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FW: Survival of generic-classes in ghc
by Simon Peyton-Jones 25 Feb '02

25 Feb '02

I had meant to send this message to the Haskell list, because I think there may be some readers who have good ideas about it. Simon -----Original Message----- From: Simon Peyton-Jones [mailto:simonpj@microsoft.com] Sent: 20 February 2002 21:16 To: Patrik Jansson; Haskell Cafe List Cc: Ralf Hinze; Johan Jeuring Subject: RE: Survival of generic-classes in ghc | and I know from testing earlier versions that the | generic-classes support was pretty buggy. Is there any hope | of a revival? Does it already work in CVS? I suspect it will | rot away if nobody works on it. | | Any comments from the implementors - does the idea "fit well" | with ghc? Is it a lot of work to make it more stable? No, it should not be a lot of work to revive, and I will do it. Keep bugging me until I do. One thing that is slowing me down is a design uncertainty: how to deal cleanly with constructors. To write read, show, etc, one needs access to the constructor name, fixity etc. We dance around that a little in the paper. I'm thinking of this design: -- An example data type data List a = Nil | Cons a (List a) -- The "ring" type derived from it -- Note the Con wrapped around the constructors data ListO a = Con NilT 1 :+: Con ConsT (a :*: List a) -- Con is part of the Generics library, like 1, :*:, :+: newtype Con c a = Con a class ConCls c where name :: c -> String arity :: c -> Arity ..etc.. data NilT -- Phantom types data ConsT -- one per constructor instance ConCls NilT where name _ = "Nil" ...etc... instance ConCls ConsT wher name _ = "Cons" ...etc... Now you can write class Show a where show :: a -> String show {| Con c t |} (Con x) = (name (undefined::c)) ++ show x ...etc.. Note the "undefined::c"; it is really just a type argument to "name", so that we get the string for the right constructor. It's a bit clunky, though. Another possiblity would be to make the ConCls class look like this class ConCls c where name :: String arity :: Int ...etc... Now we'd have to give an explicit type argument at the call site: show {| Constr c t |} (Con x) = (name {| c |}) ++ show x I quite like the thought of being able to supply explicit type arguments.... but I don't konw how to speak about the order of type parameters. What order does map takes its two type parameters in? Another question is whether we need an indication that we have reached a field of the constructor. Should we have: data ListO a = Con NilT (Field 1) :+: Con ConsT (Field a :*: (Field List a)) newtype Field a = Field a Without this, it's hard to write a generic function to count the number of fields of the data type. But in general, one might want to know when making the transition from "this" type to the argument types of the constructors. I think that the main reason I'm not making much progress is that I don't know what design choices to make. I'd be glad to discuss it with interested parties. Simon

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ANNOUNCE: HUnit 1.0
by Dean Herington 22 Feb '02

22 Feb '02

HUnit is a unit testing framework for Haskell, similar to JUnit for Java. I have made HUnit 1.0 available at <http://hunit.sourceforge.net>. >From the User's Guide: A test-centered methodology for software development is most effective when tests are easy to create, change, and execute. The JUnit tool pioneered support for test-first development in Java. HUnit is an adaptation of JUnit to Haskell, a general-purpose, purely functional programming language. With HUnit, as with JUnit, you can easily create tests, name them, group them into suites, and execute them, with the framework checking the results automatically. Test specification in HUnit is even more concise and flexible than in JUnit, thanks to the nature of the Haskell language. HUnit currently includes only a text-based test controller, but the framework is designed for easy extension. (Would anyone care to write a graphical test controller for HUnit?) Dean Herington

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Verba Volant
by request＠logos.net 21 Feb '02

21 Feb '02

We have been requested to insert the following email address, "haskell(a)haskell.org", in the Verba Volant Newsletter database. Through this daily service you will receive a quotation, selected from amongst the most celebrated philosophers, writers and poets of all time and translated into many languages and dialects by volunteers worldwide. If you would like to confirm your subscription to Verba Volant, please click on the following link: http://www.logos.net/owa-l/press.subscribe?lang=en&email=haskell@haskell.org If you do not wish to click on the link, your subscription will be cancelled. Thank you for your time. Verba Volant Il nous a été demandé d'ajouter l'adresse électronique "haskell(a)haskell.org" dans la liste des destinataires de Verba Volant, un service qui tous les jours vous adressera une citation sélectionnée parmi les uvres des meilleurs philosophes, écrivains, poètes de tous les temps et traduite en de très nombreuses langues grâce à des volontaires du monde entier. Pour confirmer l'inscription à Verba Volant, veuillez vous connecter au lien suivant: http://www.logos.net/owa-l/press.subscribe?lang=fr&email=haskell@haskell.org Si vous préférez ne pas cliquer sur le lien, vous ne recevrez rien. Merci dans tous les cas de nous avoir accordé quelques secondes. Verba Volant Se nos ha solicitado insertar la dirección de correo electrónico "haskell(a)haskell.org" en el listado de envíos de Verba Volant, un servicio que diariamente le enviará citas elegidas entre los mejores filosofos, escritores, poetas, etc., traducidas a varios idiomas y dialectos. Dichas citas están traducidas por voluntarios que se conectan a nuestra web desde todo el mundo. Si quiere confirmar la suscripción a Verba Volant, le rogamos entre en: http://www.logos.net/owa-l/press.subscribe?lang=es&email=haskell@haskell.org Si no entra en la dirección señalada no recibirá las citas. Muchas gracias por el tiempo que nos ha dedicado. Verba Volant Ci è stato chiesto di inserire l'indirizzo di posta elettronica "haskell(a)haskell.org" nellelenco dei destinatari di Verba Volant, un servizio che ogni giorno ti invierà una citazione scelta tra quelle dei migliori filosofi, scrittori, poeti di tutti i tempi e tradotta in moltissime lingue e dialetti grazie alla collaborazione di volontari da tutto il mondo. Se desideri confermare l'iscrizione, ti preghiamo di collegarti al seguente link: http://www.logos.net/owa-l/press.subscribe?lang=it&email=haskell@haskell.org Nel caso preferissi non cliccare sul link, non riceverai nulla. Grazie comunque per i secondi che ci hai dedicato. Cordiali saluti.

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declaring a generic type in type synonyms
by Andre W B Furtado 21 Feb '02

21 Feb '02

Is is possible to declare a generic type without using "data" or "newtype"? For example, I woud like that "pair" is a type synonym for "(t,t)" where t is a generic type, but just saying: > type pair = (t,t) won't work: i get a parse error. Thanks, -- Andre

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Graphs
by Wojciech Fraczak 21 Feb '02

21 Feb '02

Hi everybody, I would like to ask you, if you do not know any graph library for haskell. I would like to use haskell for prototyping some of algorithms on graphs and finite state automata/transducers. In fact what I'm looking for is a good (efficient and easy readable) data type definition for labeled graphs. Thanks, Wojtek

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Monadic Call/CC?
by Ashley Yakeley 21 Feb '02

21 Feb '02

Has anyone investigated monadic call-with-current-continuation in Haskell? Given this: class (Monad m) => PeirceMonad m where { peirceM :: ((a -> m b) -> m a) -> m a; }; ...which Monads can be made PeirceMonads? The corresponding non-monadic function: peirce :: ((a -> b) -> a) -> a; probably can't be defined. -- Ashley Yakeley, Seattle WA

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Re: forcing IO operations
by Jay Cox 21 Feb '02

21 Feb '02

On Wed, 20 Feb 2002, Andre W B Furtado wrote: > hi, you sent me "blank" email (nothing besides my own message). Were you > trying to tell something? :) > > -- Andre Heh, actually I was. I curse the day that the designers of pine put (control-X) for sending a letter next to (control-C) for killing it :) Please don't take offense to this, but do you feel unsafePerformIO is really necessary for your application? There's got to be another way... In Systematic Design of Monads [1] there is given a systematic method to wrap monads around other monads. I was going to email you an example monad which wraps around the IO monad. Granted, its a state monad wrapped around the IO monad, which may not be of much use except for organizational purposes. I've essentially stolen the design from the aforementioned paper, but since the paper appears to be for Gofer, the Haskell precursor, I decided to modernize it a bit. By the way, seems like there might be something along the lines of this in the hugs library. (If not, I'm sure I remember something about reader and writer monad wrappers) Jay Cox [1]written by John Hughes & Magnus Carlson (1996) Appologies to both since I cant give a more specific reference to the paper. John (or Magnus), you wouldn't have happened to taken the time to have updated the code from that paper, have you? If you already have, I appologize for not refering to it and using it as well :) ---- If you cut and paste the following lines I think they ought to be able to run as in a haskell.lhs source file. You proposed using some runMyMonad function. in order to combine IO actions with your code the runction returns and IO (blah). see, the IO is hidden within the new monad but IO actions are still being threaded through MyMonad. >newtype MyNewStateMonad m s a = StateMonad (s -> m (a,s)) > > >bindST (StateMonad x) f = > StateMonad (\s -> ( x s >>= \(v,s') -> let StateMonad g = f v in g s')) > >unitST v = StateMonad (\s -> return (v,s)) > > >instance Monad m => Monad( MyNewStateMonad m s) where > (>>=) = bindST > return = unitST > > >type MyMonad a = MyNewStateMonad IO Int a > >runMyMonad :: MyMonad a -> Int ->IO (a,Int) -- (a,Int) the state tuple >runMyMonad (StateMonad f) = f that's it! the remainder is mostly example code it: 1. lifts print and getLine to monadic actions under MyMonad (aka MyNewStateMonad IO Int) 2. has an example monadic action (add50toMyMonadState) which has absoultely nothing to do with the underlying IO monad. 3. lifted a sequence of IO actions to be executed under MyMonad via arbitrarysequence 4. combines all the actions under MyMonad into monad_actions so that they may be run with runMyMonad. note the line: y<- runMyMonad monad_actions 0 the 0 initializes the state of the MyMonad, (hence this is why runMyMonad is typed MyMonad a -> Int ->IO (a,Int) ) >liftIOtoMyMonad' :: (a -> IO ()) -> a -> MyMonad () >liftIOtoMyMonad' p q = > StateMonad $ \s -> do p q > return ((),s) > >liftIOtoMyMonad :: IO a -> MyMonad a >liftIOtoMyMonad p = > StateMonad $ \s -> do y <- p > return (y,s) > >liftIOtoMyMonad_ :: IO () -> MyMonad () >liftIOtoMyMonad_ m = liftIOtoMyMonad' (const m) () > > >getLineMM = liftIOtoMyMonad getLine >printMM = liftIOtoMyMonad' print >arbitrarysequence = > liftIOtoMyMonad $ > do print "What's your name?" > name <-getLine > print $ "Your name is " ++ name > > >add50toMyMonadState :: MyMonad () >add50toMyMonadState = StateMonad(\s -> return ((),50+s)) > > >monad_actions= do arbitrarysequence > printMM "What's your password" > pass <- getLineMM > printMM ("Your password is: " ++ pass) > add50toMyMonadState > return 3 > > >main = do y<- runMyMonad monad_actions 0 > print $ "State:" ++ show (snd y) ++ " Value:" ++ show (fst y) If you are reading this you should be instead copying it to some file and playing with it instead!

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ICFP 2002: write now!
by Simon Peyton-Jones 20 Feb '02

20 Feb '02

Don't miss your chance to submit a paper to ICFP02. It'll be fun! Simon PJ ICFP 2002 International Conference on Functional Programming October 4-6, 2002, Pittsburgh, USA Final call for papers -------------------------------------------- Submission deadline: 21 March 2002 18:00 UTC -------------------------------------------- Program Chair: Simon Peyton Jones (Microsoft Research) Full call for papers including submission details: http://icfp2002.cs.brown.edu/CfP/ ICFP 2002 seeks original papers on the full spectrum of the art, science, and practice of functional programming. The conference invites submissions on all topics ranging from principles to practice, from foundations to features, and from abstraction to application. The scope covers all languages that encourage programming with functions, including both purely applicative and imperative languages, as well as languages that support objects and concurrency. Topics of interest include, but are not limited to, the following: Foundations: formal semantics, lambda calculus, type theory, monads, continuations, control, state, effects. Design: modules and type systems, concurrency and distribution, components and composition, relations to object-oriented and logic programming, multiparadigm programming. Implementation: abstract machines, compile-time and run-time optimization, just-in-time compilers, memory management, foreign-function and component interfaces. Transformation and analysis: abstract interpretation, partial evaluation, program transformation, theorem proving, specification and verification. Software development techniques for functional programming: design patterns, specification, verification and validation, debugging, test generation, etc. Human productivity of functional programming: visual, graphical (etc) approaches, evaluating language usability, empirical studies of human effectiveness, etc. Applications and domain-specific languages: scientific and numerical computing, symbolic computing and artificial intelligence, systems programming, databases, graphic user interfaces, multimedia programming, Web programming. Practice and experience: functional programming in education and industry, ramifications on other paradigms and computing disciplines. Functional pearls elegant, instructive examples of functional programming. General Chair: Mitchell Wand (Northeastern University) Program Committee Matthias Blume (Lucent) Margaret Burnett (Oregon State University) Manuel Chakravarty (University of New South Wales) Matthew Flatt (University of Utah) Haruo Hosoya (Kyoto University) Uwe Nestmann (EPFL, Lausanne) Chris Okasaki (United States Military Academy) Norman Ramsey (Harvard University) David Sands (Chalmers University) Olin Shivers (Georgia Tech) Stephanie Weirich (Cornell) Joe Wells (Heriot Watt University)

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RE: foralls in class assertions
by Simon Peyton-Jones 20 Feb '02

20 Feb '02

OK, so it does look as though it's the same idea as that described in our paper. Good. I have not implemented, yet. As always my implementation priorities are strongly influenced by my perception of whether some enhancement would be used. Maybe you can outline why such a change would be useful to you? I only have the example in our paper as motivation so far. Simon | -----Original Message----- | From: Ashley Yakeley [mailto:ashley@semantic.org] | Sent: 20 February 2002 01:08 | To: Simon Peyton-Jones; Haskell List | Subject: RE: foralls in class assertions | | | At 2002-02-19 09:21, Simon Peyton-Jones wrote: | | >I don't know if it makes sense. You've written down some syntax, | >but it's not clear to me what you intend by it. | | Hmm... it should be straightforward... | | > instance (forall a. Eq a => Eq (f a)) => Eq (Rose f a) where.. | | I assume that the 'a' quantified in (forall a. ...) is not | the same as | the 'a' in 'Rose f a'? | | >| instance | >| ( | >| forall a. HasIdentity (m a a), | >| forall a b c. Composable (m b c) (m a b) (m a c) | >| ) => | >| Category m; | | This means 'if for all a, "HasIdentity (m a a)", and also for | all a b c, | "Composable (m b c) (m a b) (m a c)", then "Category m"'. | | "(forall a. HasIdentity (m a a))" as a class assertion declares a | property of m. It says that for all types a, there's an instance | "HasIdentity (m a a)". | | >| Or even allow the foralls their own context: | >| | >| foo :: (forall a. (C a b) => D a c) => T b c; | | This means foo has type (T b c), where for every type a for | which there's | an instance "C a b", there's an instance "D a c". | | >| class | >| ( | >| forall a. HasIdentity (m a a), | >| forall a b c. Composable (m b c) (m a b) (m a c) | >| ) => | >| Category m; | | Personally I think the 'superclass' arrow in class | declarations should | point the other way, as in '<='. After all, instances of the | class imply | instances of the superclasses, not the other way around. | | But that aside, this means '"Category" is a class on m, | provided that for | all types a, there's an instance "HasIdentity (m a a)", and | also for all | types a b c, there's an instance "Composable (m b c) (m a b) | (m a c)"'. | | | -- | Ashley Yakeley, Seattle WA | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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RE: Haskell 98 Report
by Simon Peyton-Jones 20 Feb '02

20 Feb '02

I don't want to do that until its finished! Which I earnestly hope will be soon. Simon | -----Original Message----- | From: David Feuer [mailto:dfeuer@cs.brown.edu] | Sent: 20 February 2002 08:43 | To: haskell(a)haskell.org | Subject: Haskell 98 Report | | | Is the revised Haskell98 report going to be put in its proper | place on the web site any time soon? As it is, it is sitting | off to the side while the old Report sits in the seat of honor. | | David Feuer | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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