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Comments requested: succ Java
by Curt Sampson 02 Oct '09

02 Oct '09

No, it's not quite what it sounds like. :-) Stuart Halloway recently posted a series of blog entries entitled "Java.next"[1], discussing the benefits of four other languages that compile to JVM bytecode and interoperate with Java: Clojure, Groovy, JRuby, and Scala. I thought I'd put my oar in and write a parallel series comparing Haskell to these. I've finished a draft of the first posting, started on the third, and made a couple of notes on the second and fourth, and I thought I'd post the drafts[2] and solicit comments here. If you have time to read and comment, I'd greatly appreciate the help; feel free either to e-mail me privately or post here. Also feel free to forward this to anybody else you feel might be interested in commenting. I'll probably be posting these about one per week, starting some time next week. [1]: http://blog.thinkrelevance.com/2008/9/24/java-next-overview [2]: http://www.starling-software.com/en/blog/drafts/2009/09/27.succ-java-summar… cjs -- Curt Sampson <cjs(a)starling-software.com> +81 90 7737 2974 Functional programming in all senses of the word: http://www.starling-software.com

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Haskell for Physicists
by edgar＠ymonad.com 02 Oct '09

02 Oct '09

Hi, I will give a seminar to physicists at USP (Universidade de São Paulo, Brazil) university and they asked me for a good title, something that can attract physicists. Anyone has some suggestions? (Will be a seminar about the use of Haskell to substitute C or Fortran in a lot of tasks, and how it can be used in some problems instead of Matlab, Mathematica, etc.) Thanks, Edgar

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Introspection on types.
by Gregory Propf 02 Oct '09

02 Oct '09

Is there a way to tell, let's say, how many constructors there are for a type? Or do I need one of the haskell extensions I've read about?

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ANN: json-b-0.0.4
by Jason Dusek 02 Oct '09

02 Oct '09

This version fixes defective handling of empty objects and arrays. http://hackage.haskell.org/package/json-b-0.0.4 Thanks to Dmitry Astapov for this fix. -- Jason Dusek

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a boring parser
by Anatoly Yakovenko 02 Oct '09

02 Oct '09

i got annoyed with Parsec and wrote a much more boring parser which allows me to parse anything with any kind of matching i want. Its basically a combination of State and Error monads. So i can use a grep like parser that matches via a regular expression over a list of lines grep re = do vv::B.ByteString <- any let (_,_,_,rv) = (vv =~ re)::(B.ByteString,B.ByteString,B.ByteString,[B.ByteString]) case (rv) of [] -> throwError "no match" _ -> return $ rv > run (grep $ C.pack "(hello)") $ [C.pack "hello world"] Right (["hello"],[]) or use the same library to scan over a string by combining regular expressions regex re = do make $ \ st -> do case (B.null st) of True -> throwError "eos" _ -> do let (_,_,after,rv) = (st =~ re)::(B.ByteString,B.ByteString,B.ByteString,[B.ByteString]) case (rv) of [] -> throwError "no match" _ -> return $ (rv,after) > run (do aa <- regex $ C.pack "(hello)"; bb <- regex $ C.pack " (world)"; return (aa,bb) ) $ C.pack "hello world" Right ((["hello"],["world"]),"") or simply match integers in a list, or anything that is of type Eq > run (many1 $ eq 1) [1,1,1,2,3,4] Right ([1,1,1],[2,3,4]) i can define lt lt cc = do vv <- any case (vv < cc) of True -> return $ vv _ -> throwError "no match" and do > run (many1 $ lt 5 <|> eq 5) [1..10] Right ([1,2,3,4,5],[6,7,8,9,10]) here is the implementation module Parser( ParserM --type alias for the parser ParserM a b is over "stream" a and returns b , make --makes a parser from a matching function of type :: stream -> m (match_data,stream) --for example any is implemented via: --any :: ParserM [a] a --any = make $ \ ll -> -- case (ll) of -- (hh:tt) -> return $ (hh,tt) -- _ -> throwError "eos --matches and returns an element from a list, which makes any of type :: ParserM [a] a , any --matches any element from [a] type stream , eq --matches an equal element from [Eq] stream, trivialy implemented in terms of any --eq :: Eq a => a -> ParserM [a] a --eq cc = do -- vv <- any -- case (vv == cc) of -- True -> return $ vv -- _ -> throwError "no match , (<|>) --or operator, tries the left one then the right one , manyTill --collects the results of parser 1 until parser 2 succeeds , many1 --collects the results of the parser, must succeed at least once , many --collects the results of a parser , run --runs the parser ) where import Control.Monad.State.Lazy import Control.Monad.Error import Test.QuickCheck import Control.Monad.Identity import Prelude hiding (any) type ParserM a c = StateT a (ErrorT [Char] Identity) c make pp = do st <- get (rv,nst) <- pp $ st put $ nst return $ rv aa <|> bb = aa `catchError` \ _ -> bb manyTill :: ParserM a c -> ParserM a d -> ParserM a ([c],d) manyTill pp ee = do do dd <- ee return $ ([],dd) `catchError` \ _ -> do cc <- pp (ccs,dd) <- manyTill pp ee return $ (cc:ccs,dd) many1 pp = do rv <- pp rest <- many1 pp `catchError` \ _ -> return $ [] return $ rv : rest many pp = do many1 pp <|> return [] any :: ParserM [a] a any = make $ \ ll -> case (ll) of (hh:tt) -> return $ (hh,tt) _ -> throwError "eos" eq :: Eq a => a -> ParserM [a] a eq cc = do vv <- any case (vv == cc) of True -> return $ vv _ -> throwError "no match" lt cc = do vv <- any case (vv < cc) of True -> return $ vv _ -> throwError "no match" run pp dd = runIdentity $ runErrorT $ runStateT pp dd run' = flip run prop_MatchA = (Right ('a',"bc")) == (run' "abc" $ eq 'a') prop_MatchEOS = (Left "eos") == (run' "" $ eq 'a') prop_MatchNoMatch = (Left "no match") == (run' ("bcd") $ eq 'a') prop_MatchABC =(Right ('c',""))== (run' "abc" $ do eq 'a' eq 'b' eq 'c') prop_MatchA_C = (run' "abc" $ do eq 'a' eq 'd' <|> eq 'b' <|> any eq 'c') == (Right ('c',"")) prop_Or = (run' "abc" $ do { eq 'a' ; do { eq 'b' ; eq 'd' } <|> do { eq 'b' ; eq 'c' } }) == (Right ('c',"")) prop_UntilC = (Right (("",'c'),"")) == (run' ("c") $ manyTill any $ eq 'c') prop_Until1 ls = let rv = run' (ls ++ [1]) $ manyTill any $ eq 1 in case (rv) of Right ((ls,1),rest) -> (elem 1 ls) == False _ -> False prop_all1 ls = let rv = run' ([1,1,1] ++ ls) $ many1 $ eq 1 in case (rv) of Right (_,(1:_)) -> False Right ((1:1:1:_),_) -> True _ -> False

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I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Casey Hawthorne 01 Oct '09

01 Oct '09

I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language. If this is true, it needs to be pushed. And if by changing a few lines of source code one can develop a whole family of similar applications, that needs to be pushed, also. :) -- Regards, Casey

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DSL and GUI Toolkits
by Günther Schmidt 01 Oct '09

01 Oct '09

Hi, I managed to abstract parts of the business logic of my application by employing a very naive DSL and it actually worked quite well. I'll probably replace that with a "Finally Tagless" version, just for the sake of it. But now I'd really try to abstract the hard part, the GUI logic. I had been switching back and forth between wxHaskell and Gtk and eventually settled for wxHaskell. And my gui code is a huge mess, it works, for now, but it'd be hell if I even had to change the tiniest bit. So I'm wondering if I could go the same way as with (parts) of the business logic, using a DSL here too. Now I could do with some input on the merits of this approach. Günther

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Can this be done?
by Cristiano Paris 01 Oct '09

01 Oct '09

I wonder whether this can be done in Haskell (see muleherd's comment): http://www.reddit.com/r/programming/comments/7wi7s/how_continuationbased_we… Cristiano

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Fwd: [Haskell-cafe] Can this be done?
by Alberto G. Corona 01 Oct '09

01 Oct '09

You can do it almost transparently with the Workflow package:: http://hackage.haskell.org/package/Workflow 2009/2/11 Cristiano Paris <cristiano.paris(a)gmail.com> On Wed, Feb 11, 2009 at 2:30 PM, Peter Verswyvelen <bugfact(a)gmail.com> > wrote: > > I haven't looked at the details, but I think this is what a library like > > Reactive from Conal Elliott could do, but as far as I understand it, it > is > > still work in progress. > > I'm interested in the possibility of > stopping/pickling/unpickling/resuming a computation. > > Cristiano > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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parsing error
by ShawnShawn 01 Oct '09

01 Oct '09

Hi there, I am trying to write some code for parsing string in "p==>q" or "p&&&q" or "p|||r", something like this.But I have trouble in reading operator like"==>""|||"... how can I fix it? I attached my code file with the email.Can some one give me some hints to fix the error and run the code?Thank you in advance!!! _________________________________________________________________ 上Windows Live 中国首页，下载最新版Messenger！ http://www.windowslive.cn

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ANNOUNCE: usb-0.1
by Bas van Dijk 01 Oct '09

01 Oct '09

Hello, I would like to announce the release of usb-0.1, a Haskell library for communicating with USB devices from userspace. usb is implemented as a high-level wrapper around Maurício C. Antunes' bindings-libusb which is a binding to the C library: libusb-1.* ( http://libusb.org ). All functionality from libusb is provided except for asynchronous IO. I'm still thinking about if and how I should add that. As usual, install usb using cabal: > cabal install usb Or download and install it manually from hackage: http://hackage.haskell.org/package/usb-0.1 Or get the development source using darcs: darcs get http://code.haskell.org/~basvandijk/code/usb Comments and patches are highly welcome. Note that this is the first release, so expect API changes in the future. Note that my brother uploaded 'usb-id-database' to hackage. A small library for associating usb identifiers like vendor, product, class, subclass or prototcol identifiers with their respective names and vice versa: http://hackage.haskell.org/package/usb-id-database This can be helpful when trying to find the right device to operate. In a short while, the package 'ls-usb' will be released. This is a simple program for showing all the usb devices currently attached to your system. It's a nice demonstration on how to use the library. regards, Bas

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New TMR editor
by Wouter Swierstra 01 Oct '09

01 Oct '09

Dear all, After several years at the helm, I've decided to step down as editor of the Monad.Reader. I am happy to announce that Brent Yorgey will take over my role as editor. I'd like to take this opportunity to thank Brent for helping to keep the Monad.Reader alive. I'm sure he'll do a fantastic job. Finally, I'd like to thank all the people who have contributed to the Monad.Reader over the last few years – you guys have made this publication, and my job as an editor, so much fun. All the best, Wouter

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c2hs, size_t, and CSize -- argh!
by John Van Enk 01 Oct '09

01 Oct '09

Hello List, I'm running into a problem with c2hs and how it parses the C typedef 'size_t'. On 32bit systems, this ends up being parsed as a CUInt. On 64bit systems, this ends up as a CULong. This gets especially sticky with function pointers. In order to make bindings with c2hs that work across the different word sizes, I have to write an indirection layer in C that defines wrappers for functions or re-defines typedef's for function pointers to include 'unsigned long' instead of 'size_t'. I see there is a ticket open for this: http://hackage.haskell.org/trac/c2hs/ticket/20 Has any one else run into this issue? Is there a good workaround that doesn't involve writing a C function/typedef for each collision? /jve

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Splitting data and function declarations over multiple files
by Peter Verswyvelen 01 Oct '09

01 Oct '09

I guess this is related to the expression problem. Suppose I have a datatype *data Actor = Ball ... | Paddle ... | Wall ...* and a function *move (Ball ...) = * *move (Paddle ...) = * *move (Wall ...) = * in Haskell one must put *Actor* and *move* into a single file. This is rather cumbersome if you work with multiple people or want to keep the files small and readable. Surely it is possible to use type classes, existentials, etc to split the data type into multiple ones, but that's already advanced stuff in a sense. But wouldn't it be possible to allow these to be put into multiple files, and let the compiler merge them back into one? A bit like C#'s partial keyword: in file Ball.hs: *partial data Actor = Ball ...* *move (Ball ...) =* in Paddle.hs *partial data Actor = Paddle ...* *move (Paddle ...) =* The compiler would then merge all partial data types and functions into one. As far as no overlap exists in the pattern matches in move, so that the order of the pattern matches does not matter at all, the partial trick should be possible no? Cheers, Peter

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ANNOUNCE: vty-ui 0.1
by Jonathan Daugherty 01 Oct '09

01 Oct '09

vty-ui is: An extensible library of user interface widgets for composing and laying out Vty user interfaces. This library provides a collection of widgets and a type class for rendering widgets to Vty Images. Get it from Hackage: http://hackage.haskell.org/package/vty-ui Or get the source with darcs: http://repos.codevine.org/vty-ui This package is motivated by the desire to create terminal user interfaces easily without having to worry about doing all of the structural book-keeping that so often comes with such an approach. Future releases will probably include more widget composition tools, more interesting widgets, and helper code for building interesting things. For now, the library contains some basic widgets (including horizontal and vertical box layout) and a List widget. See the vty-ui-demo program and source for an interactive demonstration of the available widgets. Future releases may also address performance, as I honestly don't have a very clear idea of how well my approach is going to scale for non-trivial interfaces. Please don't hesitate to send patches, feedback, and criticism directly to me at drcygnus AT gmail DOT com. Lastly, thanks a ton to the authors of Vty. It's a pleasure to use. Enjoy! -- Jonathan Daugherty

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Fwd: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Alberto G. Corona 01 Oct '09

01 Oct '09

;) Off topic: Maybe the entire space time, the universe and his history, is isomorphic to a mathematical structure. http://space.mit.edu/home/tegmark/toe_frames.html <http://space.mit.edu/home/tegmark/toe_frames.html> 2009/10/1 Peter Verswyvelen <bugfact(a)gmail.com> > > > On Thu, Oct 1, 2009 at 9:53 AM, Andrew Coppin <andrewcoppin(a)btinternet.com > > wrote: > >> Sure. But what is a computer program? It's a *list of instructions* that >> tells a computer *how to do something*. And yet, the Haskell definition of >> sum looks more like a definition of what a sum is rather than an actual, >> usable procedure for *computing* that sum. (Of course, we know that it /is/ >> in fact executable... it just doesn't look it at first sight.) >> > > Is it? The list of instruction is just an abstraction layer built on top of > purely physical process of electrons and transistors; I'm not sure how much > imperativeness remains at this level? Not to mention the quantum mechanical > processes that take place... And that are also just mathematical models... I > mean, it really depends from which angle and at which detail you look at it, > no? > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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river crossing puzzle
by pat browne 01 Oct '09

01 Oct '09

Hi, Does anyone know where there are any Haskell implementations of the the River Crossing puzzle (AKA Farmer/Fox/Goose/Grain). There are several variations but the general ideas are explained at: http://en.wikipedia.org/wiki/River_crossing_puzzle http://en.wikipedia.org/wiki/Fox,_goose_and_bag_of_beans_puzzle I have found one at: http://www.shido.info/hs/haskell9.html Thanks, Pat

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Re: [Haskell-cafe] Exceptions during exception unwinding
by Brian Bloniarz 01 Oct '09

01 Oct '09

Sorry for the garbled post, this should hopefully be plain text: > On Thu, 2009-10-01 at 03:29 +0000, Brian Bloniarz wrote: >> I.e. why does an exception raised during exception handling get >> propagated past the exception that triggered the handler? > > Because it's the obvious and sensible thing to do and it what every > other mainstream language with exceptions does. The behaviour you want, > to automatically discard any exceptions raised in the handler and to > always re-raise the original exception can be implemented using the > current semantics, but the reverse is not true. Good to know ... C++ (the mainstream language I'm most familiar with) treats throwing from a destructor during unwinding as a fatal error. I was suggesting having bracket ignore exceptions from closeSomeHandle when doAction threw, not that it'd ignore any exceptions from closeSomeHandle. It's only the case where bracket has 2 exceptions to choose from that's ambiguous, and I'm saying that there's a fair amount of code out there that'd prefer to see the first one. Your point about ^C gets us into an even grayer area -- maybe code would prefer to propagate whichever of the 2 exceptions is asynchronous, or define a criterion to choose which exception is more serious, who knows. All these behaviors are easy to implement using the building blocks that Control.Exception exports right now, you're right. So I'm free to build my own bracket variants with different behavior and that's what I'll do. I was just wondering if the default behavior that bracket/onException provide (which are themselves built from catch/block/unblock) is the one that most people want. Thanks, -Brian _________________________________________________________________ Microsoft brings you a new way to search the web. Try Bing™ now http://www.bing.com?form=MFEHPG&publ=WLHMTAG&crea=TEXT_MFEHPG_Core_tagline_… bing_1x1

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Exceptions during exception unwinding
by Brian Bloniarz 01 Oct '09

01 Oct '09

I had a question about onException & friends: what's the rationale for having: (error "foo") `onException` (error "bar") give bar and not foo? I.e. why does an exception raised during exception handling get propagated past the exception that triggered the handler? Most examples I can think for exception unwinding code would prefer the original exception be propagated -- for example, HDBC has a function which rolls back a DB transaction on exception; it implements it like so: > withTransaction conn func = > do r <- onException (func conn) doRollback > commit conn > return r > where doRollback = > -- Discard any exception from (rollback conn) so original > -- exception can be re-raised > Control.Exception.catch (rollback conn) doRollbackHandler > doRollbackHandler :: SomeException -> IO () > doRollbackHandler _ = return () IMHO, it'd be easier to just write: > withTransaction conn func = > do r <- onException (func conn) (rollback conn) > commit conn > return r This same argument applies to bracket, bracket_, bracketOnError & finally; even the common: > bracket openSomeHandle closeSomeHandle doAction If some error arises during doAction, there's a chance closeSomeHandle might fail (even a good chance, given that exception unwinding paths are usually poorly tested), and probably doAction has more accurate information about what went wrong than closeSomeHandle. This is just a thought; I hadn't seen this discussed somewhere. I know for example that Java has the same approach as the current Control.Exception, so there must be good arguments for that too. One that I can think of: using onException to rethrow an exception as a different type, though that's what mapException is for, correct? Thanks, -Brian _________________________________________________________________ Microsoft brings you a new way to search the web. Try Bing™ now http://www.bing.com?form=MFEHPG&publ=WLHMTAG&crea=TEXT_MFEHPG_Core_tagline_… bing_1x1

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Fwd: Uniplate + strict fields = fail. Why?
by Dmitry Astapov 01 Oct '09

01 Oct '09

Hi, I've been playing with generics in general (pardon the pun) and Uniplate in particular, and found out that strict data fields somehow derail Uniplate. Observe: === code === {-# LANGUAGE DeriveDataTypeable #-} module Test where import Data.Generics (Data(..),Typeable(..)) import Data.Generics.PlateData import Data.ByteString import Data.ByteString.Char8 as C data Foo = Foo String deriving (Show, Data, Typeable) tst1 = [ Foo "a", Foo "b" ] test1 = [ show x | Foo x <- universeBi tst1 ] -- *Test> test1 -- ["\"a\"","\"b\""] data Bar = Bar ByteString deriving (Show, Data, Typeable) tst2 = [ Bar (C.pack "a"), Bar (C.pack "b") ] test2 = [ show x | Bar x <- universeBi tst2 ] -- *Test> test2 -- *** Exception: Prelude.undefined === end of code === First, I thought that instance of Data for ByteString is somehow deficient, but this is not the case. If you change definition of Foo to "data Foo = Foo !String", you would get the same error with "Prelude.undefined". Since all fields in ByteString constructors are strict, I have no joy trying to use it with Uniplate. Howere, my type-foo is not strong enough to understand what's wrong. Could someone give me a hint? PS ghc 6.10.4 uniplate 1.2.0.3 bytestring 0.9.1.4 PPS Neil Mitchell is receiving a copy of this email -- Dmitry Astapov

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Re: [Haskell-cafe] Any generic serializer to String? was: Any working example of using genericserialize?
by Dimitry Golubovsky 01 Oct '09

01 Oct '09

Hi, On 9/30/09, Jason Dagit <dagit(a)codersbase.com> wrote: [skip] > Seems like using withList is wrong or the deserializer is simply buggy. It > certainly doesn't work the way I would expect SExp reading to work. I also > notice from reading the source on hackage that there may not be any tests > for this package and it is a 0.1 release. I'd contact the author, as it > seems there is a deficiency in the documentation or a bug in the > implementation. Thanks Jason for trying this. The genericserialize package is probably not finished yet. What else exists that could be used to serialize in generic way (i. e., anything that is an instance of Data and Typeable) into a string? It does not need to be very efficient as structures to be serialized are not huge (but may be pretty complex), and serialization is a part of an utility not to be used frequently. It only needs to "just work". One thing I thought of was to serialize to JSON (there is a generic serializer in one of packages although I did not test it other way) which has higher overhead than S-expressions though. Any other thoughts? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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ANNOUNCE: SourceGraph 0.5.1.0
by Ivan Lazar Miljenovic 01 Oct '09

01 Oct '09

This is an updated version of my graph-theoretic static analysis tool for Haskell, SourceGraph. This version is available from http://hackage.haskell.org/package/SourceGraph-0.5.1.0 (now with docs on the Hackage page!). This version provides the ability to pass in a single Haskell source file rather than a Cabal file for analysis purposes. Note that the Cabal version is still preferred as it is better able to determine the project name and which modules are exported: when using a single source file, that file is presumed to be the only exported module and its module name is used as the overall project name. This feature was requested by Curt Sampson. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Journals on Haskell and FP?
by Eugene Kirpichov 01 Oct '09

01 Oct '09

Hi. I was going to add a reference to the recently born Russian journal "Practice of functional programming" (http://fprog.ru/ ; had its 1st issue in July and 2nd was Sep.28) to haskellwiki into a section devoted specifically to journals, but it turned out there is no such section and, consequently, no information on journals: just books, tutorials and research papers. I think this is very valuable information and it's a shame that it's not listed on haskellwiki. So, I added such a subsection to http://haskell.org/haskellwiki/Books_and_tutorials#Journals and added a reference to Monad.Reader (the only other openly available journal on FP that I am aware of) , and I encourage the community to mention some more journals there. I also encourage the Russian readers who have not yet seen the aforementioned journal to give it a look :) -- Eugene Kirpichov Web IR developer, market.yandex.ru

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error on "--++ bla bla bla"
by Hong Yang 01 Oct '09

01 Oct '09

Hi, I got an error if one of lines reads "--++ bla bla bla" where I tried to comment, but "-- ++ bla bla bla" (notice the space after "--") is OK. Do you think this revealed a tiny bug in the GHC compiler (I am using Windows Haskell Platform 2009.2.0.2)? Thanks, Hong

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combinatorial search with running bound
by Michael Mossey 01 Oct '09

01 Oct '09

I have a combinatorial search problem that requires a running lower bound to be tracked in order to prune the search. I have enough Haskell experience to know how to do a combinatorial search, for example with list compresions or the list monad, but I don't know how to keep a running lower bound. The problem is: I have two groups of boxes, and need to figure out how closely the centers of the groups can be brought left-right. For example, 55 11111 55 11111 44 22 44 22 + <- how close? -> + 33333 6666 33333 6666 33333 The left group consists of boxes 1, 2, and 3, which have both a size and a position (the position is relative to the center of the group, represented with the +). The right group has boxes 4, 5, and 6. The problem is to determine how closely the groups can be brought together without any boxes intersection. The basic algorithm is to consider each pair of boxes and ask if they have any "vertical overlap"---if so, figure out how closely they can be brought together without intersecting, otherwise ignore them. Then take the maximum of those numbers. -- (Here assume lrSeparation returns minBound for boxes that don't have -- vertical intersection.) boxesSep :: [Box] -> [Box] -> Int boxesSep lefts rights = maximum [ lrSeparation l r | l <- lefts, r <- rights ] However, this algorithm can be improved by pruning. - Define the 'left extent' of a box by how far its left edge sticks out to the left of the group center. Similarly the 'right extent'. - Sort the list of left boxes in the order of decreasing right extent. Sort the list of right boxes in order of decreasing left extent. - Consider pairs of boxes as a kind of outer loop on the left boxes, and inner loop on the right boxes. - Track the current maximum required separation, which is a lower bound on the final answer. - If at any point in the inner loop, the right extent has gotten so small that there's no way you could find a new maximum, skip the rest of the inner loop (skip the remainder of the right boxes). Here's my attempt to write this using a state monad. There's probably a more idiomatic way to do it. -- This is state used in the state monad. data SearchState = SearchState { -- running maximum: ssMaximum :: Int -- remember the full list of right boxes -- so we can initiate a new outer loop , ssRights :: [Box] } boxesSep2 :: [Box] -> [Box] -> Int boxesSep2 lefts rights = let ls = sortBy ((flip compare) `on` rightExtent) lefts rs = sortBy ((flip compare) `on` leftExtent) rights in fst $ runState (boxesSep2' ls rs) (SearchState minBound rs) boxesSep2' :: [BoxBounds] -> [BoxBounds] -> State SearchState Int -- Termination of algorithm: boxesSep2' [] _ = gets ssMaximum -- Initiate a new inner loop: boxesSep2' (l:ls) [] = do rights <- gets ssRights boxesSep' ls rights -- Common case: boxesSep2' lss@(l:ls) (r:rs) = do -- In this way of writing the code, we distinguish between the -- left/right separation which is the sum of the extents, and the -- question of whether there is vertical overlap. let v = isVerticalOverlap l r sep = lrSeparation l r ss <- get let max = ssMaximum ss if sep <= max then boxesSep' ls (ssRights ss) --Here we "prune" (initiate new inner loop) else do -- Update max is needed: when v (put ss { ssMaximum = sep }) boxesSep' lss rs So if there is a better way to do this, I'm all ears.

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Fwd: [Haskell-cafe] Market Place for Haskell development teams?
by Alberto G. Corona 01 Oct '09

01 Oct '09

Curt, 2009/9/29 Curt Sampson <cjs(a)starling-software.com> > On 2009-09-29 13:18 +0200 (Tue), Alberto G. Corona wrote: > > > Java is part of the Java platform, that brought OS independence and > > interoperability at the right time. .Download-execution on the client > > was also a reason for the initial success of Java in the Internet era. > > I was a die-hard Java hacker from 1999 until some undetermined time in > the early-to-mid-2000s. (I abandoned it more or less completely sometime > around late 2005, if I recall correctly.) > > This may be somewhat anecdotal evidence, but I disagree with both > of your statements here. > Of course, I´m not talking about real advantages of Java or any PL. I told about the reasons that people used at the time to introduce the language in the mainstream, either is desirable this for haskell or not. I think it is. Nobody consider the runtime download of Java code important nowadays. Not even the cross-platform features. but it was marketeed at his time as such. > > Rubi and Python came with libraries targeted to Rapid development of > > Internet applications. > > No, neither originally "came" with that. > Rubi and Pyton came into existencie without their internet libraries, but they would´nt be popular without them. Although I conffess I don´t know the history in detail. > > What is the vehicle that haskell can use to enter the mainstream?. > > That may be the wrong question. "Avoid success at all costs" still > rings true to me. A year or so ago I seemed like one of the few on > the haskell-libraries list voting in favour of fixing API problems in > libraries, rather than etching in stone those problems in the name of > backwards compatibility so that we could "become more popular." > > Said above. We have different goals. > Do you really want, in 2020, to look back at the 2010 revision of the > Haskell standard and think, "we entrenched things that for a decade > everybody agreed was dumb"? > I see no problem in haskell having both. experimental and fixed versions. Haskell 2020 for you and me and haskell 2010 for my commercial code. Both woukd ve maintained and enriched by far more people. > > I can tell you, even when you're a Java enthusiast, there's nothing > more depressing than looking at java.util.Date and thinking, "That > should have been immutable, but it's going to be mutable for the rest of > eternity. We will never fix that." > > But let's try this again: > > > What is the vehicle that haskell can use to enter the mainstream?. > > Become more stupid. > > Is that a better answer? I'm not just a geek; I do marketing too (this > is what happens when you start your own company), and if you asked me, > using the utmost of my technical knowledge and marketing skills, to make > Haskell popular, this is what I'd recommend. > > Become more stupid may mean "give exactly what the people want" that transaltes to be more stable, give libraries, platforms etc. That is not a extra effor. that will come naturally as more people use the language. some people is naturally more abstract. some are more practical. > (I suppose it's a sign of my professionalism that to do this would > nearly break my heart, but if you wanted me to tell you the best way to > do this, and I couldn't tell you to get lost, that's what I'd say.) > > > Many people will play with Haskell in the spare time, and many of them > > will be permitted to develop some non critical applications at work. > > But that is all. > > Hm. So I suppose that this options trading system I'm working on, which > is the sole way our business makes money and is entirely written in > Haskell, doesn't actually exist. > > Congratulations.!! > > I think that all the current niches are filled, but new niches are > coming. > > Haskell already has a good niche. In fact, a brilliant one. We have > a whole bunch of academics doing truly wonderful stuff (imagine the > world without monads!--thank you Philip Wadler (and Eugenio Moggi)) > that the rest of us (relatively) dumb idiots can use to make our lives > better. We've got several very good implementations of the language, > one of which is a truly shit-hot compiler[1]. And we can use that to do > commercial applications quite comfortably[2]. > > Right. ;) > My personal opinion is, yes, let's let Haskell stick to the niche where > it's great, but it changes so fast that it's scary to everybody else. To > echo Paul Graham, I'm extremely happy to see my competition use Java. > > [1] Like that's so important. Ruby's standard implementation to this day > is an interpreter that implements all the popular extensions and has a > reasonably decent FFI. In Haskell-land, we call that "Hugs." It's only > because we have GHC as well that we can look down on Hugs; in the Ruby > (and Python, and PHP) worlds, they're saying that interpreters are just > fine for all sorts of "enterprise applications." > > [2] (Warning: self-promotion): > http://www.starling-software.com/misc/icfp-2009-cjs.pdf > > > Financial applications are an example of higher level programming > > where tasks usually performed by humans are now automatized and there > > is no or few traditions about that. The need to think at a higher > > level without being worried by side effects and other details are > > specially needed in such kind of areas. That's where haskell could > > have its own niche. > > Actually, I think that Haskell has a niche there not because of that > sort of thing, but merely because it's better than Java at doing what > Java does well, but scary enough that only small groups of brave > people who are comfortable with risk would ever attempt to use it. The > financial sector happens to have a lot more of those than many others. > > same concept in more plain words. I guess. > cjs > -- > Curt Sampson <cjs(a)starling-software.com> +81 90 7737 2974 > Functional programming in all senses of the word: > http://www.starling-software.com >

5 4

convert a list of booleans into Word*
by Paul.Brauner＠loria.fr 01 Oct '09

01 Oct '09

Hello, I haven't found a function in hackage or in the standard library that takes a list of booleans (or a list of 0s and 1s, or a tuple of booleans or 0s and 1s) and outputs a Word8 or Word32. I have written one which seems very inefficient : toWord8 :: [Bool] -> Word8 toWord8 bs = go 0 0 bs where go n r [] = r go n r (b:bs) = go (n+1) (if b then setBit r n else clearBit r n) bs Is there a better way to do this out there ? (If it helps, i'm writting a toy compression algorithm, which outputs binary as lists of booleans, and I'd like to output that in a file). Cheers Paul

9 11

type class question
by Ben 30 Sep '09

30 Sep '09

dear haskellers -- i'm trying this question again, in haskell-cafe. i got some responses in haskell-beginners but am looking for more guidance. also, i understand this functionality is encapsulated in the Workflow module in hackage, but i'd like to understand this myself. this email is an (il)literate haskell file. suppose i have class of computations a -> State s b. for concreteness, let's say i'm writing a library of on-line statistical summary functions, like > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-} > > module Foo where > > import Control.Monad > import Control.Monad.State > import Control.Monad.State.Class > > data RunningAverageState = S Double Int > > runningAverage :: Double -> State RunningAverageState Double > runningAverage v = do > S sum count <- get > let nsum = sum + v > ncount = count + 1 > put $ S nsum ncount > return $ nsum / (fromIntegral ncount) > > test = take 10 $ evalState (mapM runningAverage [1..]) $ S 0 0 test -> [1.0,1.5,2.0,2.5,3.0,3.5,4.0,4.5,5.0,5.5] here "on-line" means that we may be taking data from an intermittant external source, e.g. a data generator IO [Double], say, and want to be able to feed the summarizer datum one-by-one, and produce intermediate summaries. also we may want to be able to serialize our computation state (with Data.Binary, say) so that we can resume data collection and summarization later. naturally i want to create some common higher order operations on these primitives, like applying them to a stream of data, or combining them in some way. it seems that one would want some kind of type class to define a common interface to them. > class (MonadState s m) => Summarizer s m | m -> s where > initialState :: s > runOne :: Double -> m Double > where initialize puts some intial state into the system, and runOne collects and summarizes the next piece of data. an instance for runningAverage would look like > instance Summarizer RunningAverageState (State RunningAverageState) where > initialState = S 0 0 > runOne = runningAverage but how would i use this, e.g. > --summarizeMany vs = last $ evalState (mapM runOne vs) initialState is not possible as it has an ambiguous type. 1) what am i doing wrong? what are the right type class and instance declarations? 2) is there a better way of expressing this kind of "on-line" calculation, perhaps in pure (non-monadic) functions? i tried mapAccumL, but was looking for something a little cleaner. best, ben

2 1

suggestion for hslogger
by Sean McLaughlin 30 Sep '09

30 Sep '09

Hello, I have a program that does a lot of unicode manipulation. I'd like to use hslogger to log various operations. However, since hslogger uses System.IO.putX, the unicode comes out mangled. I hacked the source to use System.IO.UTF8 instead, but it would be nice if that was an option so I don't have to rehack the code whenever there is a new release. Thanks! Sean

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Re: [Haskell-cafe] Haskell for Physicists
by edgar＠ymonad.com 30 Sep '09

30 Sep '09

On Wed, 30/Sep/2009 at 22:27 +0200, Max Rabkin wrote: > I am *not* a physicist, but I imagine many physicists know at least > something of functional analysis, algebra, Lie algebras, etc. > > However, when physicists write programs (this is my inference from the > widespread use of Fortran and the computational assignments given to > undergraduate students) they are almost exclusively numerical: very > often evaluating some integrals or integrating a system of > differential equations. Although Haskell can do these things, it's not > a place where Haskell really shines (compared to symbolic > computation). Well, if you want to write all the code in Haskell, maybe this is true (some parts an imperative code still is the most efficient, but nothing that you can't do in C and use in Haskell via FFI). But in my case, Haskell really shined using as an interface to GSL/Lapack via the wonderful hmatrix lib. > Since I'm not a physicist, I can't give a good example, but think more > of the things Mathematica is good for, rather than Fortran or Matlab. > My impression is that Haskell's advantage over Mathematica is in its > generality: Mathematica is great if it has a builtin function to do > what you want, but it's not a very pleasant programming language. And speed is other advantage! The code that I wrote to solve a problem in bose condensation is dozen times fastest that the Mathematica equivalent, and much more clean and simple to expand or modify. Edgar > HTH, > Max > > On Wed, Sep 30, 2009 at 9:39 PM, Khudyakov Alexey > <alexey.skladnoy(a)gmail.com> wrote: > > В сообщении от Среда 30 сентября 2009 23:29:32 Max Rabkin написал: > >> On Wed, Sep 30, 2009 at 9:24 PM, Alberto G. Corona <agocorona(a)gmail.com> > > wrote: > >> > Haskell: mathematics beyond numerical calculus > >> > >> I'd imagine most physicists know a fair bit of mathematics beyond > >> numerical calculus; what they might not know much about is > >> *computation* beyond numerical calculus. > >> > > Could you elaborate this. As physicist I don't quite get it. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Haskell for Physicists
by edgar＠ymonad.com 30 Sep '09

30 Sep '09

Good ones! Specially the second, since I will show a real example where I used Haskell to model a boson condensate. Thanks for the suggestions. Edgar On Wed, 30/Sep/2009 at 10:52 -0700, Ted Nyman wrote: > Some ideas of highly variable quality: > > Getting Functional with Physics > Bosons, Fermions, and Monads? Haskell for Physicists > Purer Programming for Physicists > Use Haskell for Physics, and Say 'C'-You-Later > > - ted > > On Wed, Sep 30, 2009 at 10:42 AM, <edgar(a)ymonad.com> wrote: > > > Hi, > > > > I will give a seminar to physicists at USP (Universidade de São Paulo, > > Brazil) university and they asked me for a good title, something that can > > attract physicists. Anyone has some suggestions? (Will be > > a seminar about the use of Haskell to substitute C or Fortran > > in a lot of tasks, and how it can be used in some problems instead of > > Matlab, Mathematica, etc.) > > > > Thanks, > > Edgar > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] Haskell for Physicists
by Jack Norton 30 Sep '09

30 Sep '09

Khudyakov Alexey wrote: > В сообщении от Среда 30 сентября 2009 22:25:14 вы написали: > >> Khudyakov Alexey wrote: >> >>> В сообщении от 30 сентября 2009 21:42:57 edgar(a)ymonad.com написал: >>> >>>> Hi, >>>> >>>> I will give a seminar to physicists at USP (Universidade de São Paulo, >>>> Brazil) university and they asked me for a good title, something that >>>> can attract physicists. Anyone has some suggestions? (Will be a seminar >>>> about the use of Haskell to substitute C or Fortran >>>> in a lot of tasks, and how it can be used in some problems instead of >>>> Matlab, Mathematica, etc.) >>>> >>> What area of physics? They all face somewhat different problems from >>> computation. >>> >>> Could you publish your slides from seminar (if any) and even if they are >>> in Spanish (nothing is impossible for man with a dictionary) >>> >> I am also interested in seeing the slides (being a physicist myself). >> Also, that'd be portuguese, not spanish, that would be spoken in >> Brazil. The dictionary comment still applies though :) >> >> > I suppose you aimed for cafe but missed. (: > Yep, sure did. I just hit `reply' assuming haskell-cafe was in the reply-to. I do that more often than not it seems. Going back to the OP, what area of physics, and how on earth are you going to convert years of fortran users to haskell? I mean, in particle physics (were I came from) it seems as though only recently have they moved from fortran to C++ (note: C was skipped). There are things written in python (like Athena) but, well..., they are unreliable crap (I do like python though). In fact, when I was in undergraduate, not 4 years ago, a PhD student was writing his big QCD project in fortran from the ground up. I'm not even familiar enough with fortran to attempt such a thing (I would have used C). Case in point, I think there are some areas of physics that exist as a communal project (i.e. experimental particle physics) and because of this, you are limited to the tools and data used by your peers (Athena, Geant4, etc...). It is really hard to introduce anything new. So I guess my advice would be to avoid Haskell as a 'replacement' for anything to a physicist (including mathematica -- which I never liked myself). They will immediately ignore you. Approach it as a new tool, and focus on what it can do that software-x can't. That was my take on it though, so it may be a bit incorrect and/or biased. -Jack

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Fwd: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Alberto G. Corona 30 Sep '09

30 Sep '09

I would say that pure knowledge is pure and functional. but human planning and problem solving is imperative because implies sequencing of operations based on this pure knowledge. haskell express both nicely. 2009/9/30 Andrew Coppin <andrewcoppin(a)btinternet.com> > Peter Verswyvelen wrote: > >> I really doubt people tend to think in either way. It's not even sure our >> thinking can be modeled with computing no? >> > > Well, try this: Go ask a random person how you add up a list of numbers. > Most of them will say wsomething about adding the first two together, adding > the third to that total, and so forth. In other words, the step by step > instructions. Very few of them will answer that the sum of an empty list is > defined to be zero, and the sum of a non-empty list is defined to be the > first number plus the sum of the list tail. > > Then again, few non-programmers will set anything about creating a counter > variable and initialising it to zero either; this is a programming > "artifact". (Humans don't think like this internally, but most programming > languages conceptually require it.) Nobody has much difficulty with this, so > maybe the only problem with Haskell is that everybody learns to program "the > other way" first, before they get to Haskell... > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Fwd: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Alberto G. Corona 30 Sep '09

30 Sep '09

Again, i missed to forward the message to the list: I experince also the drug effect. Evolutionary psychologists would say that, because it was vital for our survival, since the stone age, we appreciate any tool powerful enough to solve many problems while at the same time remain simple. So whenever the utility versus learning.-using-,maintaining costs of a tool is low. then the tool is more appreciated and more pleasure we experiencie by using it. That applies either to a sword, a horse, a car or a programming language. Haskell has an execution strategy and a type system that cares for himself about code consistency and a reasonable optimization, It has only a few keywords, and a intuitive syntax , But combining the security that gives the first two factors and the flexibility of the other two, one can reach high levels of abstraction maintaining a high degree of confidence in the generated code, And such code can be applied to a wider variety of problems. For that matter I think that while other languages can borrow some features of haskell, they will never have the power that can be achieved by having them all. And most of them are deep in the core. 2009/9/30 Peter Verswyvelen <bugfact(a)gmail.com> Yep, LINQ makes C# more enjoyable :-) Scala and haXe also look nice, a bit > of a mix between OCaml/F#, C#/Java and Haskell. > Besides the fact that hacking in Haskell is a great deal of fun, the main > reason I see for learning Haskell: it makes you a better programmer. After > a couple of years of playing with Haskell, I can now solve problems that I > couldn't before. It's of course hard to tell if Haskell is the reason here, > or just experience, but I feel it really is Haskell (actually, functional > programming). Haskell made me see the world in a different way (and if I see > Oleg's and co's code, I still have an infinitely long road ahead. > > The main reason why you should not learn Haskell: it's a bit of a drug; > after you learned Haskell, programming in an "industrial strength" language > suddenly feels like a waste of time, time better spent learning more > Haskell... > > On Wed, Sep 30, 2009 at 10:26 AM, Deniz Dogan <deniz.a.m.dogan(a)gmail.com>wrote: > >> 2009/9/30 Andrew Coppin <andrewcoppin(a)btinternet.com>: >> > (Mr C++ argues that homo sapiens fundamentally think in an imperative >> way, >> > and therefore functional programming in general will never be popular. >> >> Sounds more like Mr C++ fundamentally thinks in an imperative way >> because that's what he is used to. >> >> I recently started working with C# and struggled for way too long with >> for/foreach loops to do things that in Haskell could be expressed >> using only folding, mapping and filtering. When I realised that those >> ideas actually exist in System.Linq I suddenly started liking the >> language a bit more. >> >> txtCommaSeparatedNames.Text.Split(',').Select(x => x.Trim()).Where(x >> => x.Length > 0).Select(x => Convert.ToInt32(x)).ToList(); >> >> Ah, the joy of FP. >> >> -- >> Deniz Dogan >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Fwd: [Haskell-cafe] frag game-compiling error
by selahaddin gerdan 30 Sep '09

30 Sep '09

---------- Forwarded message ---------- From: selahaddin gerdan <selahattin.gerdan(a)gmail.com> Date: 2009/9/28 Subject: Re: [Haskell-cafe] frag game-compiling error To: Lyndon Maydwell <maydwell(a)gmail.com> Sorry I'm just learning haskell, so I don't get your suggestion. Could you explain to me what exactly I have to do? According to this<http://cvs.haskell.org/Hugs/pages/libraries/OpenGL/Graphics-Rendering-OpenG…>page GLfloat is just a type synonym for type Float. How come the error message differentiates between them? 2009/9/28 Lyndon Maydwell <maydwell(a)gmail.com> I think this is an opengl version problem. I came across this error in > vacuum-opengl after I had upgraded my opengl. I patched it up using a > "from<numeric type>" call. > > On Mon, Sep 28, 2009 at 8:00 AM, selahaddin gerdan > <selahattin.gerdan(a)gmail.com> wrote: > > Hi there, > > when I try to install frag,I get this error: > > > > .cabal/bin/cabal install frag > > Resolving dependencies... > > Downloading frag-1.1.2... > > Configuring frag-1.1.2... > > Preprocessing executables for frag-1.1.2... > > Building frag-1.1.2... > > [ 1 of 39] Compiling IdentityList ( src/IdentityList.hs, > > dist/build/frag/frag-tmp/ > > IdentityList.o ) > > [ 2 of 39] Compiling Quaternion ( src/Quaternion.hs, > > dist/build/frag/frag-tmp/Quaternion.o ) > > > > src/Quaternion.hs:22:27: > > Couldn't match expected type `GLfloat' > > against inferred type `Float' > > In the expression: (r00 :: GLfloat) > > In the second argument of `newMatrix', namely > > `[(r00 :: GLfloat), r01, r02, r03, ....]' > > In the expression: > > newMatrix ColumnMajor [(r00 :: GLfloat), r01, r02, r03, ....] > > cabal: Error: some packages failed to install: > > frag-1.1.2 failed during the building phase. The exception was: > > exit: ExitFailure 1 > > > > Why? > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > >

6 5

Problem with result-type context restrictions in typeclasses.
by DNM 30 Sep '09

30 Sep '09

N.B. I'm a newbie to Haskell, and this problem is a bit complex, so bear with me. I'm using typeclasses to implement a sort of common interface for all things -- call them things of type 'Cls' -- that can be expected to implement a set of functions -- an 'interface' in OOP-speak. (Yes, yes, I'm aware that typeclasses are subtly different and far superior, but my Haskell-ese is still a bit rudimentary.) Essentially, I want to have a typeclass that expects its instances to have an accessor function that results in something that is an instance of another typeclass whose instances can perform some operation. The ghc type-checker doesn't seem to like my code, though, and I can't seem to figure out why. To make it concrete, I've typed up some dummy typeclasses and a dummy function that uses their instances to illustrate what I mean, as well as the form of the ghc(i) error. ------------- BEGIN CODE ------------------ class Cls c where foo :: (Bar b) => c -> b class Bar b where toNum :: b -> Int -- | One implementation of Cls data D = D {fu :: FU} data FU = FU {num :: Int} instance Cls D where foo = fu instance Bar FU where toNum f = (num f) + 47 -- | Another implementation of Cls data E = E {fi :: FI} data FI = FI {nuum :: Int} instance Cls E where foo = fi instance Bar FI where toNum f = (nuum f) + 100 -- | Yet another (this one re-uses FI) data F = F {fii :: FI} instance Cls F where foo = fii -- | And one last one, just to stress that -- I really need to implement multiple -- instances of Cls. data G = G {fuu :: FU} instance Cls G where foo = fuu -- | Good. Now, the function 'useThisStuff' need -- not know anything about it's payload -- other than that it its args are Cls's -- (hence they are foo'able things that -- can be used to construct an Int answer). useThisStuff :: (Cls x, Cls y) => x -> y -> Int useThisStuff x y = (toNum $ foo x) + (toNum $ foo y) ------------- END CODE -------------------- When I type this up in a file and try to load it in ghci, I get the following error message(s): ------------- BEGIN ERROR MSG ---------- Prelude> :load Typeclasses.hs [1 of 1] Compiling Typeclasses ( Typeclasses.hs, interpreted ) Typeclasses.hs:14:10: Couldn't match expected type `b' against inferred type `FU' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: D -> b Inferred type: D -> FU In the expression: fu In the definition of `foo': foo = fu Typeclasses.hs:23:10: Couldn't match expected type `b' against inferred type `FI' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: E -> b Inferred type: E -> FI In the expression: fi In the definition of `foo': foo = fi Typeclasses.hs:31:10: Couldn't match expected type `b' against inferred type `FI' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: F -> b Inferred type: F -> FI In the expression: fii In the definition of `foo': foo = fii Typeclasses.hs:39:10: Couldn't match expected type `b' against inferred type `FU' `b' is a rigid type variable bound by the type signature for `foo' at Typeclasses.hs:4:16 Expected type: G -> b Inferred type: G -> FU In the expression: fuu In the definition of `foo': foo = fuu Failed, modules loaded: none. ------------- END ERROR MSG ------------ It seems that ghc doesn't like the fact that I am saying 'foo' must return a class 'b' of typeclass 'Bar', while providing a function that returns a concrete data instance of 'Bar' (viz., FU or FI) later on when I implement 'foo' in each type classes. Repeated for convenience: class Cls c where foo :: (Bar b) => c -> b ... -- (e.g.) data G = G {fuu :: FU} instance Cls G where foo = fuu Does anyone have any clue as to what I'm doing wrong (language extensions that I may need, etc.)? Is is because I'm using context restrictions on the *result* type of a typeclass method? I've written other typeclasses with methods that say, essentially: class A a where blah :: (MonadPlus m) => a -> a -> m a with no issues. The restriction there is not on the return type a, but rather on some monadic 'wrapper' around it. This may be why that code works. Please advise. Any help is greatly appreciated. --D.N. (Dennis)

5 6

Any working example of using genericserialize?
by Dimitry Golubovsky 30 Sep '09

30 Sep '09

Hi, I am trying to use the genericserialize package (http://hackage.haskell.org/package/genericserialize) but cannot get things working. While buildList (sexpSerialize [1, 2, 3]) yields "(1 2 3)" as it might be expected, I cannot deserialize it back: *Main> (withList sexpDeserialize $ buildList (sexpSerialize [1, 2, 3])) :: Maybe [Integer] Nothing or *Main> (withList sexpDeserialize $ buildList (sexpSerialize [1, 2, 3])) :: Maybe [Int] Nothing while I would expect at least one of these cases result in Just [1, 2, 3] What am I missing? Thanks. -- Dimitry Golubovsky Anywhere on the Web

2 1

i am missing something really trivial with parsec
by Anatoly Yakovenko 30 Sep '09

30 Sep '09

number = do { num <- natural ; return $ num } main = do txt <- hGetContents stdin print $ parse number "stdin" txt why doesn't that work?

3 2

Type synonyms vs standard types
by Olex P 30 Sep '09

30 Sep '09

Hi everyone, Dumb question about declaring a function and type synonyms. There are to different declarations of the same function: attrNames :: String -> AttrDict -> [String] attrNames :: AttrClass -> AttrDict -> AttrNames First gives you the idea about exact types it expects (except AttrDict for which user has to take a look into the docs or sources) while the second one gives you the idea about meaning of parameters. Both reasons make sense. The question is when which one should be used? I'm using type synonyms everywhere and possibly without too much reasons... Maybe I should stop doing it? :) Cheers, Oleksandr.

4 6

[Haskell-beginners] Better way to retrieve data strucures from sql-db?
by Paolo Losi 30 Sep '09

30 Sep '09

Hi Roman, Template Haskell should be the preferred way to cope with these "boilerplate" code problems. Template Haskell: http://www.haskell.org/haskellwiki/Template_Haskell Note: I'm a beginner myself so please wait for a more informed response. Paolo 2009/9/28 Roman Naumann <roman_naumann(a)fastmail.fm>: > The following function converts the data read from an sql database to a > player data structure. > > toPlayer :: [SqlValue] -> Player > toPlayer sx = Player { > plID = fromSql (sx!!0), > plAccount = fromSql (sx!!1), > plForename = fromSql (sx!!2), > plSurename = fromSql (sx!!3), > plPos_x = fromSql (sx!!4), > plPos_y = fromSql (sx!!5) > } > > > It appears to me, that I have to write this boilerplate code every time I > introduce a new data structure that has to be read from the sql database. > Can you think of a way to automate this? > > Thanks for your replies, > Roman Naumann

1 0

Debugging Haskell code
by Paul Moore 29 Sep '09

29 Sep '09

I'm still playing round with my random dieroll generation program. In doing so, I just hit a segmentation fault (I didn't think Haskell could *cause* a segfault!) I'm sure it's my code - I got this to compile by fiddling with types until the errors (which I didn't understand) went away. Certainly not the right way to code, I know, but never mind. The problem is that I have *no idea* how to begin debugging this. In C, Python, or any other imperative language, I'd put traces in, etc. But in Haskell, I don't even know where to start. I attach the code below. While help in the form of pointers to what I did wrong would of course be appreciated, what I'm really looking for is a pointer on how I'd find out for myself. (Hey! I just read the bit in the ghc manual which says if I am not using foreign or unsafe functions, a crash is a compiler bug. Did I find a compiler bug?) My code is below. All I did is ghc --make hist_3d6.hs, then run hist_3d6.exe. This is ghc 6.10.4 on Windows Vista Home 32-bit. Thanks for any help, Paul. import System.Random.Mersenne import qualified Data.Map as Map import Data.Map (Map) import Data.List takes :: Int -> [a] -> [[a]] takes n [] = [] takes n xs = take n xs : takes n (drop n xs) sums :: Num a => Int -> [a] -> [a] sums n xs = map sum (takes n xs) simulate :: Int -> IO [Double] simulate count = do gen <- newMTGen Nothing dice <- (randoms gen :: IO [Double]) return (take count dice) histogram :: Ord a => [a] -> [(a,Int)] histogram = Map.assocs . foldl' f Map.empty where f m k = Map.insertWith' (+) k 1 m simulation = do lst <- simulate 100000 return lst --return (histogram lst) main = do s <- simulation putStrLn (show s)

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Strong duck typing / structural subtyping / type class aliases / ??? in Haskell
by oleg＠okmij.org 29 Sep '09

29 Sep '09

Alp Mestan wrote: > Indeed, OCaml has stuctural polymorphism, it's a wonderful feature. > > *# let f myobj = myobj#foo "Hi !";; > val f : < foo : string -> 'a; .. > -> 'a = <fun>* And Haskell has that too: > -- This is how we define labels. > data Field1 deriving Typeable; field1 = proxy::Proxy Field1 > > -- This is how record selection looks like. > foo f = f # field1 The inferred type of foo is *OCamlTutorial> :t foo foo :: (HasField (Proxy Field1) r v) => r -> v It doesn't seem too different from the OCaml's type; the type variable r acts as a row type. The quoted example is the first from many others described in http://darcs.haskell.org/OOHaskell/OCamlTutorial.hs The file quotes at length OCaml's Object tutorial and then demonstrates how the OCaml code can be written in Haskell. When it comes to objects, structural subtyping, width and depth subtyping, etc., Haskell does not seem to miss match compared to OCaml. In contrast, Haskell has a few advantages when it comes to coercions (one does not have to specify the type to coerce to, as Haskell can figure that out). The other files in that directory give many more example of encoding C++, Eiffel, OCaml patterns.

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Hackage bug? Autobuild failure on literate source with ignored code blocks.
by John Millikin 29 Sep '09

29 Sep '09

According to <http://www.haskell.org/haskellwiki/Literate_programming>, the following should compile properly because the second block of code will be ignored by GHC: \begin{code} main = putStrLn "Hello world!" \end{code} \begin{code}% main = -- TODO \end{code}% However, Hackage's automatic build system seems to pre-process the literate source in such a way that the ignored block is included in the .hs source, resulting in a compilation error: http://hackage.haskell.org/packages/archive/dbus-core/0.2/logs/failure/ghc-… dist/build/tmp23596/DBus/Bus/Connection.hs:119:12: Empty 'do' construct The block in question is a TODO feature that should be included in documents generated from the LaTeX source, but does not build and shouldn't be parsed as Haskell: \begin{code}% -- TODO tcp :: A.Address -> Transport tcp a@(A.Address _ params) = handleTransport a connect' where connect' = do -- check host -- check port -- check family -- return handle \end{code}% Although Hackage fails to build the package, it works fine on my local system when I run "cabal configure && cabal build && cabal haddock". Is this a bug in the Hackage auto-build mechanism, or have I done something wrong when uploading the package?

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double colon equal
by xu zhang 29 Sep '09

29 Sep '09

Hi, is there anyone can tell me what does the ::= mean?And what does the code below mean? expr ::= expr addop term | term Thank you in advance!

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Exciting job opportunity
by siki 29 Sep '09

29 Sep '09

Principal investment firm based in Manhattan is looking for an outstanding software developer to maintain and develop proprietary accounting and portfolio management systems. Job duties will include coding projects as well as management of outsourced system maintenance. Candidates should have at least a bachelor’s degree in computer science from a top university and impeccable coding style. The majority of the programming is done in JAVA at present, so candidates must have extensive experience with JAVA and related technologies but we are always experimenting with the most exciting technologies (e.g. Haskell) and you will be given freedom in choosing suitable programming languages and technologies. Experience working with SQL is a must and familiarity with functional programming is a plus. We are looking for a candidate who shows strong analytical, organizational, and communication skills. Attention to detail is essential. We are a growth oriented firm that values people who take a craftsman’s pride in their work. Candidates must have a desire to constantly improve one's knowledge of programming and the financial markets. A true love of building quality software and a team spirit is strongly recommended. Background in accounting is not necessary but is definitely a plus. This is a high-impact, high-visibility job opportunity where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. Please send your CV and cover letter to recruitment(a)karamaan.com, thank you. -- View this message in context: http://www.nabble.com/Exciting-job-opportunity-tp25667801p25667801.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Instances for Data.Text
by Pasqualino "Titto" Assini 29 Sep '09

29 Sep '09

2009/9/29 Paulo Tanimoto <tanimoto(a)arizona.edu>: > Hi Bryan and others, > > On Mon, Sep 28, 2009 at 5:29 PM, Bryan O'Sullivan <bos(a)serpentine.com> wrote: >> bytestring predates the other two libraries by several years. The underlying >> stream type for uvector and text are almost the same, so they could in >> principle be merged. There's a fair amount of duplication there, but uvector >> is in some ways more complicated and in others much less thorough than text. >> Merging them would be a lot of work! >> > > If I may free-ride on this thread: how should one go about deriving a > Data.Binary instance for text? It looks like doing it efficiently > would require using some parts of the internal module that are not > exposed, am I correct? I've been using "encodeUtf8", but that doesn't > feel right. I don't know what to do, hopefully I'm missing something > simple. This is a good point, I also need to make Data.Text an instance of a few basic classes and I am not sure that I did it correctly. So far I have: import Data.Text instance Binary Text where put = put . encodeUtf8 get = liftM decodeUtf8 get -- DOUBT: Is this correct also for Data.Text.Lazy ? instance NFData Text instance Serial Text where -- DOUBT: is this efficient? series d = [T.pack (series d :: String)] -- DOUBT: how to define this coseries rs = error "coseries" More in general: what is the right policy for instances definition? Should the library author provide them, at least for the most common and appropriate classes (at the cost of adding some additional dependencies) ? Should they go in a separate package? Should the Haskell Platform team provide some guidance on this point to library authors? titto

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ANN: atom-0.1.1
by Tom Hawkins 29 Sep '09

29 Sep '09

Atom is a Haskell DSL for designing hard real-time embedded applications. At Eaton, we use it for automotive control systems. An Atom description is composed of a set of guarded atomic actions that operate on a global program state. Atom makes it easy to manage program concurrency without the need for mutex locking and run-time task scheduling. As such, Atom can eliminate the need and overhead of an RTOS, which traditionally serves these functions. This release includes several improvements to C code generation, including a simplified rule scheduler that consumes less memory and execution time. It also ships with a new unit testing framework that allows the use of assertions and functional coverage points to assist with program verification. http://hackage.haskell.org/package/atom

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How to decide if a number is an integer?
by Magicloud Magiclouds 29 Sep '09

29 Sep '09

Hi, In other weak-type language, `round i == i` would work. But in haskell, what should I do? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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Doing people's homework?
by Michael P Mossey 29 Sep '09

29 Sep '09

I'm not really hip to the culture here so this is just an observation, but some of the recent questions posted to this list (and beginners(a)haskell.org) look a lot like someone's homework. Is anyone here concerned about avoiding giving the full answer, or maybe it's really none of our business (we aren't responsible for anyone's learning process)?

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Writing tool question (may be out of scope here)
by Saul Malesac 29 Sep '09

29 Sep '09

Hi, Excuse me if that is out of subject here, but does anyone know which tool/writing framework was used to write the "Real world Haskell" book ? I'm wondering, in particular, about the capability to write/edit/publish it online while allowing people to leave comments, then have a paperback/PDF version. Thanks in advance, -- Saul

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