- Haskell-Cafe - Haskell.org

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 30 Apr '05

30 Apr '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 29 Apr '05

29 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 28 Apr '05

28 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 28 Apr '05

28 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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Newbie question -- fromInt
by Tim Rowe 28 Apr '05

28 Apr '05

I'm working through "The craft of functional programming" , and I've come to excercise 3.14: "Give a function to return the average of three integers". My attempt at an answer is: averageThree :: Int -> Int -> Int -> Float averageThree a b c = fromInt(a + b + c) / 3 but when I try to load this file I get an error, "Undefined variable fromInt". I get the error on winHugs version Nov 2003 and on Hugs version Nov 2003 (in both cases in Hugs mode). Curiously, when I found a machine with an old (2002) version of Hugs on it the file loaded fine. Has something changed between versions of Hugs, is my code reasonable, and how /do/ I convert an Int to a Float in Hugs version Nov 2003? Thanks in advance, -- Tim Rowe

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 28 Apr '05

28 Apr '05

On 28 April 2005 14:26, John Goerzen wrote: > To do that though, we should really identify a permanent home for the > canonical fptools darcs repo. I'm not really set up to provide > accounts for those that would need write access, and I don't want to > be the gatekeeper (I suspect nobody else wants that either <g>). > > If cvs.haskell.org is up to the task, that would be an ideal location > IMHO. I haven't yet convinced tailor.py to work with the pserver for > fptools, so if it can access the repo on the local filesystem, that > would be ideal. Plus, one could then cron it to run frequently. cvs.haskell.org is the way to go - we already have all the required accounts set up on that machine, and you get local access to the CVS repo. I've asked the owner of the box if it can be upgraded. Failing that, you'll just have to make darcs more efficient ;-) There's already an old version of darcs installed (0.9.18), and darcs repos in /home/darcs. Cabal is kept there, for example. We need to upgrade that version of darcs, I presume - can you point me at an appropriate RPM? > Now, having said that, I did keep the request in mind. I figure that > this big repo can be split up into smaller ones at any time after the > CVS mirroring is stopped. For each smaller one, the process would be: > > 1. Branch (darcs get) the master repo > > 2. Delete all the files that don't apply to the smaller project > > 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? >> - commit mails to the cvs-<blah>@haskell.org lists > > I figure a cron job can do this. Every x minutes, run darcs changes > -s, and send copies of never-before-seen logs to the list. Should be > fairly trivial. I can do this too. (But only after the CVS gateway > is disabled; otherwise, you'd get two copies for every commit) Keep the cvs gateway for the time being; at some point we can seamlessly change to generating commit mails from the darcs repository. BTW, one thing I've always wanted is a hyperlink in the commit mail that takes me straight to the patch. Cheers, Simon

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