- Haskell-Cafe - Haskell.org

Introducing me
by Derek Elkins 14 Jan '05

14 Jan '05

> Hi, > > my name is Fabian Otto. Heya. Some other fixtures of the Haskell Community are covered on this page, http://haskell.org/hawiki/HaskellCommunities. Two noteworthy ones are the wiki (http://haskell.org/hawiki/) obviously, and the #haskell IRC channel on irc.freenode.net (http://haskell.org/hawiki/HaskellIrcChannel) which has about a hundred or so people on it at a time. Many of the quotes on* http://haskell.org/hawiki/QuotesPage are from the IRC channel, so that may give you an idea of what it's like, at least at the more humorous moments. * For others who haven't looked at it, it is quite entertaining.

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Introducing me
by fabian otto 14 Jan '05

14 Jan '05

Hi, my name is Fabian Otto. I'm doing my master in computer science at the technical university of Berlin (Germany). I have choosen advanced functional programming (FTFP) [1] as one of my subjects. In this course we use opal[2] with some unimplemented features. e.g: * optional laziness * parametic types * theories / typeclasses Some topics in this course are: * monads * fixpoint algorithmes / cpos * parser generatoren * optimized datastructures ( e.g. queues) * reference counting To deepen my understanding in these topics I have choosen Haskell and I am really looking forward participating in this mailinglist Thanks for reading. Otto [1]: http://uebb.cs.tu-berlin.de/lehre/2004SVftfp/ [2]: http://www.cs.nott.ac.uk/~gmh//faq.html#opal -- bookmarks: http://del.icio.us/sigsegv jabber: sigsegv(a)jabber.ccc.de wiki/blog: http://jotpee.de/blog

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RE: [Haskell-cafe] Re: Hugs vs GHC (again) was: Re: Some randomnewbiequestions
by Simon Marlow 14 Jan '05

14 Jan '05

On 08 January 2005 08:09, Aaron Denney wrote: > On 2005-01-07, Simon Marlow <simonmar(a)microsoft.com> wrote: >> - Can you use (some encoding of) Unicode for your Haskell source >> files? I don't think this is true in any Haskell compiler right >> now. > > I assume this won't be be done until the next one is done... Not necessarily; GHC doesn't use the standard IO library for reading source files. >> - Can you do String I/O in some encoding of Unicode? No Haskell >> compiler has support for this yet, and there are design decisions >> to be made. Some progress has been made on an experimental >> prototype (see recent discussion on this list). > > Many of the easy ways to do this that I've heard proposed make the > current hacks for binary IO fail. Making hacks fail isn't necessarily a bad thing :-) > IMHO, we really, really, need a > standard, supported way to do binary IO. I agree, but I think it should be part of a larger redesign of the IO library. The streams proposal includes binary I/O, by the way. I'm not keen to provide binary IO on top of the existing IO library, and then to have Unicode as a layer on top of that. Performance will be terrible. It needs to be designed properly from the ground up. > If I can read in and output > octets, then I can implement unicode handling on top of that. In fact > it would let a bunch of the proposed ideas for unicode support can > be implemented in pure haskell and have API details hashed out and > polished. > > For unix, there are couple different tacks one could take. The locale > system is standard, and does work, but is ugly and a pain to work > with. In particular, it's another (set of) global variables. And > what do you do with a character not expressible in the current locale? > > I'd like to possibility of different character sets for different > files, for example. Not a problem. Have you looked at the streams proposal? Cheers, Simon

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Re: [Haskell-cafe] Question about instance
by John Velman 14 Jan '05

14 Jan '05

Thanks, Andreas. Your example still left me without context in the show statement itself, but your message set me to look in the right place. Now I have --- Code >data Relation a i b = Rel {name::RN, arity::Int, members::(Set [EN])} > >instance (Show a, Show i, Show b) => Show (Relation a i b) > where > show (Rel a i b) = > a ++ "/" ++ (show i) ++ " " > ++ (show b) > --- with result: ---GHCI Prelude> :l test.hs Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> mkRelation1 "test" 2 [["one","two"], ["three","four"]] test/2 mkSet [["one","two"],["three","four"]] *Main> ---End GHCI It's hard to find examples like this, and the fact that it is fairly standard practice for the type name and constructor names to be the same in, for example, Gentle Haskell, and Haskell School of Expression make it more difficult for the novice to see when each is used! Best, John Velman On Fri, Jan 14, 2005 at 01:49:57AM +0100, Andreas Marth wrote: > If you replace > >data Relation = Relation {name::RN, arity::Int, members::(Set [EN])} > with > >data Relation = Rel {name::RN, arity::Int, members::(Set [EN])} > you will easy find out what is wrong and come to: > > >import Data.Set > > > >type EN = String -- element name > > > >type RN = String -- relation name > > > >instance Show a => Show (Set a) where > > show s = "mkSet " ++ show (setToList s) > > > >data Relation = Rel {name::RN, arity::Int, members::(Set [EN])} > > > >instance Show (Relation) > > where > > show (Rel a i b) = a ++ "/" ++ (show i) ++ " " ++ (show b) > > > Of course you can now change Rel back to Relation. But because of the > problems you just experienced I don't like it to name a type and its > constructor the same. > > > Happy coding, > Andreas > > > > ----- Original Message ----- > From: "John Velman" <velman(a)cox.net> > To: <haskell-cafe(a)haskell.org> > Sent: Friday, January 14, 2005 1:29 AM > Subject: [Haskell-cafe] Question about instance > > > > Instance with a context doesn't seem to work as I expect. > > > > Here's the story: > > > > I define an data type, Relation, and I want to make it an instance of > > "Show", partly so I can debug and tinker with things interactively and > have > > ghci print something. > > > > Here is my first try: > > ---- > > > > > >import Data.Set > > > > > >type EN = String -- element name > > > > > >type RN = String -- relation name > > > > > >instance Show a => Show (Set a) where > > > show s = "mkSet " ++ show (setToList s) > > > > > >data Relation = Relation {name::RN, arity::Int, members::(Set [EN])} > > > > > >instance (Show a, Show i, Show b) => Show (Relation a i b) > > > where > > > show (Relation a i b) = > > > a ++ "/" ++ (show i) ++ " " > > > ++ (show b) > > ---- > > > > When I try to load this into ghci, I get: > > > > ---GHCI: > > > > *Main> :l test.hs > > Compiling Main ( test.hs, interpreted ) > > > > test.hs:14: > > Kind error: `Relation' is applied to too many type arguments > > When checking kinds in `Relation a i b' > > When checking kinds in `Show (Relation a i b)' > > In the instance declaration for `Show (Relation a i b)' > > Failed, modules loaded: none. > > Prelude> > > ---END GHCI > > But, when I define showRelation separately, then leave the context out of > > the instance declaration with show = showRelation it works: > > > > -----------(Everything down to the instance declaration is the same) > > >instance Show Relation > > > where > > > show = showRelation > > > > > >showRelation:: Relation -> String > > >showRelation (Relation a i b) = > > > a ++ "/" ++ (show i) ++ " " > > > ++ (show b) > > > > > ----------- > > > > Now I get: > > ----------- GHCI output: > > Prelude> :l test.hs > > Compiling Main ( test.hs, interpreted ) > > Ok, modules loaded: Main. > > *Main> mkRelation1 "test" 2 [["one","two"], ["three","four"]] > > test/2 mkSet [["one","two"],["three","four"]] > > *Main> > > ---------- End GHCI > > > > Why does the original instance declaration result in failure and the > > message "Kind error: `Relation' is applied to too many type arguments" (I > > confess to not understanding 'kinds' too well.) > > > > I've done a bit of tinkering with the original version, and have tried the > > second version with a context in the instance declaration, but none of my > > attmepts work. The only one that worked was the one shown, with no > context > > in the instance declaration. Needless to say (?), I've tried to > > understand this from reading in the Haskell 98 report, `Haskell school of > > Expression', and any place else I can think of, but I'm missing the point > > somewhere. > > > > > > Thanks, > > > > John Velman > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

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Problem with wxHaskell
by Dmitri Pissarenko 14 Jan '05

14 Jan '05

Hello! I've downloaded wxHaskell, ran the wxhaskell-register.bat file and now try to build a minimal wxHaskell program. For this purpose, I tried to start GHCi using following command ghci -package wx GuiTest.hs GHCi crashed with following error messages: <error-messages> GHC Interactive, version 6.2.2, for Haskell 98. http://www.haskell.org/ghc/ Type :? for help. Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Loading package lang ... linking ... done. Loading package concurrent ... linking ... done. Loading package QuickCheck ... linking ... done. Loading package readline ... linking ... C:/ghc/ghc-6.2.2/HSreadline.o: unknown symbol `_rl_redisplay_function' ghc.exe: unable to load package `readline' </error-messages> What am I doing wrong? TIA Dmitri Pissarenko PS: Here is the code of the application module Main where import Graphics.UI.WXCore main :: IO () main = run gui gui :: IO () gui = do frame <- frameCreateTopFrame "Hello World" windowShow frame windowRaise frame return () -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Question about instance
by John Velman 14 Jan '05

14 Jan '05

Instance with a context doesn't seem to work as I expect. Here's the story: I define an data type, Relation, and I want to make it an instance of "Show", partly so I can debug and tinker with things interactively and have ghci print something. Here is my first try: ---- > >import Data.Set > >type EN = String -- element name > >type RN = String -- relation name > >instance Show a => Show (Set a) where > show s = "mkSet " ++ show (setToList s) > >data Relation = Relation {name::RN, arity::Int, members::(Set [EN])} > >instance (Show a, Show i, Show b) => Show (Relation a i b) > where > show (Relation a i b) = > a ++ "/" ++ (show i) ++ " " > ++ (show b) ---- When I try to load this into ghci, I get: ---GHCI: *Main> :l test.hs Compiling Main ( test.hs, interpreted ) test.hs:14: Kind error: `Relation' is applied to too many type arguments When checking kinds in `Relation a i b' When checking kinds in `Show (Relation a i b)' In the instance declaration for `Show (Relation a i b)' Failed, modules loaded: none. Prelude> ---END GHCI But, when I define showRelation separately, then leave the context out of the instance declaration with show = showRelation it works: -----------(Everything down to the instance declaration is the same) >instance Show Relation > where > show = showRelation > >showRelation:: Relation -> String >showRelation (Relation a i b) = > a ++ "/" ++ (show i) ++ " " > ++ (show b) > ----------- Now I get: ----------- GHCI output: Prelude> :l test.hs Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> mkRelation1 "test" 2 [["one","two"], ["three","four"]] test/2 mkSet [["one","two"],["three","four"]] *Main> ---------- End GHCI Why does the original instance declaration result in failure and the message "Kind error: `Relation' is applied to too many type arguments" (I confess to not understanding 'kinds' too well.) I've done a bit of tinkering with the original version, and have tried the second version with a context in the instance declaration, but none of my attmepts work. The only one that worked was the one shown, with no context in the instance declaration. Needless to say (?), I've tried to understand this from reading in the Haskell 98 report, `Haskell school of Expression', and any place else I can think of, but I'm missing the point somewhere. Thanks, John Velman

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Re: [Haskell-cafe] Parsec and type (IO t) error
by Greg Buchholz 13 Jan '05

13 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 13 Jan '05

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

11 Jan '05

On 11 January 2005 06:52, Andre Pang wrote: > Is there a Wiki page or URL with the steram proposal? There is now: http://www.haskell.org/hawiki/HaskellStreamIO Simon

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Re: Hugs vs GHC (again)
by Dimitry Golubovsky 11 Jan '05

11 Jan '05

Ketil, Ketil Malde wrote: > Dimitry Golubovsky <golubovsky(a)gmail.com> writes: > > (Did you intend this for the list?) Yes, and I re-posted similar message on the list. > > >>>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. >> > >>And then there might be a bunch of (perhaps autogenerated) >>locale-matching modules like ISO8859_1, KOI_8 etc. > > > Well - maybe. Another option is for these to require conversion from > to Unicode. > Right, because input of "encoding-specific" functions will be Unicode anyway. And AFAIK conversion from/to unicode may be also contained in localedef files (or for instance in Java CharToXXXX, XXXToChar classes) - I mean that there must be enough information around to build those modules automatically. Except for Unicode itself which would be autogenerated from the Unicode database files like multiple examples of doing this (including mine) that were posted on this list. Dimitry Golubovsky Middletown, CT

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

11 Jan '05

On 07 January 2005 17:10, Malcolm Wallace wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: >>>> - Can the Char type hold the full range of Unicode characters? >>>> This has been true in GHC for some time, and is now true in >>>> Hugs. I don't think it's true in nhc98 (please correct me if >>>> I'm wrong). >>> >>> You're wrong :-). nhc98 has always had 32-bit characters >>> internally. >> >> I checked the nhc98 sources, and it seems that maxBound::Char is >> '\255'. > > Yes, but nothing prevents you from creating a larger character by e.g. > (toEnum 0x12345678) :: Char Then I respectfully submit a bug report :-) It shouldn't be possible to create a value of Char outside minBound..maxBound. It's possible the report doesn't say this explicitly, but see for example section 6.3.4. Cheers, Simon

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

11 Jan '05

On 11 January 2005 09:00, Sebastian Sylvan wrote: > On Mon, 10 Jan 2005 17:12:44 -0000, Simon Marlow > <simonmar(a)microsoft.com> wrote: > >> Not a problem. Have you looked at the streams proposal? >> > > I've missed most of the discussion on this, so if someone could just > clarify the reasons for a few things I find peculiar: > > * Prefixing function names with their types, not necessary with > qualified imports. I agree with this. > * Why not put isEOS in the InputStream class since it doesn't make > sense for OutputStreams? I think so, and there's a comment to that effect in the prototype code. > * Why not introduce an additional class BufferedStream containing the > the Buffering-functions (instead of having them in Stream and > resorting to weird failure-patterns for non-buffered streams): > setBufferMode :: s -> BufferMode -> IO () > getBufferMode :: s -> IO BufferMode > flush :: s -> IO () > sync :: s -> IO Bool > Where setBufferMode is not allowed to "fail" (by returning False). If > for some reason it does fail an exception is thrown. Same thing with > getBufferMode. It will return a Buffering or throw an exception (not > return "NoBuffering" instead of a failure). The prototype code has a separate Buffered class, with TextOutputStream an instance of it. I've been wondering whether having a generic buffering layer might be possible/desirable too, so you could do buffered binary I/O. Anyway, it would be good to record comments and suggestions like this on the wiki page (http://www.haskell.org/hawiki/HaskellStreamIO) Cheers, Simon

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Re: [Haskell-cafe] The Implementation of Functional Programming Languages
by Graham Klyne 11 Jan '05

11 Jan '05

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

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Typing question
by Dmitri Pissarenko 11 Jan '05

11 Jan '05

Hello! I want to create two data types: a) Purchase, which represents purchase by a customer (two attributes - amount of the purchase and rebate, both Double's) b) Customer, which represents a customer and his/her purchases. This tuple has two attributes - customer ID and a list of all purchases. Customer ID is Int, list of all purchases is a list of instances of Purchase (see above item a)). So, I wrote this code: <code-snippet> data Purchase (price :: Double) (rebate :: Double) = -- error occurs here Purchase price rebate deriving (Show, Eq) data Customer (id :: Int) (purchases :: [Purchase]) = Participant id purchases deriving (Show, Eq) </code-snippet> When I try to load it into GHCi, I'm getting this error message: <error-message> Prelude> :reload Compiling Gs ( Gs.hs, interpreted ) Gs.hs:15: parse error on input `Double' Failed, modules loaded: none. </error-message> Line 15 is the first line of the code snippet above. I have three questions: a) How should I define the types of the attributes correctly? b) Is it possible to impose a constraint on the "constructor" of tuple Purchase, so that the compiler throws an error, if one tries to create a purchase where rebate is greater than price (rebate and price are given in absolute numbers; rebate is NOT a fraction of price) ? c) What is a correct name for "attributes" of tuples (such as price and rebate in Purchase) and "constructors" of tuples? Many thanks in advance Dmitri Pissarenko

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RE: [Haskell-cafe] Re: Some random newbie questions
by Simon Peyton-Jones 11 Jan '05

11 Jan '05

| -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of John | Hughes | Sent: 10 January 2005 09:16 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Re: Some random newbie questions | | I seriously considered switching frlom Hugs to GHC for my introductory | programming class this year, but in the end stayed with Hugs because of | a single feature. | | I'm teaching beginning programmers, and for them at least, there is an | overwhelming volume of names to learn -- "what's that function?" is a | question they ask themselves often, as is "what's that type?". I teach | them that, whenever they see a name they don't recognise, they can find | out more about it using the :i command. This is a simple trick to learn, | that helps them understand points they've missed and catches | misapprehensions. | | My students also see type classes very early. I'll bet yours will too. | Even if one is very careful to restrict the examples in lectures so as | to avoid them (which is a bind), as soon as students try out Hugs for | themselves, they will make mistakes that generate error messages | referring to type classes. No problem: the question "what's that class?" | can ALSO be answered by :i. | | Now, at the beginning students have only a very rudimentary | understanding of classes. A class is a collection of types to them, | nothing more. In particular, the class definition itself is of little | use to them, since it often contains a very subtly chosen collection of | methods (just type :i Show, for example, which students do very early). | What IS useful, right from the beginning, is the list of instances. What | are Num types? Oh, integers and reals. What are Show types? Oh, pretty | much everything. Particularly when debugging "missing instance" errors, | this is just the information you need. | | Unfortunately, while Hugs prints the list of instances of a class in | response to :i, GHCi does not. It only prints the class definition -- | which, for my students, contains no useful information. For that reason | alone, I stuck with Hugs last year. | How about changing the behaviour of :i, Simon, so I can use GHCi | throughout next year? Done, some while ago. You want GHC 6.4! See below Simon Prelude> :i Show class Show a where showsPrec :: Int -> a -> String -> String show :: a -> String showList :: [a] -> String -> String -- Imported from `GHC.Show' instance Show IOMode -- Imported from `GHC.IOBase' instance Show IOException -- Imported from `GHC.IOBase' instance Show IOErrorType -- Imported from `GHC.IOBase' instance Show HandleType -- Imported from `GHC.IOBase' instance Show Handle -- Imported from `GHC.IOBase' instance Show ExitCode -- Imported from `GHC.IOBase' instance Show Exception -- Imported from `GHC.IOBase' instance Show BufferMode -- Imported from `GHC.IOBase' instance Show AsyncException -- Imported from `GHC.IOBase' instance Show ArrayException -- Imported from `GHC.IOBase' instance Show ArithException -- Imported from `GHC.IOBase' instance Show SeekMode -- Imported from `GHC.Handle' instance Show HandlePosn -- Imported from `GHC.Handle' instance Show CWchar -- Imported from `Foreign.C.Types' instance Show CUShort -- Imported from `Foreign.C.Types' instance Show CULong -- Imported from `Foreign.C.Types' instance Show CULLong -- Imported from `Foreign.C.Types' instance Show CUInt -- Imported from `Foreign.C.Types' instance Show CUChar -- Imported from `Foreign.C.Types' instance Show CTime -- Imported from `Foreign.C.Types' instance Show CSize -- Imported from `Foreign.C.Types' instance Show CSigAtomic -- Imported from `Foreign.C.Types' instance Show CShort -- Imported from `Foreign.C.Types' instance Show CSChar -- Imported from `Foreign.C.Types' instance Show CPtrdiff -- Imported from `Foreign.C.Types' instance Show CLong -- Imported from `Foreign.C.Types' instance Show CLLong -- Imported from `Foreign.C.Types' instance Show CLDouble -- Imported from `Foreign.C.Types' instance Show CInt -- Imported from `Foreign.C.Types' instance Show CFloat -- Imported from `Foreign.C.Types' instance Show CDouble -- Imported from `Foreign.C.Types' instance Show CClock -- Imported from `Foreign.C.Types' instance Show CChar -- Imported from `Foreign.C.Types' instance Show (GHC.Ptr.Ptr a) -- Imported from `Foreign.Ptr' instance Show (GHC.Ptr.FunPtr a) -- Imported from `Foreign.Ptr' instance Show Float -- Imported from `GHC.Float' instance Show Double -- Imported from `GHC.Float' instance Show Int8 -- Imported from `GHC.Int' instance Show Int64 -- Imported from `GHC.Int' instance Show Int32 -- Imported from `GHC.Int' instance Show Int16 -- Imported from `GHC.Int' instance Show Word8 -- Imported from `GHC.Word' instance Show Word64 -- Imported from `GHC.Word' instance Show Word32 -- Imported from `GHC.Word' instance Show Word16 -- Imported from `GHC.Word' instance Show Word -- Imported from `GHC.Word' instance Show [a] -- Imported from `GHC.Show' instance Show (a, b, c, d, e) -- Imported from `GHC.Show' instance Show (a, b, c, d) -- Imported from `GHC.Show' instance Show (a, b, c) -- Imported from `GHC.Show' instance Show (a, b) -- Imported from `GHC.Show' instance Show () -- Imported from `GHC.Show' instance Show Ordering -- Imported from `GHC.Show' instance Show (Maybe a) -- Imported from `GHC.Show' instance Show Int -- Imported from `GHC.Show' instance Show (Either a b) -- Imported from `GHC.Show' instance Show Char -- Imported from `GHC.Show' instance Show Bool -- Imported from `GHC.Show' Prelude>

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

11 Jan '05

Here's a summary of the state of Unicode support in GHC and other compilers. There are several aspects: - Can the Char type hold the full range of Unicode characters? This has been true in GHC for some time, and is now true in Hugs. I don't think it's true in nhc98 (please correct me if I'm wrong). - Do the character class functions (isUpper, isAlpha etc.) work correctly on the full range of Unicode characters? This is true in Hugs. It's true with GHC on some systems (basically we were lazy and used the underlying C library's support here, which is patchy). - Can you use (some encoding of) Unicode for your Haskell source files? I don't think this is true in any Haskell compiler right now. - Can you do String I/O in some encoding of Unicode? No Haskell compiler has support for this yet, and there are design decisions to be made. Some progress has been made on an experimental prototype (see recent discussion on this list). - What about Unicode FilePaths? This was discussed a few months ago on the haskell(-cafe) list, no support yet in any compiler. Cheers, Simon On 07 January 2005 00:52, Dimitry Golubovsky wrote: > Hi, > > Looks like Hugs and GHC are being compared again ;) > > I am just interested to know, what is the current status of Unicode > support in GHC? Hugs has had it for about a year (or more, in CVS) at > least at the level of recognizing character categories and simple case > conversions based on the Unicode database files. Also UTF-8 or > locale-based I/O encoding conversion to internal Unicode is available. > Does GHC has similar support? > > Some time ago (about 1.5 years) I tried to play with Unicode I/O in > GHC, and it looked like it did not have much Unicode support back > then (at least on I/O level). Has anything progressed in this regard > since then? > > Most of this list subscribers seem to be GHC users, so can anybody > answer? > > BTW when answering the original post (brief quote below) different > aspects were mentioned, but not internationalization ones. Is it > really not that important? > > Dimitry Golubovsky > Middletown, CT > > Benjamin Pierce wrote: > > >> * What are the relative advantages of Hugs and GHC, beyond the >> obvious (Hugs is smaller and easier for people not named Simon to >> modify, while GHC is a real compiler and has the most up-to-date >> hacks to the type checker)? Do people generally use one or the >> other for everything, or are they similar enough to use Hugs at >> some moments and GHC at others? > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: [Haskell-cafe] Implementing computations with timeout
by Simon Marlow 10 Jan '05

10 Jan '05

On 07 January 2005 20:00, Sebastian Sylvan wrote: > On Fri, 7 Jan 2005 20:56:42 +0100, Sebastian Sylvan > <sebastian.sylvan(a)gmail.com> wrote: >> On Fri, 07 Jan 2005 15:31:10 +0200, Einar Karttunen >> <ekarttun(a)cs.helsinki.fi> wrote: >>> Hello >>> >>> What is the best way of doing an computation with a timeout? >> >> I like the approach taken in "Tackling the ackward squad": >> > > I should also state that this isn't safe when it comes to asynchronous > exceptions. > If one were to raise an exception in a timeout'd computation it would > simply abort the takeMVar which means the two child processes won't > get killed. Yes, making a timeout combinator that (a) can be nested and (b) is invisible with respect to asynchronous exceptions is quite a challenge. I'm not sure that we managed to do it. STM will almost certainly make it easier, though. Cheers, Simon

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RE: [Haskell-cafe] Implementing computations with timeout
by Simon Marlow 10 Jan '05

10 Jan '05

On 07 January 2005 22:17, Tomasz Zielonka wrote: > On Fri, Jan 07, 2005 at 11:00:27PM +0100, Tomasz Zielonka wrote: >> Hmmm, TMVar's seem to be significantly (ie. 6 times) faster than >> MVars in some simple tests :) >> >> Is it expected? > > If it is, we can reimplement MVars using STM, when STM becomes stable > :) No, TMVars are quite a bit slower than MVars if you use the basic takeMVar/putMVar ops. However, if you use the safer withMVar/modifyMVar combinators, then you'll probably find that performance is about the same. A simple test I did comparing TChan with Chan came up with roughly the same performance, and STM isn't really tuned yet. Cheers, Simon

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Using Haskell as a database
by Dmitri Pissarenko 10 Jan '05

10 Jan '05

Hello! I am thinking about writing a system, using which it is possible to store information about the structure of a bureaucratic system. Coarse-grained features of the program include: 1) The program should allow the user to enter the hierarchy (like: authority X has a board of Y members, each of them has Z departments under his/her control etc) of a governmental organization 2) The program should visualize the hierarchy in form of a tree 3) It should be possible to enter information about what areas of public administration a particular civil servant is responsible for. In other words: If things go wrong in some domain, controlled by the government, the user should be able to figure out, who is responsible for that mess. My first idea was to use a relational database for storing this information. However, a relational database may be inefficient due to the complexity of the data structure. At the moment, I think that it makes more sense to store the data in form of facts (not tables as in relational database). I mean that one is able to enter statements like (this is PROLOG syntax) isOrganization(X). hasBoard(X, Members) :- isOrganization(X), isList(Members). civilServant(X). isResponsibleFor(X, Y) :- civilServant(X), domain(Y). and then run queries, using which one can find out, which organizations are there, what members they have etc. Can one do this in Haskell? Thanks in advance Dmitri Pissarenko

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Re: Some random newbie questions
by John Hughes 10 Jan '05

10 Jan '05

I seriously considered switching frlom Hugs to GHC for my introductory programming class this year, but in the end stayed with Hugs because of a single feature. I'm teaching beginning programmers, and for them at least, there is an overwhelming volume of names to learn -- "what's that function?" is a question they ask themselves often, as is "what's that type?". I teach them that, whenever they see a name they don't recognise, they can find out more about it using the :i command. This is a simple trick to learn, that helps them understand points they've missed and catches misapprehensions. My students also see type classes very early. I'll bet yours will too. Even if one is very careful to restrict the examples in lectures so as to avoid them (which is a bind), as soon as students try out Hugs for themselves, they will make mistakes that generate error messages referring to type classes. No problem: the question "what's that class?" can ALSO be answered by :i. Now, at the beginning students have only a very rudimentary understanding of classes. A class is a collection of types to them, nothing more. In particular, the class definition itself is of little use to them, since it often contains a very subtly chosen collection of methods (just type :i Show, for example, which students do very early). What IS useful, right from the beginning, is the list of instances. What are Num types? Oh, integers and reals. What are Show types? Oh, pretty much everything. Particularly when debugging "missing instance" errors, this is just the information you need. Unfortunately, while Hugs prints the list of instances of a class in response to :i, GHCi does not. It only prints the class definition -- which, for my students, contains no useful information. For that reason alone, I stuck with Hugs last year. Of course, later in the course there is no problem in introducing GHC as well. Students coping well are happy to learn there is a compiler available too, while those who are struggling can stay with Hugs throughout the course. I demonstrated GHC in order to show them wxHaskell (which was very popular with the students), but I didn't REQUIRE them to use it. How about changing the behaviour of :i, Simon, so I can use GHCi throughout next year? John

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Re: [Haskell-cafe] Guards (Was: Some random newbie questions)
by Jon Cast 10 Jan '05

10 Jan '05

Henning Thielemann <lemming(a)henning-thielemann.de> wrote: <snip> > What about dropping Guards? :-) Are they necessary? Do they lead to > more readable source code? Absolutely. In Haskell's syntax, if-then-else-if interacts badly with do notation, and Haskell lacks a direct analogy to Lisp's cond. case () of () | p1 -> e1 | p2 -> e2 ... works beautifully as a replacement. Also, GHC's pattern guards are a nice feature, and frequently seem clearer than case. Compare, e.g., parseCmd ln | Left err <- parse cmd "Commands" ln = BadCmd $ unwords $ lines $ show err | Right x <- parse cmd "Commands" ln = x with the Haskell-98 alternative parseCmd ln = case parse cmd "Commands" ln of Left err -> BadCmd $ unwords $ lines $ show err Right x -> x The trade-off: using pattern guards makes it harder to verify (and ensure) that the exact same expression is being matched against; using case makes it harder to see exactly what is being matched against. Furthermore, guards are an extension of pattern matching, which means you can write code like this: xn !! n | n < 0 = error "Prelude.(!!): Negative index" [] !! n = error "Prelude.(!!): Index overflow" (x:xn) !! n | n == 0 = x (x:xn) !! n = xn !! (n - 1) Exactly one equation for each edge in the control-flow graph, which is nice and not easily done (I'm not sure it's even possible) without guards. Pattern guards are also nice for implementing ‘views’: -- | Convert an 'XMLData' into an equivalent application of -- 'Balanced', if possible. In any case, return an equivalent data -- structure. balance (Balanced es) = Balanced es balance (LeftLeaning (LeftBalanced e:es)) | Balanced es' <- balance (LeftLeaning es) = Balanced (e:es') balance (LeftLeaning []) = Balanced [] balance (RightLeaning [("", "", es)]) = Balanced es balance (RightLeaning []) = Balanced [] balance e = e Where XMLData can store a (nearly) arbitrary fragment of an XML document. The problem being solved by the pattern guard in the second equation is that the data type is ambiguous; there is more than one way to represent a ‘balanced’ XML fragment (that is, the concatenation of a sequence of well-formed XML fragments and CDATA sections). This function attempts to coerce the data structure passed in into a canonical representation; it succeeds if the data is in fact balanced and fails otherwise. The pattern guard illustrates how to use this function as a replacement for pattern matching on Balanced, to catch all cases where the argument is in fact balanced (we can't use it in this case as a replacement for the first equation, since that create an infinite loop, but in other functions we could). I'm sure there are uses I'm forgetting, but I think that's enough. > Do they lead to more efficient code? I could perfectly live without > them up to now. Well, I could never do without them. Jonathan Cast

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RE: [Haskell-cafe] RE: Enum class
by Arjun Guha 10 Jan '05

10 Jan '05

Sorry, that was a stupid error. I meant to use fromEnum: > next:: (Enum a, Bounded a) => a -> a > next v = if (fromEnum v) == (fromEnum maxBound) > then minBound > else succ v I don't think this will quite work if fromEnum:: a -> Int for type a, were not injective. However, that might be part of the specification... I don't remember. It should be fine, so long as the preimage of (fromEnum maxBound) is {maxBound}. -Arjun -- Arjun Guha <guhaarju(a)grinnell.edu>

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Some random newbie questions
by Benjamin Pierce 09 Jan '05

09 Jan '05

OK, I'm taking the plunge and using Haskell in a course I'm teaching this semester. To get ready, I've been doing quite a bit of Haskell programming myself, and this has raised a few questions... * What are the relative advantages of Hugs and GHC, beyond the obvious (Hugs is smaller and easier for people not named Simon to modify, while GHC is a real compiler and has the most up-to-date hacks to the type checker)? Do people generally use one or the other for everything, or are they similar enough to use Hugs at some moments and GHC at others? * As far as I can determine, there is no way to check pattern matches for exhaustiveness. Coming from OCaml, this feels like losing a significant safety net! How do people program so as not to be getting dynamic match failures all the time? * HUnit and QuickCheck seem to offer very nice -- but different -- testing facilities. Has anyone thought of combining them? (In fact, is HUnit actually used? The last revision seems to be a couple of years ago.) * I wrote a little program for generating Sierpinkski Carpets, and was astonished to find that it runs out of heap under Hugs (with standard settings -- raising the heap size with -h leads to a happier result). module Main where import SOEGraphics fillSquare w x y s = drawInWindow w (withColor Blue (polygon [(x,y), (x+s,y), (x+s,y+s), (x,y+s), (x,y)])) carpet w x y s = if s < 8 then fillSquare w x y s else let s' = s `div` 3 in do carpet w x y s' carpet w (x+s') y s' carpet w (x+s'*2) y s' carpet w x (y+s') s' carpet w (x+s'*2) (y+s') s' carpet w x (y+s'*2) s' carpet w (x+s') (y+s'*2) s' carpet w (x+s'*2) (y+s'*2) s' main = runGraphics ( do w <- openWindow "Carpet" (700,700) carpet w 50 50 600 k <- getKey w closeWindow w ) I've clearly got a lot to learn about space usage in Haskell... can someone give me a hint about what is the problem here and how it might best be corrected? Thanks for any comments, Benjamin ----------------------------------------------------------------------------- BENJAMIN C. PIERCE, Professor Dept. of Computer & Information Science University of Pennsylvania +1 215 898-2012 3330 Walnut St. Fax: +1 215 898-0587 Philadelphia, PA 19104, USA http://www.cis.upenn.edu/~bcpierce -----------------------------------------------------------------------------

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RE: Enum class
by Arjun Guha 09 Jan '05

09 Jan '05

Well, you already have succ:: (Enum a) => a -> a defined for all data of the Enum class. You also need Bounded so that you can check maxBound. This (untested) code should do it: > next:: (Enum a, Bounded a) => a -> a > next v = if (toEnum v) == (toEnum (maxBound)) > then minBound > else succ v -- Arjun Guha <guhaarju(a)grinnell.edu>

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Re: Typed Lambda-Expressions withOUT GADTs
by oleg＠pobox.com 09 Jan '05

09 Jan '05

Ashley Yakeley wrote on the first day of 2005: > This compiled with today's CVS GHC 6.3. I don't think you can do this > without GADTs. It seems we can do without GADT -- roughly with the same syntax (actually, the syntax of expressions is identical -- types differ and we have to write `signatures' -- i.e., instance declarations). Tested with GHC 6.2.1. I especially liked the example test3 = (runExpression s) (runExpression k) (runExpression k) below. Its inferred type is -- :t test3: -- test3 :: forall r. r -> r so, SKK must be the identity (if it terminates, that is) {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} module Expression where -- data Sym env r where -- FirstSym :: Sym (r,xx) r -- NextSym :: Sym env r -> Sym (a,env) r class SymT t env r data FirstSym = FirstSym deriving Show data NextSym t = NextSym t deriving Show instance SymT FirstSym (r,xx) r instance SymT t env r => SymT (NextSym t) (a,env) r class SymT t env r => RunSym t env r | t env -> r where runSym :: t -> env -> r -- runSym FirstSym (r,xx) = r; instance RunSym FirstSym (r,xx) r where runSym _ (r,xx) = r instance RunSym var env r => RunSym (NextSym var) (a,env) r where runSym (NextSym var) (a,env) = runSym var env -- the code literally -- data OpenExpression env r where -- Lambda :: OpenExpression (a,env) r -> OpenExpression env (a -> r); -- Symbol :: Sym env r -> OpenExpression env r; -- Constant :: r -> OpenExpression env r; -- Application :: OpenExpression env (a -> r) -> OpenExpression env a -> -- OpenExpression env r -- Frankly speaking, the class OpenExpression below isn't necessary -- We could have gotten by with just RunOpenExpression -- However, the class OpenExpression makes it easy to formulate -- the constraint for well-formed expressions. -- We can use that constraint elsewhere. class OpenExpression t env r data Lambda a t = Lambda t deriving Show data Symbol t = Symbol t deriving Show data Constant r = Constant r deriving Show data Application a t1 t2 = Application t1 t2 deriving Show instance OpenExpression t (a,env) r => OpenExpression (Lambda a t) env (a->r) instance SymT t env r => OpenExpression (Symbol t) env r instance OpenExpression (Constant r) env r instance (OpenExpression t1 env (a->r), OpenExpression t2 env a) => OpenExpression (Application a t1 t2) env r class OpenExpression t env a => RunOpenExpression t env a | t env -> a --class RunOpenExpression t env a | t env -> a where runOpenExpression :: t -> env -> a instance RunOpenExpression (Constant r) env r where runOpenExpression (Constant r) _ = r instance (RunOpenExpression t1 env (b->a), RunOpenExpression t2 env b) => RunOpenExpression (Application b t1 t2) env a where runOpenExpression (Application t1 t2) env = (runOpenExpression t1 env) (runOpenExpression t2 env) instance RunSym var env r => RunOpenExpression (Symbol var) env r where runOpenExpression (Symbol var) env = runSym var env instance RunOpenExpression t (a,env) r => RunOpenExpression (Lambda a t) env (a->r) where runOpenExpression (Lambda exp) env = \a -> runOpenExpression exp (a,env) newtype Expression t r = MkExpression (forall env. RunOpenExpression t env r => t) runExpression (MkExpression e) = runOpenExpression e () sym0 = Symbol FirstSym sym1 = Symbol (NextSym FirstSym) sym2 = Symbol (NextSym (NextSym FirstSym)) -- k = \x.\y -> x = \.\.1 k = MkExpression (Lambda (Lambda sym1)) -- s :: Expression ((a -> b -> c) -> (a -> b) -> a -> c); -- -- s = \x.\y.\z -> x z (y z) = \.\.\.2 0 (1 0) s = MkExpression (Lambda (Lambda (Lambda (Application (Application sym2 sym0) (Application sym1 sym0))))) test1 = (runExpression k) sym0 sym2 test2 = (runExpression k) sym2 sym0 test3 = (runExpression s) (runExpression k) (runExpression k) -- Note: That can only be the identity! -- :t test3: -- test3 :: forall r. r -> r test3' = test3 "It's the identity!!! Look at the type!!!"

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Two fun things with GADTs
by Ashley Yakeley 08 Jan '05

08 Jan '05

1. Reifying class instances Easy to do, and useful sometimes: data MyMonad m where MkMyMonad :: (Monad m) => MyMonad m myReturn :: MyMonad m -> a -> m a myReturn MkMyMonad = return myBind :: MyMonad m -> m a -> (a -> m b) -> m b myBind MkMyMonad = (>>=) Note this may not yet be available in GHC CVS, see bug #1097046. 2. Casting among a limited set of types data Witness a where BoolWitness :: Witness Bool IntWitness :: Witness Int ListWitness :: Witness a -> Witness [a] data ComposeT p q a = MkComposeT {unComposeT p (q a)} witnessCast :: Witness a -> Witness b -> p a -> Maybe (p b) witnessCast BoolWitness BoolWitness pa = Just pa; witnessCast IntWitness IntWitness pa = Just pa; witnessCast (ListWitness wa) (ListWitness wb) pla = fmap unComposeT (witnessCast wa wb (MkComposeT pla)) witnessCast _ _ _ = Nothing; class Castable a where witness :: Witness a instance Castable Bool where witness = BoolWitness instance Castable Int where witness = IntWitness instance (Castable a) => Castable [a] where witness = ListWitness witness cast :: (Castable a,Castable b) => p a -> Maybe (p b) cast = witnessCast witness witness castOne :: (Castable a,Castable b) => a -> Maybe b castOne a = fmap runIdentity (cast (Identity a)) -- Ashley Yakeley, Seattle WA

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The Implementation of Functional Programming Languages
by Simon Peyton-Jones 08 Jan '05

08 Jan '05

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

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RE: [Haskell-cafe] Unicode: Hugs vs GHC (again) was: Re: Some random newbie questions
by Simon Marlow 08 Jan '05

08 Jan '05

On 07 January 2005 13:01, Dimitry Golubovsky wrote: > Lennart Augustsson wrote: >> Simon Marlow wrote: >> >>> Here's a summary of the state of Unicode support in GHC and other >>> compilers. There are several aspects: >>> >>> - Can the Char type hold the full range of Unicode characters? >>> This has been true in GHC for some time, and is now true in Hugs. >>> I don't think it's true in nhc98 (please correct me if I'm >>> wrong). > > I remember, it was in GHC. But any attempt to output Unicode > characters using standard I/O functions always ended up outputting > only low 8 bits. Has anything changed since then? No, that's still the case (I mentioned it in a separate point in that message). >>> - Do the character class functions (isUpper, isAlpha etc.) work >>> correctly on the full range of Unicode characters? This is true >>> in Hugs. It's true with GHC on some systems (basically we were >>> lazy and used the underlying C library's support here, which is >>> patchy). > > Which basically means that one with older or underconfigured system > where they do not have permissions/technical possibilities to > configure locales in the C library properly is out of luck... True. Anyone care to take Hugs' implementation of the character class functions and put it in GHC? > The reason I asked this question was: I am trying to understand, where > is internationalization of Haskell compilers on their developers' list > of priorities, and also how high is demand from users to have at least > basic internationalization. We're keen to have it. The more people that complain about the lack of it, the faster it'll get done, probably :-) Cheers, Simon

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Preventing space leaks without seq (was: Re: [Haskell-cafe] Some random newbie questions)
by Henk-Jan van Tuyl 07 Jan '05

07 Jan '05

L.S., Olaf <O.Chitil(a)kent.ac.uk> wrote: > I'm constantly surprised hearing from so many people about their space > problems. I cannot remember having space problems with my programs. I > don't know what everybody else is doing wrong I do disagree with > people recommending strictness annotations (seq etc). In contrast, I make > my programs as lazy as possible. Can you give some rules and/or examples how you do this? Henk-Jan van Tuyl -- Using Opera's revolutionary e-mail client: https://secure.bmtmicro.com/opera/buy-opera.html?AID=789433

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Implementing computations with timeout
by Einar Karttunen 07 Jan '05

07 Jan '05

Hello What is the best way of doing an computation with a timeout? A naive implementation using two threads is easy to create - but what is the preferred solution? withTimeout :: forall a. Int -> IO a -> IO (Maybe a) withTimeout time fun = do mv <- newEmptyMVar tid <- forkIO (fun >>= tryPutMVar mv . Just >> return ()) forkIO (threadDelay time >> killThread tid >> tryPutMVar mv Nothing >> return ()) takeMVar mv btw How would I do the same with the new STM abstraction? - Einar Karttunen

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RE: [Haskell-cafe] Hugs vs GHC (again) was: Re: Some randomnewbiequestions
by Simon Marlow 07 Jan '05

07 Jan '05

On 07 January 2005 12:30, Malcolm Wallace wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: > >> Here's a summary of the state of Unicode support in GHC and other >> compilers. There are several aspects: >> >> - Can the Char type hold the full range of Unicode characters? >> This has been true in GHC for some time, and is now true in Hugs. >> I don't think it's true in nhc98 (please correct me if I'm wrong). > > You're wrong :-). nhc98 has always had 32-bit characters internally. I checked the nhc98 sources, and it seems that maxBound::Char is '\255'. >> - Can you use (some encoding of) Unicode for your Haskell source >> files? I don't think this is true in any Haskell compiler right >> now. > > Many years ago, hbc claimed to be the only compiler with support for > this. What encoding(s) did hbc allow in source files? The docs only mention unicode characters inside character & string literals. Cheers, Simon

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RE: [Haskell-cafe] Some random newbie questions
by Simon Peyton-Jones 07 Jan '05

07 Jan '05

| * As far as I can determine, there is no way to check pattern matches for | exhaustiveness. Coming from OCaml, this feels like losing a significant | safety net! How do people program so as not to be getting dynamic match | failures all the time? GHC has -fwarn-incomplete-patterns and -fwarn-overlapped-patterns. But the code implementing these checks is old and crufty, and the warnings are sometimes a bit wrong -- at least when guards and numeric literals are involved. I think they are accurate when you are just using "ordinary" pattern matching. Cleaning up this bit of GHC is a long-standing to-do item, if anyone feels motivated to undertake it. It's a well-defined task, with plenty of well-written papers explaining how to do it -- but it's tricker than it seems at first! Simon

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RE: [Haskell-cafe] The implementation of Functional
by Simon Peyton-Jones 07 Jan '05

07 Jan '05

The online-readable copy is all ready to go. I'm just awaiting a final "ok" from Marnie and I'll announce it. For a paper copy we'll await John's Cafepress efforts. Much thanks to both Marnie and John. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | robert dockins | Sent: 06 January 2005 19:22 | To: haskell-cafe | Subject: [Haskell-cafe] The implementation of Functional | | | The following discussion occurred last September. Is there any kind of | update on any version of this book? | | ----------------------------------------- | | John Meacham writes: | > > >I am looking for the book "The implementation of Functional | > > >Programming languages" by S. L. Peyton Jones. | | > > This book is out of print and currently there is no electronic version | > > of it. The Haskell bookstore folk are working on reconstructing it and | > > making it available for print-on-demand, | > > http://www.cafepress.com/haskell_books/, but it's not clear when | > > exactly it will be available. | > > | > > Your other option is to try to find a used copy, but they are pretty | > > expensive. | > | > I am working on getting that book available in the haskell bookstore. I | > searched quite a while before I found a used printed copy at a | > reasonable price and my search was part of my motivation for creating | > the bookstore. | > | > It is a bit trickier than the other books on the site because I only | > have a scanned in copy of the print version to work with, rather than | > LaTeX source. but I should have time this week to get it online. | > John | | My wife (mainly) and I, with Simon's permission, have been working on | getting a web-enabled version of this available for quite some time. It | hovers on the brink of completion, and should be there Real Soon Now as | well. This will include a web enabled table of contents and next and back | buttons. | | If I'd known how much time she would put in, I'd have never asked her for "a | small favor"....... | | Dave Barton | EDAptive Computing | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Some random newbie questions
by Derek Elkins 07 Jan '05

07 Jan '05

> OK, I'm taking the plunge and using Haskell in a course I'm teaching > this semester. To get ready, I've been doing quite a bit of Haskell > programming myself, and this has raised a few questions... > > * What are the relative advantages of Hugs and GHC, beyond the obvious > (Hugs is smaller and easier for people not named Simon to modify, > while GHC is a real compiler and has the most up-to-date hacks to > the type checker)? Do people generally use one or the other for > everything, or are they similar enough to use Hugs at some moments and > GHC at others? All the below is my personal opinion: My impression is that GHC is by far the most used Haskell implementation. In my opinion, the only reason Hugs should be used is if it's the only implementation that will run on your system, otherwise GHC or NHC will likely make a better choice. Some of the differences between GHC and the last version of Hugs I've looked at are: 1) GHCi views it's repl as being in a do-block, with the most important consequence being the ability to define functions interactively, Hugs views the input as an expression so functions can only be defined locally. Neither are rather impressive interactive environments (not when compared to Squeak or CL listeners), but GHCi's is definitely more convenient. 2) GHC is generally acknowledged to do a (significantly) better job with error messages (both type and run-time errors). To me, the difference is so significant that, all other things being equal, GHC would still win hands-down. 3) Most development of third-party libraries and tools target GHC first, part of that has to do with 4) Beyond enhancements to type checking, GHC has many other extensions such as: template haskell, preemptive concurrency (Hugs does have cooperative concurrency), asynchronous exceptions, built-in arrow notation, support for generics, and more. 5) And of course, (3) is also caused by second order effects of itself, e.g. Yi uses GHC because it uses hs-plugins which is intimately related to GHC. 6) GHC has support for profiling.

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The implementation of Functional
by robert dockins 06 Jan '05

06 Jan '05

The following discussion occurred last September. Is there any kind of update on any version of this book? ----------------------------------------- John Meacham writes: > > >I am looking for the book "The implementation of Functional > > >Programming languages" by S. L. Peyton Jones. > > This book is out of print and currently there is no electronic version > > of it. The Haskell bookstore folk are working on reconstructing it and > > making it available for print-on-demand, > > http://www.cafepress.com/haskell_books/, but it's not clear when > > exactly it will be available. > > > > Your other option is to try to find a used copy, but they are pretty > > expensive. > > I am working on getting that book available in the haskell bookstore. I > searched quite a while before I found a used printed copy at a > reasonable price and my search was part of my motivation for creating > the bookstore. > > It is a bit trickier than the other books on the site because I only > have a scanned in copy of the print version to work with, rather than > LaTeX source. but I should have time this week to get it online. > John My wife (mainly) and I, with Simon's permission, have been working on getting a web-enabled version of this available for quite some time. It hovers on the brink of completion, and should be there Real Soon Now as well. This will include a web enabled table of contents and next and back buttons. If I'd known how much time she would put in, I'd have never asked her for "a small favor"....... Dave Barton EDAptive Computing

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urls
by Tristan Wibberley 06 Jan '05

06 Jan '05

http://research.microsoft.com/Users/simonpj/papers/assoc-types/index.htm http://www.haskell.org/hawiki/GHC_206_2e4

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Begginer question
by Maurício 06 Jan '05

06 Jan '05

Hi, When compiling this: ************ import Complex; complex_root :: (Float, Float, Float) -> (Complex Float, Complex Float) complex_root (a,b,c) = (x1,x2) where { delta = b * b - 4 * a * c :: Float; sqr_delta = if delta >= 0 then (sqrt delta) :+ 0 else 0 :+ (sqrt delta) :: (Complex Float); x1 = (b + sqr_delta)/(2 * a); x2 = (b - sqr_delta)/(2 * a); } *********** I get this error message: ********** Couldn't match `Float' against `Complex Float' Expected type: Float Inferred type: Complex Float In the second argument of `(+)', namely `sqr_delta' In the definition of `x1': x1 = (b + sqr_delta) *********** Can you help me finding what is wrong? Shouldn't "b" be converted to Complex Float and be summed to sqr_delta? Thanks for your help, Maurício

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RE: [Haskell-cafe] Seeking comments on this IO proposal
by Simon Marlow 06 Jan '05

06 Jan '05

On 17 December 2004 16:49, John Goerzen wrote: > First, if someone were to make a working, useful package out of this, > is it likely that it would become the "standard" (whatever that > means) IO system in Haskell anytime in the near future? I ask > because I don't want to put a lot of time into developing an IO > library, and code that works with it, only to have nobody use my code > because it's incompatible with everything they're doing. I'm keen to transition over to a more general IO framework, and I believe Bens/my proposal is heading in the right direction. When we have a more complete implementation, I'd be happy to include it with GHC for experimentation, and over time transition code to use the new framework while leaving the old System.IO in place for the time being. The design isn't by any means set in stone at this stage though. > Second is my own level of expertise. I frankly don't understand how > much of that code could even compile (example: I couldn't find > setNonBlockingFD anywhere in my docs; maybe it's from one of those > GHC.* areas), and I don't really understand the whole array/buffer > situation either. I spent some time reading docs, and I'm still not > sure exactly how one builds a mutable, resiable array. I've also > never done anything but the most trivial FFI work. Much of the reason for the complexity is because I was paying careful attention to performance (perhaps too much attention). I don't want the addition of a text encoding/decoding layer to the IO subsystem to affect performance more than is necessary, and that means doing the translation on raw character buffers rather than strings. Unfortunately this does mean that porting to other compilers is going to be more work. Cheers, Simon

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RE: [Haskell-cafe] Re: Utility functions
by Simon Marlow 06 Jan '05

06 Jan '05

On 06 January 2005 12:20, Ketil Malde wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: > >> There are already a couple of bits of (L)GPL under fptools: GMP and >> readline. GMP we'd like to replace because it is necessarily a part >> of every compiled Haskell program; readline isn't so important but >> it would be nice to have a BSD-licensed replacement. > > Readline is GPL and GMP LGPL, aren't they? Yes. > So GMP only restricts > what you can do with modifications to that library (and thus only > affects developers who want to release a modified GHC without source), > while linking with readline will potentially affect regular users of > GHC. Strictly speaking you have to dynamically link GMP to avoid your program being affected by the LGPL. Most GHC installations use a dynamically linked GMP, but some do not. Readline's license affects GHC (because GHC itself links with it), but it doesn't affect programs you compile with GHC unless you explicitly request the readline package (unlike GMP, which is always linked in). Cheers, Simon

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RE: [Haskell-cafe] Re: Utility functions
by Simon Marlow 06 Jan '05

06 Jan '05

On 30 December 2004 05:25, Philippa Cowderoy wrote: > On Wed, 29 Dec 2004, John Goerzen wrote: > >> But like I've said, I am willing to negotiate with people that >> require code under a license that lets them use it without releasing >> the code. I have also stated that I will put any part of the code >> I've written under the fptools license if that code will be added to >> fptools. (I perceive that as a greater good than license pragmatism >> in this instance.) > > This is the bit I was worried about, the idea of GPLed library code in > fptools disturbs me somewhat. There are already a couple of bits of (L)GPL under fptools: GMP and readline. GMP we'd like to replace because it is necessarily a part of every compiled Haskell program; readline isn't so important but it would be nice to have a BSD-licensed replacement. The policy for Haskell libraries in fptools is that licenses are assigned no finer than package granularity, so that a user of a package can have a reasonable chance of telling whether they are complying with the relevant licenses. Cheers, Simon

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