December 2006 - Haskell-Cafe

Re: Request for a Simple Pretty Printing library
by Doaitse Swierstra 14 Dec '06

14 Dec '06

I am sorry, I might have confused you. It appears that we have just a function for doing what you want; in UU.Pretty.Basic there is a function "invisible". From the manual page I quote: invisible ppd Makes the formatted element invisible (all its attributes are forgotten in order to always succeed, even if there is no space left!) In exceptional cases we want to escape to the page width limit, for example when you want to generate additional tags for markup languages such as LaTeX or HTML: > res_word rw = invisible (pp "{\\bf ") >|< rw >|< invisible (pp "}") and try render (res_word "let") 3. Doaitse On Dec 14, 2006, at 9:57 AM, Christian Maeder wrote: > Doaitse Swierstra schrieb: >> The Prettyprint library you can download from: >> >> http://www.cs.uu.nl/wiki/HUT/Download > > I've looked into your module UU.Pretty.Basic starting with: > > -- $Header: /data/cvs-rep/uust/lib/pretty/UU/Pretty/Basic.hs,v 1.2 > 2003/02/26 11:18:27 uust Exp $ > > And I did not find the type Doc that you describe below. > > Christian > >> and is described on: >> >> http://www.cs.uu.nl/wiki/HUT/PrettyPrintingCombinators >> >> contains amongst others the following code: >> >> data Doc = Empty >> | Char Char -- invariant: char is not >> '\n' >> | Text !Int String -- invariant: text doesn't >> contain '\n' >> | Line !Bool -- True <=> when undone by >> group, do not insert a space >> | Cat Doc Doc >> | Nest !Int Doc >> | Union Doc Doc -- invariant: first lines of >> first doc longer than the first lines of the second doc >> | Column (Int -> Doc) >> | Nesting (Int -> Doc) >> >> text "" = Empty >> text s = Text (length s) s >> >> so if you define now: >> >> wrap l t r = Text 0 l <|> text t <|> Text 0 r >> >> you should be set and done, >> >> Doaitse Swierstra >

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Re: [Haskell] interpreter ouput in color
by Neil Mitchell 14 Dec '06

14 Dec '06

Hi Walter, > Hi all. I was wondering if some people miss the colored output > of some applications, such like the IPython enhanced shell. I've > been googling for similar options for Haskell but I found nothing. WinHugs already colours (to some degree) and hyperlinks error messages. http://www-users.cs.york.ac.uk/~ndm/projects/winhugs.php (screenshot on that page) GuiHaskell does it a lot more, with full syntax colouring of error messages and hyperlinks. Unfortunately it can't be used reliably at the moment due to threading issues with Gtk+Gtk2Hs+GHC+Windows. http://www-users.cs.york.ac.uk/~ndm/projects/guihaskell.php GuiHaskell merely wraps GHCi (and Hugs and other stuff), so allows syntax colouring to be implemented without diving into the GHC source code. Thanks Neil PS. The Haskell-cafe mailing list is better for this sort of question, haskell tends to be more used for announcements, so i've sent the follow up there.

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type variable question
by Nicolas Frisby 14 Dec '06

14 Dec '06

I'm developing a typesafe record library (akin to HList but starting with key/val pairs and enforcing uniqueness and sorted order of keys). I'm having a GHC problem I've had with other projects and seen comments regarding it in other people's code (HList for instance: "GHC doesn't like it's own type" to paraphrase). I have a function |update| > --update :: forall sel val rec val' rec' . > -- SetField sel val rec val' rec' => sel -> (val -> val') -> rec -> rec' > update sel f rec = rec' > where --val :: val > val = value sel rec > --val' :: val' > val' = f val > --rec' :: rec' > rec' = set sel val' rec The commented out signature is the signature that GHC infers for the function. When I uncomment that signature, it will no longer type check. Why does this happen? Even with the forall and the explicit signatures in the where clause, it chokes. I don't plan on sharing the code for |SetField| because it's that wild looking type class hackery, and the problem isn't specific to this example--I remember having it happen in far simpler cases which I can't seem to find right now... Is there a canonical example example that exhibits this behavior? Or a ticket for it already? I'd like to understand what's happening. Thanks for your time, Nick

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Request for a Simple Pretty Printing library
by Neil Mitchell 13 Dec '06

13 Dec '06

Hi, I've been using the HughesPJ pretty printing library, but I need a new combinator: wrap :: String -> Doc -> String -> Doc wrap prepend doc append = ... The semantics of this would be that the text is prepended and appended to the doc when rendered, but does not alter the pretty printing at all. The idea of this is to support printing with HTML rendering: wrap "<span class='keyword'>" (text "case") "</span>" The HughesPJ library is 1000 lines long, so will be quite a lot of work to change this to get it working properly. Is there a simpler pretty printing library, with source code available somewhere? Ideally using the same interface. Of course, if anyone knows how to make this combinator using the existing library, or using another existing library, I'd appreciate it. Thanks Neil

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Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Andrew Wagner 13 Dec '06

13 Dec '06

I think there are some great ideas here, and it would be a fantastic project to do as a community, via a wikibook. I, for one, have been studying haskell for several months, and am just starting to see a little bit of light when it comes to monads. I think it would be beneficial to work through a non-trivial construction of a new monad, and the larger examples given would be good opportunities to do that. Andrew

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c2hs examples?
by Magnus Therning 13 Dec '06

13 Dec '06

Does anyone know of any good source of examples if one wants to learn to use c2hs? The documentation that comes with c2hs is a little too sparse when it comes to examples to actually work out how things work. I don't mind looking at some reasonably sized project that uses c2hs (gtk2hs is a little too overwhelming). /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. "Zaphod grinned two manic grins, sauntered over to the bar and bought most of it." -- Zaphod in paradise.

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cpphs and #pragma
by Dimitry Golubovsky 13 Dec '06

13 Dec '06

Hi, Is this a bug or a feature that cpphs does not pass the #pragma cpp directive (they disappear in the output)? In version 0.6 of cpphs: * Recognise and ignore the #pragma cpp directive. Or what command line options could be used to preserve #pragma's in cpphs output? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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Re: Haskell and .NET
by Monique Monteiro 13 Dec '06

13 Dec '06

Hi Justin, I've runned a research project about this topic (in fact, it was the subject of my MSc dissertation). Please see the Haskell.NET Project (http://www.cin.ufpe.br/~haskell/haskelldotnet) We have compiled a subset of the Haskell language to .NET, but we still don't have an available release (we don't support the full Haskell prelude yet). The compiler is an extension to GHC. In the future, we intend to provide full interoperability with .NET in this way. There are some interesting references in the website about our compilation strategies. If you need further information about the project's status, please contact Prof. André Santos (alms(a)cin.ufpe.br) and/or Guilherme Avelino (gaa(a)cin.ufpe.br) Best regards, -- _____________________________________________ Monique Monteiro, MSc http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br +55 81 34198137 Project Manager Recife Microsoft Innovation Center

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Re: [Haskell] ANNOUNCE: Phooey -- a Functional UI library for Haskell
by Brian Hulley 13 Dec '06

13 Dec '06

Brian Hulley wrote: > Conal Elliott wrote: > >> GUIs are usually programmed in an "unnatural" style, in that ... > > Also, suppose you have a gui consisting of an edit widget such that > when the user types something it gets lexed, parsed, and fontified as > a Haskell program, ie from the user's point of view the widget > maintains a syntax highlighted view of the code which is > "continuously" updated. Would your system do the lexing/parsing for > every key that was pressed or only once, when the widget needs to be > re-rendered (a typical gui for Windows would only propagate changes > and re-draw the gui (ie lex/parse/fontify) when there are no event > messages pending)? Thinking about this more, a better example would be a gui with 2 widgets: an edit widget to type in code and a list widget which displays a list of top level functions defined in that code. (The code could be Haskell or some simple toy language.) Then the question becomes: how to set things up such that the code in the edit widget is only parsed when the list widget (and/or edit widget) is asked to render it's contents rather than every time the user presses a key. I think perhaps the problem would be solved by the edit widget passing a lazy parse tree (ie an expression evaluating to the parse tree) to the list widget. But an interesting thing is if you compare this to imperative guis, in the functional gui each widget passes lazy expressions eagerly whereas in imperative guis each widget passes eagerly evaluated expressions lazily ie functional is "push messages" whereas imperative is "pull changes when I'm asked to pick or render something etc and I'm dirty". (Have I got this wrong?) I imagine that the overhead of eager passing in the functional gui is a small price to pay for the ease of using a functional gui, since the important thing is that in both guis, the hard work of parsing the code in the edit buffer would only be done on demand, unless I'm much mistaken. If you're still thinking of examples for your paper it would also be really great to see how you'd create a widget that displays the result of another process (eg a window that displays the output as ghc compiles a program) or some other example of how to use the IO monad inside a widget for those unfamiliar with combining arrows with monads. It would also be useful to know more about the relationship between the functional gui and WxWidgets to see what characteristics an imperative gui toolkit must have in order to be usable with it. (Like the extremely useful discussion of low level details in the Fudgets thesis.) Best regards, Brian. -- http://www.metamilk.com

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Good Haskell introduction for an Ocaml programmer?
by Brian Hurt 13 Dec '06

13 Dec '06

Greetings, all. I'm an experienced Ocaml programmer, looking to broaden my horizons yet further and pick up Haskell, and I'm wondering if there's a good introduction to Haskell for me. I have Simon Thompson's "Haskell: The Craft of Functional Programming", which isn't a bad book, but I'm something of a special case. I'm already familiar with and comfortable with a lot of concepts which are new to your average C++/Java programmer- things like symbolic computation and recursion as looping and applicative data structures. So churning through introductions to these concepts looking for the rare nugget of new information is, well, kinda boring. On the other hand there are a lot of Haskell concepts I'm not comfortable with, like monads. So I was wondering if there was a better introduction for me out there? I'm willing to pay for a book or read something online, whichever. Thanks, Brian

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