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Re: [Haskell-cafe] Parsec and type (IO t) error
by Greg Buchholz 14 Jan '05

14 Jan '05

Mike Gunter wrote: > > I'd guess that > let (cam, sh) = parse_text shape_parse input > is what you want? (Completely untested ...) > Yep. That did it. Thanks, Greg Buchholz

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Parsec and type (IO t) error
by Greg Buchholz 14 Jan '05

14 Jan '05

This is probably an easy question, but I'm having a problem with parsec in the IO monad. The essential parts of my program looks like this... >import Text.ParserCombinators.Parsec > >main = do input <- getContents > putStr $ show $ parse_text shape_parse input > --(cam, sh) <- parse_text shape_parse input > --putStr $ (show cam) ++ "\n" ++ (show sh) > putStr "\n" > >parse_text p input = case (parse p input) of > Left err -> error $ "Invalid input"++(show err) > Right x -> x > >shape_parse = do cam <- camera_parse > shapes <- many1 (sphere_parse <|> plane_parse) > return (cam, shapes) > >-- blah, blah, blah, etc. This works fine in GHC. The types for parse_text and shape_parse are... *Main> :t parse_text parse_text :: forall a tok. GenParser tok () a -> [tok] -> a *Main> :t shape_parse shape_parse :: forall st. GenParser Char st (Camera, [Shape]) *Main> Now when I change main to... >main = do input <- getContents > --putStr $ show $ parse_text shape_parse input > (cam, sh) <- parse_text shape_parse input > putStr $ (show cam) ++ "\n" ++ (show sh) > putStr "\n" I get the following message from GHCi... p2.hs:38: Couldn't match `IO t' against `(Camera, [Shape])' Expected type: GenParser Char () (IO t) Inferred type: GenParser Char () (Camera, [Shape]) In the first argument of `parse_text', namely `shape_parse' In a 'do' expression: (cam, sh) <- parse_text shape_parse input I'm probably missing something silly. Any hint would be appreciated. Thanks, Greg Buchholz

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RE: [Haskell-cafe] Re: I/O interface
by Simon Marlow 13 Jan '05

13 Jan '05

On 12 January 2005 01:27, Ben Rudiak-Gould wrote: > First of all, I don't think any OS shares file pointers between > processes. Otherwise it would be practically impossible to safely use > an inherited filehandle via any API. Different threads using the same > filehandle do share a file pointer (which is a major nuisance in my > experience, because of the lack of an atomic seek-read/write), but a > Posix fork duplicates the file pointer along with all other state. I > can't believe I'm wrong about this, but someone please correct me if > I am. I'm afraid you're wrong. Believe me, I'm as surprised as you. See the definition of "Open File Description" in POSIX: http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#t ag_03_253 > This limits the problem to a single process. If you're only using > GHC's lightweight threads, there's no problem at all. If you're using > OS > threads, the worst thing that could happen is that you might have to > protect handle access with a critical section. I don't think this > would lead to a noticeable performance hit when combined with the > other > overhead of file read/write operations (or lazy evaluation for that > matter). > > >pread requires that the file is seekable, which means that it can't > >be used for all file handles: not for pipes, sockets, terminals nor > >various other devices. > > The file interface in this library is only used for files, which are > always seekable (by definition). If you want to treat a file as a > stream, you create an InputStream or OutputStream backed by the file. > Such streams maintain internal (per-stream) file pointers. Unfortunately, they don't (at least in my prototype implementation). I assumed that dup()'ing file descriptors would be enough to produce separate file pointers, but no. So you can only safely make a single stream from a File. Making multiple streams would require re-opening the file for each subsequent one, or keeping a cached copy of the file pointer and seeking when necessary. Cheers, Simon

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Signature of a function
by Dmitri Pissarenko 13 Jan '05

13 Jan '05

Hello! I have a function activityIndicator, which has an argument of class Customer and should return a numeric value. The module is defined as follows. <module-definition> data Purchase = Purchase { price, rebate :: Double } deriving (Show, Eq) data Customer = Customer { id :: Int, purchases :: [Purchase] } activityIndicator :: Customer -> Num activityIndicator (Customer id purchases) = length purchases </module-definition> When I try to load this module into GHCi, I get this error: <error-message> Class `Num' used as a type In the type: Customer -> Num While checking the type signature for `activityIndicator' </error-message> When I remove the line activityIndicator :: Customer -> Num the error disappears. What is wrong in the signature above? TIA Dmitri Pissarenko

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Re: implementation of functional languages
by Matthew Roberts 13 Jan '05

13 Jan '05

> I am currently working my way through the tutorial book that has been > online for some time. It is perhaps the most valuable book I have and > I am really looking forward to reading this one. > > I would like to suggest we need more things like this - the Haskell > cookbook is a great initiative, but I would love books on any number > of other things. I find the conference papers are pitched just a > little too high to be educational and are often very specific. > > One example of what I mean - I had looked at Haskell parser libraries > and I know there were problems a, b and c and X was a solution to a, > etc... However, I did not "really" understand how they worked - until > I did the parser for the Core Language. Now it is all so clear! > > I am rambling a bit, but I am jut really pleased this has been made > available. > > Matt > > On 11/01/2005, at 11:32 PM, Graham Klyne wrote: > >> I think this is most welcome... I have the original printed version >> and have found it to be of great inspiration at various times. >> >> #g >> -- >> >> At 18:04 07/01/05 +0000, you wrote: >>> I'm happy to announce that my out-of-print 1987 book, >>> >>> The Implementation of Functional Programming Languages >>> >>> is now available online at >>> >>> >>> http://research.microsoft.com/%7Esimonpj/papers/slpj-book-1987/ >>> index.htm >>> >>> Very many thanks to Marnie Montgomery, who did all the work. >>> >>> Happy reading >>> >>> Simon >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> ------------ >> Graham Klyne >> For email: >> http://www.ninebynine.org/#Contact >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

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Re: [Haskell-cafe] Unit testing in Haskell
by Matthew Roberts 12 Jan '05

12 Jan '05

> > I find that I don't need unit testing frameworks. A few features of > Haskell and the associated interpreters (ghci and hugs) combine to > make "unit testing as you go" really easy. I just write a few tests > for each function I write and then some more module wide tests once > the whole module is finished. Sometimes I need a little scaffolding > to be able to output complex data types (or type synonyms), but often > just deriving Show does the job! > > To me, unit testing is two things > - testing at a low level (each and every function_ > - regression testing by running all the unit tests again > > The second may benefit more from the frameworks, but I find the first > can be done very effectively on an "ad-hoc" basis. > > Matt > > On 12/01/2005, at 6:05 AM, Dmitri Pissarenko wrote: > >> Hello! >> >> When programming in an imperative language like Java, unit tests are >> a very >> important development tool IMHO. >> >> I want to try out unit testing in Haskell and wonder what experienced >> Haskellers >> think about unit testing in Haskell in general and the hUnit testing >> framework >> (see URL below) in particular? >> >> http://hunit.sourceforge.net/ >> >> What other unit testing frameworks for Haskell do you use? >> >> TIA >> >> dap >> -- >> Dmitri Pissarenko >> Software Engineer >> http://dapissarenko.com >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Building GUIs for Haskell programs
by Dmitri Pissarenko 12 Jan '05

12 Jan '05

Hello! I want to learn to create GUIs with Haskell. Which GUI frameworks can you recommend? Thanks Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Re: [Haskell-cafe] Building GUIs for Haskell programs
by Matthew Roberts 12 Jan '05

12 Jan '05

> Can anyone attest to the sense (or otherwise) of the following > programming "pattern" > > - program all back end stuff in Haskell and compile to library > - program all GUI stuff in *insert imperative language of choice" and > link to the library for back end functionality > > It seems the ultimate decoupling of interface from logic. > > Matt > > On 12/01/2005, at 2:33 PM, Duncan Coutts wrote: > >> On Tue, 2005-01-11 at 21:05 +0100, Dmitri Pissarenko wrote: >>> Hello! >>> >>> I want to learn to create GUIs with Haskell. >>> >>> Which GUI frameworks can you recommend? >> >> wxHaskell is good for building portable GUIs. It has an extensive >> selection of widgets and has a good API. >> >> gtk2hs is good for building GUIs that target the Linux/Gtk/Gnome >> platform (though it is portable to Windows). It is also an extensive >> toolkit. It allows you to visually design GUIs with the Glade user >> interface builder. The API style is currently more low level than that >> of wxHaskell or FLTK. >> >> There is also a binding to FLTK which is a much simpler, more light >> weight portable graphics toolkit. >> >> As far as I know, these are the only actively maintained GUI toolkits >> for Haskell at the moment. >> >> Duncan >> (gtk2hs developer) >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-cafe] Hugs vs GHC (again) was: Re: Some random newbiequestions
by Dimitry Golubovsky 12 Jan '05

12 Jan '05

-------- Original Message -------- Subject: Re: [Haskell-cafe] Hugs vs GHC (again) was: Re: Some random newbiequestions Date: Mon, 10 Jan 2005 20:47:26 -0500 From: Dimitry Golubovsky <dimitry(a)golubovsky.org> To: Marcin 'Qrczak' Kowalczyk <qrczak(a)knm.org.pl> References: <3429668D0E777A499EE74A7952C382D102F30BF6(a)EUR-MSG-01.europe.corp.microsoft.com> <877jmmv2zz.fsf(a)qrnik.zagroda> <59BACBDC-6273-11D9-8389-000A95E8B0DA(a)etu.upmc.fr> <87acrgssem.fsf(a)qrnik.zagroda> Hi, Let me add a column for Hugs (summarized by looking at recent checkout from CVS, contained im several C ahd Haskell files): |Sebastien's| Marcin's | Hugs -------+-----------+----------+------ alnum | L* N* | L* N* | L*, M*, N* <1> alpha | L* | L* | L* <1> cntrl | Cc | Cc Zl Zp | c < ' ' || c >= '\DEL' && c <= '\x9f' digit | N* | Nd | c >= '0' && c <= '9' lower | Ll | Ll | Ll <1> punct | P* | P* | P* upper | Lu | Lt Lu | Lu Lt <1> blank | Z* \t\n\r | Z*(except| ' ' \t\n\r\f\v U+00A0 U+00A0 U+2007 U+202F) \t\n\v\f\r U+0085 <1>: for characters outside Latin1 range. For Latin1 characters (0 to 255), there is a lookup table defined as "unsigned char charTable[NUM_LAT1_CHARS];" I also like Ketil's idea about defining predicates like isUpper or isSpace in multiple files, quoting this: >> It's not obvious what the predicates should really mean, e.g. should >> isDigit and isHexDigit include non-ASCII digits or should isSpace >> include non-breaking space characters. > I think perhaps the answer is all of the above. The functions could > be defined in multiple modules, so that 'ASCII.isSpace' would match > the "normal" space character only, while 'Unicode.isSpace' could match > all the weird and wonderful stuff in the standard. So there might be a bunch of (perhaps autogenerated, from localedef files) modules for each locale/encoding, like ISO8859_1 or KOI_8. These modules might be imported into applications as needed. Also there would be one module autogenerated from the Unicode data files. Dimitry Golubovsky Middletown, CT

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RE: [Haskell-cafe] Information given by :info (Was: Some randomnewbie questions)
by Simon Marlow 12 Jan '05

12 Jan '05

On 10 January 2005 10:26, Sebastian Sylvan wrote: > On Mon, 10 Jan 2005 10:30:46 +0100 (MEZ), Henning Thielemann > <iakd0(a)clusterf.urz.uni-halle.de> wrote: >> What I also would like to see is the Haddock documentation >> string of a function printed by ":info" or some other command. >> > > Now _that_ would be truly useful. It's not perfect, but it's better than nothing: :def doc (\s -> let (rvar,rmod) = break (=='.') (reverse s); var = reverse rvar in System.Cmd.system ("mozilla http://www.haskell.org/ghc/docs/latest/html/libraries/base/" ++ reverse rmod ++ "html#" ++ (if (Data.Char.isUpper (head var)) then "t:" else "v:") ++ var) >> return "") eg. Prelude> :doc Prelude.head You have to use a qualified name, it only works in package base, and you can't look up data constructors. Any hackers out there want to try lifting these restrictions? Cheers, Simon

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