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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 12 Jan '05

12 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 11 Jan '05

11 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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Unit testing in Haskell
by Dmitri Pissarenko 11 Jan '05

11 Jan '05

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

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RE: [Haskell-cafe] Re: Hugs vs GHC (again) was: Re: Somerandomnewbiequestions
by Simon Marlow 11 Jan '05

11 Jan '05

On 11 January 2005 11:39, Marcin 'Qrczak' Kowalczyk wrote: > Ben Rudiak-Gould <Benjamin.Rudiak-Gould(a)cl.cam.ac.uk> writes: > >> http://www.haskell.org/~simonmar/io/System.IO.html > > "fileRead :: File -> FileOffset -> Integer -> Buffer -> IO ()" > > This is unimplementable safely if the descriptor is read concurrently > by different processes. The current position is shared. There's a big lock on File. If you want to do truly concurrent reading, you can make multiple FileInputStreams, each of which has its own file descriptor (the Unix implementation uses dup(2)). Cheers, Simon

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