July 2009 - Haskell-Cafe

Pattern matching does not work like this?
by Magicloud Magiclouds 16 Jul '09

16 Jul '09

Hi, I do not notice this before. "fun ([0, 1] ++ xs) = .." in my code could not be compiled, parse error. -- 竹密岂妨流水过山高哪阻野云飞

11 13

Re: Problems with nested Monads
by John Lato 15 Jul '09

15 Jul '09

If M is a monad transformer (e.g. StateT) and able to be parameterised over an arbitrary monad, you can do something related, which may actually be more useful to you. Given the definition of StateT from mtl: newtype StateT s m a = StateT { runStateT :: s -> m (a, s) } you could define this join-like function: join2 :: (Monad n) => n (StateT s n a) -> StateT s n a join2 m = StateT (\state -> m >>= flip runStateT state) (I don't know a good name for this function; it's called "joinIM" in iteratee) Perhaps this would apply to your situation? Cheers, John Lato > > Message: 18 > Date: Fri, 10 Jul 2009 17:25:55 -0400 > From: Job Vranish <jvranish(a)gmail.com> > Subject: Re: [Haskell-cafe] Problems with nested Monads > To: Edward Kmett <ekmett(a)gmail.com> > Cc: Haskell Cafe mailing list <haskell-cafe(a)haskell.org> > Message-ID: > <ed4cefb50907101425w118990e2x5d6585582157e08f(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Yeah, I think the problem with my case is that while M is a specific monad > (essentially StateT), N can be an arbitrary monad, which I think destroys my > changes of making a valid joinInner/joinOuter/distribute. > Maybe someday Haskell will infer valid joinInner/joinOuter for simple cases > :D > Thanks for you help. I'll definitely have to see if I can find that paper. > > - Job Vranish > > On Fri, Jul 10, 2009 at 3:09 PM, Edward Kmett <ekmett(a)gmail.com> wrote: > >> The problem you have is that monad composition isn't defined in general. >> You would need some form of distributive law either for your monads in >> general, or for your particular monads wrapped around this particular kind >> of value. >> >> What I would look for is a function of the form of one of: >> >> distribute :: N (M a) -> M (N a) >> joinInner :: M (N (M a)) -> M (N a) >> joinOuter :: N (M (N a)) -> M (N a) >> >> that holds for your partiular monads M and N. >> >> IIRC Mark P. Jones wrote a paper or a lib back around '93 that used these >> forms of distributive laws to derive monads from the composition of a monad >> and a pointed endofunctor. >> >> -Edward Kmett >> >> On Fri, Jul 10, 2009 at 11:34 AM, Job Vranish <jvranish(a)gmail.com> wrote: >> >>> I'm trying to make a function that uses another monadic function inside a >>> preexisting monad, and I'm having trouble. >>> Basically my problem boils down to this. I have three monadic functions >>> with the following types: >>> f :: A -> M B >>> g :: B -> N C >>> h :: C -> M D >>> (M and N are in the monad class) >>> I want a function i where >>> i :: A -> M (N D) >>> >>> the best I can come up with is: >>> i :: A -> M (N (M D)) >>> i a = liftM (liftM h) =<< (return . g) (f a) >>> >>> I'm starting to feel pretty sure that what I'm going for is impossible. Is >>> this the case? >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>

1 0

homomorphic encryption and monads?
by Jason Dagit 15 Jul '09

15 Jul '09

Hello, I have just a very vague understanding of what homomorphic encryption is, but I was wondering if homomorphic encryption behaves as a one-way monad (like IO). So for example, suppose you wrote a function that worked on encrypted email to determine if it spam, maybe it looks like this: isSpam :: Cypher String -> Cypher Bool I could be mistaken about the way this works, but it seems that isSpam can't return a plain 'Bool' because then you could know things about the encrypted data without having to decrypt it first. So that is why I think it has to return "Cypher Bool". And because it's a homomorphic encryption, you could have something like doWork: doWork :: Cypher a -> (a -> b) -> Cypher b Which we could use to implement isSpam: isSpam s = doWork s spamTest where spamTest :: String -> Bool I initially thought that doWork should be (>>=), but it seems the type of the function should either be (a -> b) or (Cypher a -> Cypher b). That is, return :: a -> Cypher a, is not available. All the data should already be encrypted. Thinking about it a bit more, since we never have to decrypt the data to work with it, it seems that (a -> b) is wrong as well, because we don't have the key or whatever to do the decrypting. So, then it seems reasonable that the only type for doWork is this: doWork :: Cypher a -> (Cypher a -> Cypher b) -> Cypher b Which doesn't really seem all that useful now. On the other hand, maybe we have an algorithm which can take a function on normal values and gives us a function on Cypher values: enCypher :: (a -> b) -> Cypher a -> Cypher b Or perhaps, it should be: enCypher :: (a -> b) -> Cypher (a -> b) Which would match the type of return. If that is the type of return, then we probably want: (>>=) :: Cypher (a -> b) -> ((a -> b) -> Cypher (c -> d)) -> Cypher (c -> d) At this point, I'm not actually sure if the second type for enCypher makes any sense. But, if it does, then I think it says the purpose of the monad is to act on the transformations and not the data. That is, instead of focusing on the data as being held in the cypher, we are thinking of the functions as being transformed into a cypher space. Has anyone else been thinking about this? Jason

3 2

ANNOUNCE: generator
by Yair Chuchem 15 Jul '09

15 Jul '09

A new "generator" package has been uploaded to Hackage. It implements an alternative list monad transformer, a list class, and related functions. The difference from mtl/transformers's ListT is that mtl is a monadic action that returns a list: newtype ListT m a = ListT { runListT :: m [a] } generator's is a monadic list: data ListItem l a = Nil | Cons { headL :: a, tailL :: l a } newtype ListT m a = ListT { runListT :: m (ListItem (ListT m) a) } A short example program which reads numbers from the user and interactively sums them up: import Control.Monad.ListT (ListT) import Data.List.Class (execute, joinM, repeat, scanl, takeWhile) import Prelude hiding (repeat, scanl, takeWhile) main = execute . joinM . fmap print . scanl (+) 0 . fmap (fst . head) . takeWhile (not . null) . fmap reads . joinM $ (repeat getLine :: ListT IO (IO String)) I also wrote an example/blog-post about using ListT to add an undo option to the classic game of "hamurabi": http://mashebali.blogspot.com/2009/07/charlemagne-disraeli-and-jefferson.ht… Another interesting observation is that "ListT [] a" is a tree of "a"s. The module Data.List.Tree includes functions to prune and search such trees (dfs, bfs, bestFirstSearchOn, etc). This can be useful for modularizing code that uses the list monad for combinatoric search by decoupling tree creation from processing and pruning.

1 0

ANN: shelltestrunner 0.6 released
by Simon Michael 15 Jul '09

15 Jul '09

(and, the original which I didn't cc to -cafe.) I'm pleased to announce the first release of shelltestrunner: a small tool for testing any command-line program by running it through "shell" tests defined with a simple file format. Each test can specify the command-line arguments, input, expected output, expected stderr output, and/or expected exit code. This was extracted from the hledger project, inspired by the tests in John Wiegley's ledger project, and uses Max Bolingbroke's test- framework. In some cases you will get a big speedup by using test- framework's parallelising feature. Example: $ shelltestrunner shelltestrunner *.test -- -j8 :args.test:1: [OK] :args.test:2: [OK] :args.test:3: [OK] :args.test:4: [OK] :early-j-option.test: [OK] :help-flag.test: [OK] etc. I hope you find it useful. Feedback, patches, or alternate packaging suggestions are welcome! Best, -Simon

1 0

Re: ANN: shelltestrunner 0.6 released
by Simon Michael 15 Jul '09

15 Jul '09

And the urls: home - http://hackage.haskell.org/package/shelltestrunner darcs repo - http://joyful.com/repos/shelltestrunner

1 0

Problems with nested Monads
by Job Vranish 15 Jul '09

15 Jul '09

I'm trying to make a function that uses another monadic function inside a preexisting monad, and I'm having trouble. Basically my problem boils down to this. I have three monadic functions with the following types: f :: A -> M B g :: B -> N C h :: C -> M D (M and N are in the monad class) I want a function i where i :: A -> M (N D) the best I can come up with is: i :: A -> M (N (M D)) i a = liftM (liftM h) =<< (return . g) (f a) I'm starting to feel pretty sure that what I'm going for is impossible. Is this the case?

5 5

ANN: darcs 2.3 beta 4
by Petr Rockai 15 Jul '09

15 Jul '09

Hi again! We have decided to delay the release cycle slightly for 2.3 and release another beta. The primary reason for this is our Windows support -- I would like to invite all Windows users to install darcs-beta and give it a ride. If you have a working cabal-install, all you need to do is run: $ cabal update $ cabal install darcs-beta (this works on all platforms; you may need to use -f-curl, if you don't have the cURL headers available -- this is often the case on Windows). If you do not have cabal-install, please follow the instructions near the end of this message, "Installing on Windows". In addition to using cabal-install, you can also download a tarball from http://repos.mornfall.net/darcs/darcs-2.2.98.4.tar.gz and build manually (see the build instructions in README inside the tarball). Feedback -------- To make the darcs 2.3 release a good one, we still need testing feedback: please drop a note to darcs-users@ if you have installed darcs-beta, or failed to install it. If you run into any bugs, we need to know about them. Thanks! The question of GHC 6.8 ----------------------- Using GHC 6.10.3 or newer is *strongly recommended*. You may compile darcs with GHC 6.8, but there are several caveats. If you are using 6.8.2 or older, please disable mmap support (pass -f-mmap to cabal install or runghc Setup configure below). Note that the GHC 6.8.2 that ships with Debian Lenny is not affected and it should be safe to keep mmap enabled. It is also recommended to disable use of Hackage zlib when compiling with GHC 6.8.2 (including the Debian Lenny version): pass -f-zlib to cabal. When using zlib, we have seen occasional crashes with error messages like "openBinaryFile: file locked" -- this is a known GHC 6.8.2 bug (and is fixed in GHC 6.8.3). Last, if you are using a 64-bit system, darcs may hang when you exit a pager when compiled with GHC older than 6.10.3. Although this is harmless, it is quite inconvenient. Overall, the status of GHC 6.8 is "semi-supported": for many cases, things will work just fine, especially if you take a little extra caution with compilation flags. Installing on Windows --------------------- To install darcs on Windows systems from scratch, please download the Haskell Platform and MSYS: * http://hackage.haskell.org/platform/2009.2.0.1/HaskellPlatform-2009.2.0.1-s… * http://sourceforge.net/projects/mingw/files/MSYS+Base+System/MSYS-1.0.11-rc… After installing both, you should have an "MSYS" icon: run MSYS and in the terminal window type: $ cabal update $ cabal install darcs-beta -f-curl This should download, compile and install all required dependencies and darcs-beta itself. The resulting darcs executable will be placed into the Haskell Platform executables folder, and should be accessible from the MSYS shell (just type "darcs --version" to check). Yours, Petr.

1 0

Build 32-bit apps on 64-bit Windows?
by Lyle Kopnicky 15 Jul '09

15 Jul '09

If I am running GHC on 64-bit Windows, do I have a choice of building a 32-bit or 64-bit app? On a cursory glance through the command-line options, I didn't find anything. - Lyle

2 1

Circular pure data structures?
by John Ky 15 Jul '09

15 Jul '09

Hello, Is it possible to create a circular pure data structure in Haskell? For example: a :: Data let b = getNext a let c = getNext b c == a -- Gives True Thanks, -John

10 11