- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 02 May '05

02 May '05

On 28 April 2005 04:52, John Goerzen wrote: > I am pleased to announce that I have used tailor.py to successfully > convert the entire history of fptools HEAD branch, dating back to > 1996, from CVS to darcs. For those of you that don't know, fptools > represents the development area for the GHC Haskell compiler as well > as many other related projects. It contains 21,949 changesets. Great news, thanks John. Is it possible to set up a two-way synch so we can move over to darcs gradually? It's not really practical for us to move over in one go, we've simply accumulated too many dependencies on CVS, and there are lots of people using the repo with CVS. If we had a two-way synch, we can experiment with darcs non-destructively. Off the top of my head, a few other things we need before we can even think of switching over: - split up the full fptools repo into pieces (as we discussed on #haskell). - a web interface to the repo - commit mails to the cvs-<blah>@haskell.org lists Cheers, Simon

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 01 May '05

01 May '05

On 28 April 2005 16:02, John Goerzen wrote: >>> 3. Rename the smaller project's files as appropriate >>> >>> 4. Checkpoint here >> >> Won't that leave a lot of useless history in each individual project? >> When I do 'darcs get' locally to check out a few sub-repositories, >> won't I get N copies of all the old history? > > Yes, but they'll all be hardlinked together, so no matter how many > copies you get, the old history is only stored on disk once. If I do 'darcs get' to get a bunch of different repositories from cvs.haskell.org to my local filesystem, they won't all end up hard-linked together, surely? > I bet some darcs hacker could come up with a script that would even > let us remove the patches from a descended repo that don't apply > anymore. Now that sounds like a plan. > I know it's not ideal, but trying to manually convert each part of the > CVS repo, one at a time, is far, far less ideal. Not something I care > to attempt, anyway :-) I don't understand why converting each part of the repo separately is so hard? (but I've never used the tools - I'm just curious about why it's so difficult). > The other thing is that having a big repo in darcs may turn out to be > not as bad as we all thought at first. The only really annoying thing > there is how long it takes to do an initial darcs get (~5 min for me > if I use --partial). Other than that, darcs performs quite nicely. I agree with your list of pros/cons, and just to be clear, I'm not stuck on the idea of having a split repo. In fact, the simplicity of a single repo is very attractive. But what worries me is: if I just want to check out e.g. Haddock, I have to get the entire fptools repo (350M+, wasn't it?). I can build a source distribution with just the bits I want, but I can't get a darcs tree with anything but the whole lot. So, here's two potential solutions: 1. Make it possible to 'darcs get' just part of a tree. Patches that don't touch any files in the "live" parts of the tree are discarded. (I don't know if this is possible, or how difficult it is). 2. Create separate repositories for GHC, Happy, Haddock etc., and duplicate the shared fptools structure in each project. Each time we modify something in the shared part of the tree, we pull the patch into the other trees. (is it possible to cherry-pick from a tree that doesn't have a common ancestor? If not, can we make the repositories appear to have common ancestry?). Cheers, Simon

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Random State Monad and Stochastics
by Lemming 01 May '05

01 May '05

When working with QuickCheck (the framework for tests with randomised input) I encountered some interesting connections between random variables in stochastics and their implementation in Haskell. I see the following correspondence between notions in programming and in mathematics: random generator ~ random variable / probabilistic experiment result of a random generator ~ outcome of a probabilistic experiment Thus the signature rx :: (Random a, RandomGen g) => State g a can be considered as "rx is a random variable". In the do-notation the line x <- rx means that "x is an outcome of rx". In a language without higher order functions and using a random generator "function" it is not possible to work with random variables it is only possible to compute with outcomes, e.g. rand()+rand(). In a language where random generators are implemented as objects computing with random variables is possible but still cumbersome. In Haskell we have both options either computing with outcomes do x <- rx y <- ry return (x+y) or computing with random variables liftM2 (+) rx ry This means that liftM like functions convert ordinary arithmetic into random variable arithmetic. But there is also some arithmetic on random variables which can not be performed on outcomes. For example, given a function that repeats an action until the result fulfills a certain property (I wonder if there is already something of this kind in the standard libraries) untilM :: Monad m => (a -> Bool) -> m a -> m a untilM p m = do x <- m if p x then return x else untilM p m we can suppress certain outcomes of an experiment. E.g. if State (randomR (-10,10)) is a uniformly distributed random variable between -10 and 10, then untilM (0/=) (State (randomR (-10,10))) is a random variable with a uniform distribution of {-10, ..., -1, 1, ..., 10}.

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combining HUnit and QuickCheck
by Lemming 01 May '05

01 May '05

I want to run randomized tests and others and I thought it would be a good idea to run QuickCheck tests in the HUnit framework. Eventually if a test fails the program shall fail. I couldn't figure out how to obtain the test result and a meaningful diagnostic message from QuickCheck. The functions I've found (quickCheck, test, runTests) report everything to the standard output. There is also QuickCheck.Batch.run which returns a TestResult. I wonder if there is already a function which decides whether the TestResult is a success or not and a function which turns the TestResult into a error string.

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Writing functions in Haskell.
by Daniel Carrera 30 Apr '05

30 Apr '05

Hello, I'm trying to get started with Haskell. I must say that as good as the language must be, the documentation was been a source of frustration. Only one document actually showed me how to get started (ie. run hugs or ghci), and I was asked to give out my email address before getting it. Sigh... Well, in any event, I did get that document in the end and I've been reading through it. I've compiled Hugs on my Solaris workstation. There were no errors at compilation. Yet, I think there's something seriously wrong with my installation. I can't seem to be able to defin functions: my/prompt $ hugs .... Hugs.Base> fibs 1 = 1 ERROR - Syntax error in input (unexpected `=') And some functions seem to be missing. Hugs.Base> Char.toUpper 'a' ERROR - Undefined qualified variable "Char.toUpper" I was hoping that someone could confirm or deny that my Hugs installation is broken. Thank you for your time. Cheers, Daniel.

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RE: [Haskell-cafe] RFE: Extensible algebraic user-defined data types?
by Simon Peyton-Jones 29 Apr '05

29 Apr '05

| Suppose I want to extend this datatype to the one including | possibility of neither Left or Right (i. e. None). Currently I have to | use Maybe Either, so my options are: | | Just (Left a) | Just (Right b) | Nothing | | If I could extend the Either datatype I might have (syntax may be | different, this is just an example) | | data NEither a b = <Either a b> | None As Ben says, this is altogether a bigger deal. You can imagine two main approaches 1. NEither simply "macro-includes" the constructors of Either in NEither, but they are different constructors. So Either.Left and NEither.Left would be different, and incompatible constructors. This is simple, but clunky; for example, now you'd need to use a qualified name to disambiguate which you meant, whenever you used Left. 2. Use sub-typing, so that a value (Left x) is *both* in type Either and in type NEither. This opens a very large and complicated design space, as Ben mentioned. Simon

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RFE: Extensible algebraic user-defined data types?
by Dimitry Golubovsky 29 Apr '05

29 Apr '05

Hi, Is it possible to have such a feature in the future versions of the Haskell language? For example, there is an Either datatype which is Left a| Right b. Suppose I want to extend this datatype to the one including possibility of neither Left or Right (i. e. None). Currently I have to use Maybe Either, so my options are: Just (Left a) Just (Right b) Nothing If I could extend the Either datatype I might have (syntax may be different, this is just an example) data NEither a b = <Either a b> | None where datatype in angles is a parent datatype, and all its possible data constructors are included, and their list is extended with None. which gave me possibilities: Left a | Right b | None Probably I wouldn't expect to be able to reuse Nothing here (although I might want to) because Nothing already has been defined to be of the type Maybe. This is just a suggestion, that's why it is posted in the Cafe. PS Or is there a similar feature in the language already? Regarding reusing constructor names across several datatypes: is it possible to qualify them with their enclosing datatype name, like Maybe.Nothing where there is a name conflict? Then I might reuse Nothing in my hypothetical data type, and it would be NEither.Nothing if conflicting with Maybe.Nothing PPS I may be missing something again, as always ;) -- Dimitry Golubovsky Anywhere on the Web

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about Ghci
by SCOTT J. 29 Apr '05

29 Apr '05

Hi, I use Windows XP and I like to start Ghci in extended mode. What do I have to do in order not having to type always :set -fglasgow-exts thanks Jan

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RE: [Haskell-cafe] How to speed things up
by Simon Marlow 29 Apr '05

29 Apr '05

On 28 April 2005 19:21, Greg Buchholz wrote: > Dmitry Vyal wrote: >> By the way, how to use Unboxed arrays and unsafeAccumArray Greg >> Buchholz mentioned? I can't find them in GHC 6.2 documentation. > > > http://www.haskell.org/~simonmar/haddock-example/Data.Array.Base.html Please use the documentation on the GHC site. The latest is here: http://www.haskell.org/ghc/docs/latest/html/libraries/ if you specifically need documentation for the libraries that came with GHC 6.2, it can be found here: http://www.haskell.org/ghc/docs/6.2/html/libraries/ Cheers, Simon

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How to speed things up
by Dmitry Vyal 29 Apr '05

29 Apr '05

Hello everybody. I have a long list consisted of a small number (about a dozen) of elements repeating in random pattern. I know all the possible elements. I need to count number of occurences of each particular element and to do i quickly. For example quick_func Eq a => [a] -> [(a,Int)] quick_func [1,2,3,1,2,9,1,9] == [(1,3),(2,2),(3,1),(9,2)] According to profiler this function is the bottle-neck in my sluggish program so I really need to speed it up. Any proposals?

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