October 2007 - Haskell-Cafe

Can every monad can be implemented with Cont? (was: New slogan for haskell.org)
by apfelmus 13 Oct '07

13 Oct '07

Don Stewart wrote: > allbery: > >> Didn't someone already prove all monads can be implemented in terms >> of Cont? >> > > Cont and StateT, wasn't it? > And the schemers have no choice about running in StateT :) You sure? I want to see the proof :) Last time I stumbled upon something like this, the "proof" was to embed every monad m in the type type Cont m a = forall b . (a -> m b) -> m b with an instance Monad (Cont m) where ... _independent_ of whether m is a monad or not. The problem I see with it is that we didn't really encode m with it since we're still dependent on return and (>>=) via project :: Monad m => Cont m a -> m a project f = f return and inject :: Monad m => m a -> Cont m a inject x = (x >>=) I mean, the starting point for a concrete monad M are some primitive operations like get :: M s put :: s -> M () and a function observe :: M a -> (S -> (a,S)) together with laws for the primitive operations (= operational semantics) observe (put s >>= x) = \_ -> observe (x ()) s observe (get >>= x) = \s -> observe (x s ) s and for return observe (return a) = \s -> (a,s) Now, implementing a monad means to come up with a type M and functions (>>=) and return that fulfill the monad laws. (In general, the result type of observe is _not_ M a !) This can be done with the standard trick of implementing stuff as constructors (see Unimo for details http://web.cecs.pdx.edu/~cklin/papers/unimo-143.pdf) But - and that's the problem - I don't see how it can be done with Cont in all cases. It works for the above state monad (*) but what about primitives like mplus :: m a -> m a -> m a callcc :: ((a -> m r) -> m r) -> m a that have monadic arguments in a contravariant position (possibly even universally quantified)? Regards, apfelmus *: Here you go: type Observe s a = s -> (a,s) type State s a = Cont (Observe s) a get = \x -> (\s -> observe (x s ) s) -- law for get put s = \x -> (\_ -> observe (x ()) s) -- law for put observe f = f $ \a -> (\s -> (a,s)) -- law for observe (return a)

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Dual Parser Failure???
by PR Stanley 13 Oct '07

13 Oct '07

Hi failure :: (Parser a) failure = \inp -> [] The code might contain some syntax errors and I'd be grateful for any corrections. What is a "dual parser failure"? Thanks, Paul

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WinAmp plugin?
by Andrew Coppin 13 Oct '07

13 Oct '07

Does anybody here know WinAmp? [I feel sure the answer must be yes!] How hard would it be to write a visualisation plugin in Haskell? I think this would be a neat way of demonstrating that "Haskell isn't slow". Also, WinAmp plugins (and, actually, WinAmp) are notoriously buggy and unstable. Would be a nice place to show off how reliable Haskell programs are. OTOH, I have no idea about this kind of thing, so...

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question about throwDyn
by Michael Vanier 12 Oct '07

12 Oct '07

In ghci, why does throw $ ArithException DivideByZero print *** Exception: divide by zero while throwDyn $ ArithException DivideByZero print *** Exception: (unknown) ? Mike

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Defining Tree
by PR Stanley 12 Oct '07

12 Oct '07

Hi I'm reading the chapter on parsers in the Hutton book. The text refers to the data type tree which doesn't seem to be in prelude. So, I was wondering, what would be asuitable tree for a parser? A binary tree perhaps? Are there different types of tree for different parsers? Thanks, Paul

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Re: [Haskell-cafe] WinAmp plugin?
by Andrew Coppin 12 Oct '07

12 Oct '07

pierre wrote: > > > Wouldn't it be better to write not just a visualization plugin, but a whole player from scratсh? :-) > ...this idea also occurred to me. ;-) Why, do *you* know how to decode MP3 data?

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Re: Type-level arithmetic
by Andrew Coppin 12 Oct '07

12 Oct '07

I was actually thinking more along the lines of a programming language where you can just write head :: (n > 1) => List n x -> x tail :: List n x -> List (n-1) x (++) :: List n x -> List m x -> List (n+m) x and so forth. You know, instead of the elaborate simulations crafted out of systems that weren't meant to do this stuff. Of course, the integer case is probably fairly easy. I suspect the issue is that as soon as you try to do this kind of thing, you develop a terminal case of wanting to hyper-generalise everything which then directly results in an incomprehensible mess... ;-)

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[libraries-owner@haskell.org: darcs patch: fix typo in docs.]
by David Roundy 12 Oct '07

12 Oct '07

It seems a little unfriendly to reject contributions from anyone who isn't subscribed to the libraries mailing list... I suppose one solution is to have maintainers for packages. But in this case it should be deprecated having the email of packages set to libraries(a)haskell.org. (Yes, I know I could register and then stop delivery of messages, but that's a bit much to ask random contributors to do.) David ----- Forwarded message from libraries-owner(a)haskell.org ----- Envelope-to: droundy(a)darcs.net Delivery-date: Thu, 11 Oct 2007 14:28:43 -0700 Subject: darcs patch: fix typo in docs. From: libraries-owner(a)haskell.org To: droundy(a)darcs.net You are not allowed to post to this mailing list, and your message has been automatically rejected. If you think that your messages are being rejected in error, contact the mailing list owner at libraries-owner(a)haskell.org. To: libraries(a)haskell.org From: David Roundy <droundy(a)darcs.net> Subject: darcs patch: fix typo in docs. DarcsURL: http://darcs.haskell.org/QuickCheck This fixes a typo in the docs of QuickCheck 2. David Thu Oct 11 17:26:19 EDT 2007 David Roundy <droundy(a)darcs.net> * fix typo in docs. Content-Description: A darcs patch for your repository! New patches: [fix typo in docs. David Roundy <droundy(a)darcs.net>**20071011212619] < > { hunk ./Test/QuickCheck/Arbitrary.hs 92 arbitrary :: Gen a arbitrary = error "no default generator" - -- | Produces a (possibly) empty list of all the possible + -- | Produces a (possibly empty) list of all the possible -- immediate shrinks of the given value. shrink :: a -> [a] shrink _ = [] } Context: [Haddock tweak. bringert(a)cs.chalmers.se**20061126234053] [Added a version of sampling that returns the test cases dpt(a)math.columbia.edu**20061126220704] [Added LICENSE file. bringert(a)cs.chalmers.se**20061126232241] [Fixed silly missed use of Positive constructor. Reported by Audrey Tang. bringert(a)cs.chalmers.se**20061124212824] [Added README. bringert(a)cs.chalmers.se**20061123144917] [Some haddock mark-up fixes to support haddock 0.7. bringert(a)cs.chalmers.se**20061123144831] [Removed BUGS. bringert(a)cs.chalmers.se**20061123144204] [Changed distribution of NonNegative to 1:5 for 0:everything else (was 9:1). bringert(a)cs.chalmers.se**20061120210126] [Added NonZero and reimplemented Positive in terms of NonZero and NonNegative. bringert(a)cs.chalmers.se**20061120205417] [Added BUGS file. bringert(a)cs.chalmers.se**20061120205405] [Added Integral and Real instances for Positive and NonNegative. bringert(a)cs.chalmers.se**20061120204544] [Some haddock tweaks. bringert(a)cs.chalmers.se**20061120112922] [More haddock comments. bringert(a)cs.chalmers.se**20061120112532] [More haddock and export list fixes. bringert(a)cs.chalmers.se**20061117201830] [Added lots of Haddock comments and cleaned up export lists. bringert(a)cs.chalmers.se**20061117200346] [Commented out generics import that Haddock choked on. bringert(a)cs.chalmers.se**20061117152818] [Removed shrinkSub workarounds for old GHC bugs. bringert(a)cs.chalmers.se**20061117152630] [Get rid of the last uses of modules from the haskell98 package, replace with base modules. bringert(a)cs.chalmers.se**20061117152400] [Added Cabal build system. bringert(a)cs.chalmers.se**20061117152230] [Removed Koen's CVS update script. bringert(a)cs.chalmers.se**20061117151819] [Renamed modules to Test.QuickCheck.*. bringert(a)cs.chalmers.se**20061117151417] [More fixes to get examples to compile. bringert(a)cs.chalmers.se**20061114164526] [Fixed some imports in examples. bringert(a)cs.chalmers.se**20061114163012] [Fixed Chalmers/QuickCheck/Monadic.hs to compile with GHC 6.6. bringert(a)cs.chalmers.se**20061114162953] [Moved examples to a top-level directory. bringert(a)cs.chalmers.se**20061114160643] [Fixed ExSet2 and ExLambda to compile with GHC 6.6. bringert(a)cs.chalmers.se**20061114160531] [Comment out generics stuff to compile with GHC 6.6. bringert(a)cs.chalmers.se**20061114160414] [Renamed QuickCheck to Chalmers to get module names right. bringert(a)cs.chalmers.se**20061114160326] [Tailorization of QuickCheck tailor(a)momo.medic.chalmers.se**20061114165514 Import of the upstream sources from the repository /users/cs/koen/CvsRoot as of revision 2006-05-03 18:37:52 by koen ] Patch bundle hash: a1cf7ea46740918443b7069e6caff5a51cff90aa ----- End forwarded message ----- -- David Roundy Department of Physics Oregon State University

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Fast Paced Haskell Tutorial
by Paulo J. Matos 11 Oct '07

11 Oct '07

Hello all, I'm interested in a freely available fast paced haskell tutorial. By fast paced, I means I want something that goes through basic in a very fast pace, presents a couple of examples and then talks about more advanced features. A set of tutorials would be also good. References to these kind of tutorials would be great. Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Type Synonyms
by Tom Davies 11 Oct '07

11 Oct '07

Newbie question: I was wondering the other day if type synonyms might be more useful if they were more restricted, that is, with the definitions: type Foo = String type Bar = String foo :: Foo foo = "a foo" bar :: Bar bar = "a bar" x :: Foo -> ... x f b = ...only valid for Foo Strings... both 'x foo' and 'x bar' type check correctly. Wouldn't it be useful if Foo and Bar were both equivalent to String, but Foo and Bar were not equivalent themselves? For instance, if you are using Strings as properties of something and want to associate the type of the property with its value, without wrapping the String. Would this break a transitivity property of the type system? Am I just suffering from laziness?

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