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Re: [Haskell] IVar
by Simon Marlow 06 Dec '07

06 Dec '07

Jan-Willem Maessen wrote: >> Consider this: >> >> do >> x <- newIVar >> let y = readIVar x >> writeIVar x 3 >> print y >> >> (I wrote the let to better illustrate the problem, of course you can >> inline y if you want). Now suppose the compiler decided to evaluate y >> before the writeIVar. What's to prevent it doing that? Nothing in >> the Haskell spec, only implementation convention. > > Nope, semantics. If we have a cyclic dependency, we have to respect > it---it's just like thunk evaluation order in that respect. Ah, so I was thinking of the following readIVar: readIVar = unsafePerformIO . readIVarIO But clearly there's a better one. Fair enough. Cheers, Simon

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Re: [Haskell-cafe] ghc overlapping instances -> solved
by Steffen Mazanek 06 Dec '07

06 Dec '07

Hello, Isaac, this works for me. Thx a lot, Steffen 2007/12/5, Isaac Dupree <isaacdupree(a)charter.net>: > > Steffen Mazanek wrote: > > Hi, > > > > Stefan and Isaac, thx for providing quick advice. > > > > @Stefan: Unfortunately I have to use a list. > > @Isaac: I do not get it. Could you please provide a short example of > your > > approach? > > > > The question still remains. Which arguments do I have ghc to start with > to > > get the same behavior than hugs with -98 +o (here it works). > > > > I provide my example for testing purposes: > > > > module Test where > > import Test.QuickCheck > > import Monad(liftM,liftM2) > > > > type Program = [Stmt] > > data Stmt = Text | IfElse Program Program | While Program deriving (Eq, > > Show) > > > > instance Arbitrary [Stmt] where > > arbitrary = sized genProg > > instance Arbitrary Stmt where > > arbitrary = sized genStmt > > > > genStmt::Int->Gen Stmt > > genStmt 0 = return Text > > genStmt 1 = return Text > > genStmt 2 = oneof [return Text, return (While [Text])] > > genStmt n | n>2 = oneof ([return Text, > > liftM While (genProg (n-1))]++ > > [liftM2 IfElse (genProg k) (genProg > > (n-k-1))|k<-[1..n-2]]) > > > > genProg::Int->Gen Program > > genProg 0 = return [] > > genProg 1 = return [Text] > > genProg n | n>1 = oneof ((liftM (\x->[x]) (genStmt n)):[liftM2 (:) > (genStmt > > k) (genProg (n-k))|k<-[1..n-1]]) > > > > prop_ConstructParse progr = True > > where types = progr::Program > > > > main = mapM_ (\(s,a) -> putStrLn s >> a) [("flowchart construct and > parse", > > test prop_ConstructParse)] > > is prop_ConstructParse the only thing that breaks when you remove the > instance Arbitrary [Stmt] where arbitrary = sized genProg, or have I > missed something? If that's all, try this (untested) : > > prop_ConstructParse = forAll (sized genProg) (\progr -> True) > > and similarly for other properties. > > Or, you _can_ use a newtype for quickcheck-only, something like this: > > newtype P = P { unP :: Program } > instance Show P where show = show . unP > instance Arbitrary P where arbitrary = sized genProg . unP > prop_ConstructParse (P progr) = True > > > Isaac > -- Dipl.-Inform. Steffen Mazanek Institut für Softwaretechnologie Fakultät Informatik Universität der Bundeswehr München 85577 Neubiberg Tel: +49 (0)89 6004-2505 Fax: +49 (0)89 6004-4447 E-Mail: steffen.mazanek(a)unibw.de

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matching
by Ryan Bloor 06 Dec '07

06 Dec '07

hi I have a matching problem... I am wanting to identify whether or not a string is an opening substring of another (ignoring leading spaces). I have this: word is a single word and str is a string. match :: String -> String -> (Bool, String)match word str | if removeSpace str `elem` (removeSpace word) ++ rest = (True, rest) | otherwise == (False,str) where rest = str Any help? Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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New slogan for haskell.org
by Don Stewart 06 Dec '07

06 Dec '07

It was raised at CUFP today that while Python has: Python is a dynamic object-oriented programming language that can be used for many kinds of software development. It offers strong support for integration with other languages and tools, comes with extensive standard libraries, and can be learned in a few days. Many Python programmers report substantial productivity gains and feel the language encourages the development of higher quality, more maintainable code. With the links from the start about using Python for various purposes, along with reassuring text about licenses and so on. Note its all about how it can help you. The Haskell website has the rather strange motivational text: Haskell is a general purpose, purely functional programming language featuring static typing, higher order functions, polymorphism, type classes, and monadic effects. Haskell compilers are freely available for almost any computer. Which doesn't say why these help you. Any suggestions on a 2 or 3 sentence spiel about what's available? Here's some quick points: General purpose: applications from OS kernels to compilers to web dev to ... Strong integration with other languages: FFI, and FFI binding tools Many developer tools: debugger, profiler, code coverage, QuickCheck Extensive libraries: central library repository, central repo hosting Productivity, robustness, maintainability: purity, type system, etc Parallelism! -- Don

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Re: Why is this strict in its arguments? (Jules Bean)
by John Lato 05 Dec '07

05 Dec '07

Wow, thanks. I had a similar function (long if/then/else chain, fromJust) that I haven't been happy with, but couldn't see how to improve it. Now I have a much better idea for how to fix that function. Thanks again, John Lato > the general pattern is : replace isNothing with a case match on Nothing, > replace fromJust with a case match on Just, don't be afraid to case two > expressions at once. > > Hope someone finds that useful, > > Jules

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Re: [Haskell-cafe] Clean Dynamics and serializing code to disk
by Sterling Clover 05 Dec '07

05 Dec '07

In an email to the HAppS listt today, alexj described that HAppS-State "provides a way to serialize function calls for replay either on-restart or on other replicated boxes." (which actually helped me to understand somewhat what's going on behind all its TemplateHaskell magic. This is somewhat more constrained than perhaps you're looking for, given that A) it requires TemplateHaskell and B) it requires that the functions operate only on "other replicated boxes" but it might be a place to start looking? --S On Dec 5, 2007 9:56 AM, John van Groningen <johnvg(a)cs.ru.nl> wrote: > gwern0(a)gmail.com wrote: > > >Hey everyone; recently I've been toying around with various methods of > writing a shell and reading the academic literature on such things. The best > prior art on the subject seems to be the ESTHER shell (see < > http://citeseer.ist.psu.edu/689593.html>, < > http://citeseer.ist.psu.edu/744494.html>, < > ftp://ftp.cs.kun.nl/pub/Clean/papers/2003/vWeA2003-Esther.pdf>). > > > >Now, ESTHER is a really cool looking shell, but it has two main problems > for me: > >1) Source doesn't seem to be available anywhere online > >... > > The source code of ESTHER is include with Clean 2.2 in the directory > Libraries/Hilde of the windows 32 bit binary zip and the sources zip and > tar. > > Kind regards, > > John van Groningen > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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ghc overlapping instances
by Steffen Mazanek 05 Dec '07

05 Dec '07

Hello, I want to quickcheck a property on a datatype representing programs (=[Stmt]) and need to define a specific instance instance Arbitrary [Stmt] (mainly to restrict the size of the list). In quickcheck an instance Arbitrary of lists is already defined. Which parameters do I have to give ghc such that it accepts such an instance? In hugs -98 +o is enough. I have tried -XOverlappingInstances, -XFlexibleInstances and also -XIncoherentInstances, however I still got an overlapping instances error for this declaration. Regards, Steffen

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Parsing unstructured data
by Olivier Boudry 05 Dec '07

05 Dec '07

Hi all, This e-mail may be a bit off topic. My question is more about methods and algorithms than Haskell. I'm looking for links to methods or tools for parsing unstructured data. I'm currently working on data cleaning of a Customer Addresses database. Addresses are stored as 3 lines of text without any structure and people made used lots of imagination to create the data (20 years of data using no rules at all). Postal code, street, city, state, region, country and other details as suite, building, dock, doors, PO box, etc... are all stored in free form in those 3 lines. I already wrote a haskell program to do the job. It correctly parses about 2/3 addresses and parses much of the rest but with unrecognized parts left. The current program works by trying to recognize words used to tag parts like STE, SUITE, BLDG, street words (STR, AVE, CIRCLE, etc...) and countries from a list (including typos). It uses regular expressions to recognize variation of those words, lookup tables for countries, street words, regular expression rules for postal codes, etc... The most difficult task is splitting the address parts. There is no clearly defined separator for the fields. It can be dot, space, comma, dash, slash, or anything you can imagine using as a separator and this separator can of course also be found inside an address part. In the current application when part of an address is recognized it will not be parsed again by the other rules. A system trying all rules and tagging them with probabilities would probably give better results. Any link to tools or methods that could help me in that task would be greatly appreciated. I already searched for fuzzy, probabilistic or statistical parsing but without much success. Thanks, Olivier. PS: just in case someone's interested I attached the code and partial data to this e-mail.

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a positive challenge for the Haskell effort .....
by Galchin Vasili 05 Dec '07

05 Dec '07

http://code.enthought.com/enthon/ .. how do Haskell libraries/packages stack up against this challenge? Kind regards, Vasya

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An unusual prime number generator...
by Andrew Coppin 05 Dec '07

05 Dec '07

Hi guys. I just wrote this prime number generator. It's... unusual. It seems to go increadibly fast though. Indeed, I asked it to write the first 1,000,000 prime numbers to a text file, and scanning the bit array seemed to take longer than constructing the bit array! Odd... Anyway, for your amusement: module Primes2 (compute_primes) where import Data.Word import Data.Array.IO seive :: IOUArray Word32 Bool -> Word32 -> Word32 -> IO () seive grid size p = mapM_ (\n -> writeArray grid n False) [p, 2*p .. size] next_prime :: IOUArray Word32 Bool -> IO Word32 next_prime grid = work grid 2 where work grid n = do p <- readArray grid n if p then return n else work grid (n+1) copy :: Word32 -> IOUArray Word32 Bool -> Word32 -> IO (IOUArray Word32 Bool) copy p grid size = do let size' = size * p grid' <- newArray (1,size') False mapM_ (\n -> do b <- readArray grid n if b then mapM_ (\x -> writeArray grid' (n + size*x) True) [0..p-1] else return () ) [1..size] return grid' init_grid :: IO (IOUArray Word32 Bool) init_grid = do grid <- newArray (1,6) False writeArray grid 1 True writeArray grid 5 True return grid compute_primes :: Word32 -> IO [Word32] compute_primes n = do g0 <- init_grid ps <- work n g0 6 return (2:3:ps) where work top grid size | size >= top = do p <- next_prime grid putStrLn $ "Found prime: " ++ show p if p > top then return [p] else do seive grid size p ps <- work top grid size return (p:ps) | otherwise = do p <- next_prime grid putStrLn $ "Seiving prime: " ++ show p let size' = p*size grid' <- copy p grid size seive grid' size' p ps <- work top grid' size' return (p:ps) -- Debug... show_grid :: IOUArray Word32 Bool -> IO () show_grid grid = do (b0,b1) <- getBounds grid mapM_ (\n -> do x <- readArray grid n; if x then putChar '-' else putChar '#') [b0..b1] putStrLn ":"

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