a type? Further on, a resource monad is described. Here, a lift1 function is given to allow one to lift a normal computation into a resource aware computation, with the following type signature: lift1 :: (a -> b) -> (R a -> R b) A few lines down, the inc operation is defined as follows: inc i = lift1 (i+1) To me, this seems wrong. Even without going into the semantics, it seems that the (i+1) expression does not satisfy the type of the first argument to lift1. I suspect this should have been: inc = lift1 (+1) Gr. Matthijs [1]: http://www.haskell.org/tutorial/monads.html#sect9.3

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Why monoids will abide...
by Don Stewart 27 Jan '09

27 Jan '09

http://apfelmus.nfshost.com/monoid-fingertree.html Thanks Apfelmus for this inspiring contribution!

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ANN: testpack (first release)
by John Goerzen 27 Jan '09

27 Jan '09

Hi, I've just uploaded testpack to Hackage. It's a collection of a few utilities for tests: some tools to convert QuickCheck properties into HUnit test cases, and various shortcuts and tools to increase verbosity while running tests in both QuickCheck and HUnit. It is pulled partly from MissingH (a few modules being split off), and partly from functions I find myself writing over and over again. API docs and downloads at: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/testpack Like convertible, it should be compatible with Hugs, GHC 6.8, and GHC 6.10. -- John

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Ann: Progress with IDE
by Jürgen Nicklisch-Franken 27 Jan '09

27 Jan '09

Leksah 0.4.0* pre-release is now available. This will become the first beta when it is stable enough. The current feature list: * Haskell customized editor with candy * Project management support based on Cabal * Visual editor for Cabal files * Location of compilation errors * Import helper, which writes the import statements * Navigation to definition * Module browser * Information about types and comments * Search for identifiers * Session support * Configurable visual appearance * Configurable keymaps Get more info at: http://www.leksah.org We now have a mailing list for Leksah: http://projects.haskell.org/cgi-bin/mailman/listinfo/leksah You need ghc 6.10 and gtk2hs 0.10 (which is not yet released, but you get it here: http://code.haskell.org/~pgavin/gtk2hs-0.10.0/ ) Please test Leksah and give me feedback. Contributors welcome. Jürgen --keksa leksa haksel--

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foreign function types
by Eyal Lotem 27 Jan '09

27 Jan '09

I have encountered quite a bit of bugs in foreign bindings where "Int" was used in place of "Cint". As far as I know, no foreign function should ever take a Haskell Int, only CInts. Would it be possible to create an empty type-class of which various C-acceptable types are instances (e.g CInt, CString) but Haskell types aren't (Ints)? I am not sure this is the right solution, but I think the problem of accidentally using Int in place of CInt should be solved so that compile-time errors are received, as for x86-32 systems, no compile time or even runtime error will be issued. It will wait and crash when run on an x86-64 system. Eyal

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