- Haskell-Cafe - Haskell.org

haskell.org memory upgrade
by Paul Hudak 14 Dec '06

14 Dec '06

Dear Haskellers -- Haskell.org will go down today at 1500 EST for about 10 minutes for a memory upgrade. Sorry for the inconvenience, -Paul Hudak

1 0

typeclass constraint question
by Jeff Polakow 14 Dec '06

14 Dec '06

Hello, I'm trying to understand what happens when a typeclass constraint with an uninstantiated type variable gets duplicated. Does the type checker treat the separate copies of the constraint as two constraints (with the same type variable) which are checked separately? Or does the type checker only consider one constraint which is checked (simultaneously) in different contexts? Also does it matter if the constraints are on datatype, or even gadt, constructors rather than functions? thanks, Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

1 0

Re: Designing an object model in Haskell (RESOLVED)
by John Ky 14 Dec '06

14 Dec '06

Hi all, I'm now using existential types. I avoided learning about them because the name sounded so highly technical and obscure it did not occur to me they could be related to OO. Thanks, -John On 12/7/06, John Ky <newhoggy(a)gmail.com> wrote: > > Hi, > > I've got an object model that I have a difficult time conceptualising how > it might look like in Haskell: > > class Element { } > > class Inline : Element { } > > class ParentInline : Inline { > List<Inline> children; > } > > class Bold : ParentInline { } > class Underline : ParentInline { } > > class Link : ParentInline { > String link; > } > > class Text : Inline { > String text; > } > > class Block : Element { } > > class Paragraph : Block { > List<Inline> paragraph; > } > > class Heading : Block { > List<Inline> heading; > } > > class Document : Element { > List<Block> blocks; > } > > How best to represent this OO data model in Haskell? > > Thanks > > -John > >

2 1

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 >

1 0

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.

1 0

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

2 1

Request for a Simple Pretty Printing library
by Neil Mitchell 14 Dec '06

14 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

4 5

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

9 15

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.

1 0

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

2 1