- Haskell-Cafe - Haskell.org

building HSQL MySQL on windows (Was: FFI imported function names)
by Daniil Elovkov 06 Jan '09

06 Jan '09

Ok, enough talking to myself :) If anybody ever wants to build hsql-mysql on windows and has the same problems as I had, here's how it should be done. The problem I had seemed to be that libmysql.dll uses stdcall, but names its functions without @<n> decoration. Thus, when linking a Haskell program against it, names with @<n> cannot be resolved. The solution is pretty much a hack. Somebody from the Ruby community used to run into the same problem, from the same situation -- namely building ruby mysql bindings. Here is the procedure one person suggests http://rubydotnetproxy.rubyforge.org/mysql_win.html It worked perfectly for me. Of course, with some cabal specifics, which is obviously not described in the above page. The solution was to create a def file listing the names which could not be resolved and creating a .a file based on the dll and this def file. 2008/12/24 Daniil Elovkov <daniil.elovkov(a)googlemail.com>: > Mm, actually I didn't change the calling convention ffi imports when I > thought I did. I tried to do it through defines... > > Well, by explicitly saying ccall I get the names without @<n> > decoration. And it all links well. But I get segault when I run the > code, which should mean that calling conventions didn't match. > > Actually I found some ramblings on the internet that there's something > wrong with libmysql.dll and libmysql.lib in this respect, but they > were from 2004. > > Thus the question: how do I link against a lib file in ghc? When I say > -lmysql on command line it strictly searches for dll and complains if > it's not found. Adding lib exlpicitly on the cmd line doesn't seem to > have any effect. > > Thanks > > > 2008/12/24 Daniil Elovkov <daniil.elovkov(a)googlemail.com>: >> Hello >> >> How is decided whether the name of imported function gets the ending >> of the form @4 in ghc? >> >> I'm having this problem on Windows trying to use HSQL MySQL on windows. >> >> I compile HSQL Oracle backend and I get names without that. It's ok. >> With HSQL MySQL I get names with that stuff. It prevents me from >> further linking to dll. Supplying in the ghc command-line the "lib" >> file where names have those marks doesn't have any effect. >> >> The linker says that fname@NN cannot be resolved. >> >> Can I control that? >> >> The ffi calling convention doesn't (and shouldn't as I understand) >> affect this. The only difference is that in the case of mysql the >> header file itself where functions are described marks it STDCALL. Is >> that the reason? >> >> Also I changed ffi import line from "hsmysql.h func" to just "func", >> to no avail. >> >> ghc 6.8.2 >> >> I'm lost. Please help. >> >> -- >> Daniil Elovkov >> > > > > -- > Daniil Elovkov > -- Daniil Elovkov

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Can I destructive rebind a local variable in haskell?
by Wang, Chunye (NSN - CN/Beijing) 06 Jan '09

06 Jan '09

Dear haskeller, Can I destructive rebind a local variable like this import System.Directory test filename = do is_dir <- doesDirectoryExist filename let filename = if not is_dir then filename else filename putStrLn $ " filename " ++ filename main = test "." in GHCi 6.10.1 on WinXP, the ghci aborts silencely when I executes "main" after compile it into a executable file. c:\USERS\home\learning_haskell\>testDirectory testDirectory C stack overflow in generated code I have some code in scheme which works very well (define (func x) (let ((x x)) x)) By rebinding a formal variable, we can save naming some varaibles. I am not good at naming a variable. Thank you in advance Best Regards Chunye Wang <chunye.wang(a)nsn.com>

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ANN bytestring-trie 0.1.2 (bugfix)
by wren ng thornton 06 Jan '09

06 Jan '09

-------------------------------------------- -- bytestring-trie 0.1.2 (bugfix) -------------------------------------------- Another bugfix release for efficient finite maps from (byte)strings to values. Release early, release often. -------------------------------------------- -- Changes -------------------------------------------- * Fixed a bug in alterBy pointed out by Mark Wotton. Same bug as the previous one pointed out by Maxime Henrion (a missing case). * Added an Eq instance, in case anyone cares. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie Darcs: http://code.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://code.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring-trie/ -- Live well, ~wren

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GHC 6.10.1 for Solaris i86
by Donald Halomoan 06 Jan '09

06 Jan '09

I am waiting for GHC 6.10. 1 binary for Solaris i86. Thanks.

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Portability of MonadError
by Peter Robinson 06 Jan '09

06 Jan '09

Hello, One thing that's been bothering me about MonadError monads is the non-portability of code that uses a custom Error type. Meaning, if I have libraries A and B that use different error types, I won't be able to write a function func: func = (funcA >> funcB) `catchError` (\e -> ...) funcA :: ErrorT MyErrorA m () funcB :: ErrorT MyErrorB m () So I'm wondering whether there's a reason not to introduce a type class hierarchy instead of custom error types to make the code more portable. Something like this: -------------------- import Control.Exception(IOException) import Control.Monad.Error class (Eq e,Show e,Error e) => MyExc e where myExc1 :: e myExc1 = strMsg "myExc1" -- Now we can simply extend MyExc and catch all errors defined in MyExc and -- MyExc2 in the same error-handler (see handler2 below): class MyExc e => MyExc2 e where myExc2 :: e myExc2 = strMsg "myExc2" -- Uses the error class MyExc test1:: (MonadError e m, Monad m, MonadIO m, MyExc e) => m () test1 = do liftIO $ putStrLn "############ Throwing myExc1: " throwError myExc1 `catchError` handler1 -- Uses the error class MyExc2 that extends MyExc test2 :: (MonadError e m,MonadIO m, MyExc2 e) => m () test2 = do liftIO $ putStrLn "\n############ Throwing myExc2: " throwError myExc2 `catchError` handler2 -- Uses the error type class MyExc2 but throws an error -- already defined in MyExc test3 :: (MonadError e m,MonadIO m,MyExc2 e) => m () test3 = do liftIO $ putStrLn "\n############ Throwing myExc1 within context MyExc2: " throwError myExc1 `catchError` handler2 -- Error handler for class MyExc handler1 :: (MonadError e m, MonadIO m,Monad m,MyExc e) => e -> m () handler1 e = do when (e == myExc1) $ liftIO $ putStrLn $ "Caught a MyExc1 " ++ show e -- Error handler for class MyExc2 (catches errors in MyExc1) handler2 :: (MonadError e m, MonadIO m,Monad m,MyExc2 e) => e -> m () handler2 e = do when (e == myExc1) $ do liftIO $ putStrLn $ "Caught a MyExc1 " ++ show e throwError e when (e == myExc2) $ liftIO $ putStrLn $ "Caught a MyExc2 " ++ show e -- To run the code in the IO monad we need: instance MyExc IOException where myExc1 = userError "myExc1 has occurred" instance MyExc2 IOException where myExc2 = userError "myExc2 has occurred" -- Run test1 and test2 in the IO monad mymain :: IO () mymain = (test1 >> test2 >> test3) `catchError` (\e -> putStrLn $ "Something went wrong...\n" ++ show e) -- Now let's try a custom monad: newtype MyMonad e a = MyMonad { runMM :: ErrorT e IO a } deriving(Monad,MonadError e,MonadIO) runMyMonad :: Error e => MyMonad e a -> IO (Either e a) runMyMonad = runErrorT . runMM mymainT :: IO () mymainT = do res <- runMyMonad (test1 >> test2 >> test3 :: MyMonad IOException ()) case res of Left e -> putStrLn $ "Something went wrong...\n" ++ show e _ -> return () ---------------------- Maybe I'm missing something but is there any advantage of using custom data types rather than the typeclass approach? Cheers, Peter PS: Please be frank if I'm reinventing the wheel here... :-)

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Infinite grid
by Martijn van Steenbergen 06 Jan '09

06 Jan '09

Hello, I would like to construct an infinite two-dimensional grid of nodes, where a node looks like this: > data Node = Node > { north :: Node > , east :: Node > , south :: Node > , west :: Node > } in such a way that for every node n in the grid it doesn't matter how I travel to n, I always end up in the same memory location for that node. I suspect another way of saying that is that > let f = f . north . east . south . west in f origin should run in constant space. I hope this makes the problem clear. :-) How do I do this? Thanks in advance, Martijn.

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Trouble with interact in ghci
by Adrian Neumann 06 Jan '09

06 Jan '09

Hi, I have a strange problem with "interact" on OS X (ghc 6.10.1). It seems to garble stdin. I have some code here http://hpaste.org/13135#a2 , for testing purpose: *Main> main 1 > 1.0 2 > 1.5 3 > 2.0 *Main> setNonBlockingFD: invalid argument (Bad file descriptor) 11:40:45 ~/Desktop> (I hit ctrl-D) *Main> main 1 > 1.0 2 > 1.5 3 > 2.0 ^CInterrupted. *Main> main *** Exception: <stdin>: hGetContents: illegal operation (handle is closed) *Main> Someone on #haskell suspected my editline to be the culprit. Does anyone of you know what I can do about this problem? Greetings, Adrian

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nested function application question
by Galchin, Vasili 06 Jan '09

06 Jan '09

Hello, I have the following: B.intercalate $ B.intercalate ByteString [ByteString] [ByteString] I get a type error with this. If I comment out the 2nd B.intercalate and the third parameter I get no type errors. Regards, Vasili

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Is Arrow based FRP a subset of Classic FRP, at least conceptually/semantically?
by Peter Verswyvelen 06 Jan '09

06 Jan '09

Classic FRP ("CFRP", like Fran, FAL, Reactive, Grapefruit?...) exposes signals as first class values. Arrow based FRP ("AFRP", like Fruit, Yampa...) hides signals as first class values; instead the signal transformers are the first class values. Can I conclude that it would theoretically be possible to translate an AFRP function into a CFRP function? I.e. is AFRP a subset of CFRP? Thanks, Peter

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Inferno and Haskell?
by Mauricio 06 Jan '09

06 Jan '09

Hi, Do you guys know the Inferno platform? en.wikipedia.org/wiki/Inferno_(operating_system) Wouldn't it be a nice place to run GHC? All written in a type safe language (what kind of safety I'm not smart enough to say), seems to have a much simpler design than regular unix, virtual machine runs everywhere and, from my non-expert point of view, probably good for embedded software. This is, however, one of those I-have-no-idea-on- how-much-effort-is-needed-and-have-no-knowledge-to- help questions... Sorry for that. Best, Maurício

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Use of abbreviations in Haskell
by Felix Martini 05 Jan '09

05 Jan '09

Hi all, There is currently a discussion on reddit/programming about Haskell. One complaint is that Haskell functions often use abbreviated names. I tend to agree with that. In my personal experience it generally takes more time to learn a third party Haskell library than libraries written in other languages. I am not sure why but it could be because of function names. It seems to me that Haskell's current record syntax enhances this. Take for example the new xml library, data Element = Element { elName :: QName elAttribs :: [Attr] elContent :: [Content] elLine :: Maybe Line } data Attr = Attr { attrKey :: QName attrVal :: String } data QName = QName { qName :: String qURI :: Maybe String qPrefix :: Maybe String } Personally i would prefer it to be something like data Element = Element { name :: QualifiedName attributes :: [Attribute] content :: [Content] line :: Maybe Line } data Attribute = Attribute { key :: QualifiedName value :: String } data QualifiedName = QualifiedName { name :: String uri :: Maybe String prefix :: Maybe String } but the global scope of the record field names doesn't allow that and therefore all kinds of abbreviations are inserted in front of the record field names which are hard to remember. So a better record syntax would be welcome. Perhaps the constructor could be used to limit the scope of the record field name e.g. QualifiedName.prefix? Regards, Felix

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Re: [Haskell-cafe] Type Family Relations
by Matt Morrow 05 Jan '09

05 Jan '09

Hi, > I think that > >>> class HeaderOf addr hdr | addr -> hdr > does not enforce that there must be a corresponding instance > AddressOf hdr addr. Hence, the type checker cannot use that information > either. Do you have a way to remedy that? I've often wanted something similar, and experimenting with this the following two options seem to be equivalent and work as desired: ----------------------------------------------------------------------------- -- both options share this code: -- | The crucial assertion ipv4 :: AddrHdrPair IPv4 IPv4Header ipv4 = AddrHdrPair data IPv4Header = C1 data IPv4 = C2 data AppAddress = C3 data AppHeader = C4 ----------------------------------------------------------------------------- -- OPTION(1): -- type families + GADT with equality constraints type family HeaderOf addr type family AddressOf hdr data AddrHdrPair hdr addr where AddrHdrPair :: (hdr ~ HeaderOf addr ,addr ~ AddressOf hdr) => AddrHdrPair addr hdr type instance HeaderOf IPv4 = IPv4Header type instance AddressOf IPv4Header = IPv4 ----------------------------------------------------------------------------- -- OPTION(2): -- classes + GADT with instance constraints class HeaderOf addr hdr | addr -> hdr class AddressOf hdr addr | addr -> hdr data AddrHdrPair hdr addr where AddrHdrPair :: (HeaderOf addr hdr ,AddressOf hdr addr) => AddrHdrPair addr hdr instance AddressOf IPv4Header IPv4 instance HeaderOf IPv4 IPv4Header ----------------------------------------------------------------------------- -- And commenting out the above instances in turn -- to verify that everything indeed works, and to -- compare error message content: {- -- type instance HeaderOf IPv4 = IPv4Header Cafe0.hs:9:0: Couldn't match expected type `HeaderOf IPv4' against inferred type `IPv4Header' When generalising the type(s) for `ipv4' Failed, modules loaded: none. -- type instance AddressOf IPv4Header = IPv4 Cafe0.hs:9:0: Couldn't match expected type `AddressOf IPv4Header' against inferred type `IPv4' When generalising the type(s) for `ipv4' Failed, modules loaded: none. -} {- -- instance AddressOf IPv4Header IPv4 Cafe0.hs:9:7: No instance for (AddressOf IPv4Header IPv4) arising from a use of `AddrHdrPair' at Cafe0.hs:9:7-17 Possible fix: add an instance declaration for (AddressOf IPv4Header IPv4) In the expression: AddrHdrPair In the definition of `ipv4': ipv4 = AddrHdrPair Failed, modules loaded: none. -- instance HeaderOf IPv4 IPv4Header Cafe0.hs:9:7: No instance for (HeaderOf IPv4 IPv4Header) arising from a use of `AddrHdrPair' at Cafe0.hs:9:7-17 Possible fix: add an instance declaration for (HeaderOf IPv4 IPv4Header) In the expression: AddrHdrPair In the definition of `ipv4': ipv4 = AddrHdrPair Failed, modules loaded: none. -} -- endcode ----------------------------------------------------------------------------- I'm not sure if there are any circumstances under which these two don't behave equivalently. All the best, Matt

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#haskell IRC channel reaches 600 users
by Don Stewart 05 Jan '09

05 Jan '09

A small announcement :) 7 years after its inception, under the guiding hand of Shae Erisson (aka shapr), the #haskell IRC channel[1] on freenode has reached 600 concurrent users! It's now in the top 3 language channels by size. To chart the growth, we can note that the channel was founded in late 2001, and had slow growth till 2006, reaching 200 users in January of that year. Since then growth in the user base has been far more rapid, reaching 300 users in Dec 2006, 400 users in August 2007, 500 users by July 2008, and 600 on January 2, 2009. This puts the channel at the 7th largest community of the 7000 freenode channels, and in the top 3 language communities. For comparision, a sample of the state of the other language communities, with comments comapred to their status a year ago: #php 612 #python 604 > #haskell 602 -- up 4 ##c++ 558 ##c 506 -- down 1 #perl 502 -- down 3 #ruby-lang 288 -- down #lisp 264 ##javascript 241 #erlang 146 -- unchanged #perl6 129 -- unchanged #scheme 123 -- down #lua 102 -- unchanged #clojure 78 #ocaml 70 -- unchanged You can see the growth of the channel over here: http://haskell.org/haskellwiki/IRC_channel If you've not dropped by the channel yet, feel free to come and chat, and toss around some lambdas! :) Cheers, Don

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How to check object's identity?
by Xie Hanjian 05 Jan '09

05 Jan '09

Hi, I tried this in ghci: >Prelude> 1:2:[] == 1:2:[] True Does this mean (:) return the same object on same input, or (==) is not for identity checking? If the later is true, how can I check two object is the *same* object? Thanks Jan -- jan=callcc{|jan|jan};jan.call(jan)

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Thoughts on Haskell and OOP
by Kevin Van Horn 05 Jan '09

05 Jan '09

Haskell has been around in one form or another for nearly two decades now, yet has never been extended with explicit support for object- oriented programming. I've been thinking about why this is so. I've come to the conclusion that Haskell simply doesn't need any explicit OOP support -- algebraic datatypes, modules, lazy evaluation, and first-class functions give you everything you need to do the kinds of things you would use classes and inheritance for in OO languages. To see why this is so, let's think about what OOP is. At the most basic level we have what some call object-based programming. This amounts to support for data hiding and abstract data types. Haskell's module system handles this function quite well, without any need to introduce a special concept of "class" (in the OO sense) nor private vs. public class members. Object-oriented programming is then object-based programming plus class hierarchies and inheritance. Why are these useful? Properly used, OOP is all about interface inheritance, not implementation inheritance. (At least in C++, implementation inheritance -- inheriting the data members and method implementations of a base class -- usually leads to bad design, and is discouraged by the experts.) (For those more familiar with Python, "duck typing" is the analog of interface inheritance for a dynamically-typed language.) Interface inheritance allows you to write procedures that operate on the base-class interface, but can be applied to objects of any type derived from the base class. Can we do this in Haskell? Yes, we can. Let's consider the Haskell analog of an immutable C++ base class: struct Base { virtual ~Base(); virtual int foo() const; virtual int bar(int n) const; }; class derived :: public Base { ... data members ... public: derived(T_1 arg1, ..., T_k argk); ... implementations of the virtual functions ... }; Haskell has no direct analog of object classes and virtual methods, but you can use lazy evaluation and first-class functions to achieve the same result: data Base = Base { foo :: Int, bar :: Int -> Int } derived :: T_1 -> ... -> T_k -> Base derived a_1 ... a_k = Base { foo = ...; bar = bar } where bar n = ... The analog of a mutable C++ base class is a little bit more involved, but not much. Suppose that we change bar(int) to be a mutating method in the C++ Base class: virtual int bar(int n); The Haskell analog then changes to data Base = Base { foo :: Int, bar :: Int -> (Int, Base) } derived :: T_1 -> ... -> T_k -> Base derived a_1 ... a_k = Base { foo = ...; bar = bar } where bar n = (..., (derived a_1' ... a_k')) (Here a_1', ..., a_k' are k expressions involving n and a_1, ..., a_k.) What do the rest of you think? Is my analysis correct?

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[Fwd: Re: ANN: bytestring-trie 0.1.1 (bugfix)]
by wren ng thornton 05 Jan '09

05 Jan '09

-------- Original Message -------- Message-ID: <496176CF.3090708(a)freegeek.org> Date: Sun, 04 Jan 2009 21:56:15 -0500 From: wren ng thornton <wren(a)freegeek.org> User-Agent: Thunderbird 2.0.0.16 (Macintosh/20080707) MIME-Version: 1.0 To: ChrisK <haskell(a)list.mightyreason.com> Subject: Re: ANN: bytestring-trie 0.1.1 (bugfix) References: <494C8B52.2040209(a)freegeek.org> <4960303E.8010102(a)freegeek.org> <4960FDA6.3020708(a)list.mightyreason.com> In-Reply-To: <4960FDA6.3020708(a)list.mightyreason.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit ChrisK wrote: > Question and suggestion: > > looking at > http://hackage.haskell.org/packages/archive/bytestring-trie/0.1.1/doc/html/… > > I am questioning your choice of foldr in fromList: > > So fromList forces the whole call chain above to be traversed until it > hits the Empty. For a large input list this will force the whole list > to be allocated before proceeding AND the call chain might overflow the > allowed stack size in ghc. For a large trie (which is a likely use > case) this is a poor situation. > > If you use foldl' then the input list is only forced one element at a > time. A small change to the lambda that insert passes to adjustBy will > retain the same semantics of earlier key wins (which are an especially > good idea in the foldl' case). The reason for foldr is to play nice with build/foldr list fusion. Though you're right that foldl' is often nicer when fusion isn't used. As noted (in the code) both toList and fromList are just quick mockups for now. Have you noticed issues with stack overflows or bad performance? My quick tests don't see any problems: $> cat FromListTest.hs ---------------------------------------------------------------- module Main (main) where import qualified Data.Trie as T import Data.Trie.Convenience (insertIfAbsent) import Data.List (foldl') import qualified Data.ByteString as S import Data.ByteString.Internal (c2w) import Microbench import Control.Exception (evaluate) getList, getList' :: T.KeyString -> Int -> [(T.KeyString, Int)] getList xs n = map (\k -> (k,0)) . S.inits . S.take n $ xs getList' xs n = map (\k -> (k,0)) . S.tails . S.take n $ xs main :: IO () main = do -- 100000 is large enough to trigger Microbench's stop condition, -- and small enough to not lock up the system in trying to create it. xs <- evaluate $ S.replicate 100000 (c2w 'a') microbench "fromListR obverse" (T.null . fromListR . getList xs) microbench "fromListL obverse" (T.null . fromListL . getList xs) microbench "fromListR reverse" (T.null . fromListR . getList' xs) microbench "fromListL reverse" (T.null . fromListL . getList' xs) ---------------------------------------------------------------- $> ghc --make -O2 FromListTest.hs [1 of 5] Compiling Data.Trie.ByteStringInternal ( Data/Trie/ByteStringInternal.hs, Data/Trie/ByteStringInternal.o ) [2 of 5] Compiling Data.Trie.BitTwiddle ( Data/Trie/BitTwiddle.hs, Data/Trie/BitTwiddle.o ) [3 of 5] Compiling Data.Trie ( Data/Trie.hs, Data/Trie.o ) [4 of 5] Compiling Data.Trie.Convenience ( Data/Trie/Convenience.hs, Data/Trie/Convenience.o ) [5 of 5] Compiling Main ( FromListTest.hs, FromListTest.o ) Linking FromListTest ... $> ./FromListTest * fromListR obverse: ............... 75.664ns per iteration / 13216.30 per second. * fromListL obverse: ............. 283.380ns per iteration / 3528.84 per second. * fromListR reverse: ............. 630.597ns per iteration / 1585.80 per second. * fromListL reverse: ............... 74.894ns per iteration / 13352.28 per second. Don't mind the 4x speed difference, it's an artifact of the order of the keys being inserted (the cost of splitting an arc vs appending to it). *Main> let run f l n = T.null . f . l (S.replicate n (c2w 'a')) $ n *Main> *Main> run fromListR getList 10 (0.00 secs, 0 bytes) *Main> run fromListR getList 100 (0.00 secs, 0 bytes) *Main> run fromListR getList 1000 (0.01 secs, 599116 bytes) *Main> run fromListR getList 10000 (0.47 secs, 5198576 bytes) *Main> run fromListR getList 100000 (45.36 secs, 51067440 bytes) *Main> *Main> run fromListL getList 10 (0.00 secs, 0 bytes) *Main> run fromListL getList 100 (0.00 secs, 1063168 bytes) *Main> run fromListL getList 1000 (0.06 secs, 74587280 bytes) *Main> run fromListL getList 10000 (10.62 secs, 7438453480 bytes) *Main> run fromListL getList 100000 Interrupted. *Main> *Main> run fromListR getList' 10 (0.00 secs, 0 bytes) *Main> run fromListR getList' 100 (0.00 secs, 1063196 bytes) *Main> run fromListR getList' 1000 (0.09 secs, 74587208 bytes) *Main> run fromListR getList' 10000 (18.19 secs, 7438854556 bytes) *Main> run fromListR getList' 100000 Interrupted. *Main> *Main> run fromListL getList' 10 (0.00 secs, 0 bytes) *Main> run fromListL getList' 100 (0.00 secs, 0 bytes) *Main> run fromListL getList' 1000 (0.01 secs, 533072 bytes) *Main> run fromListL getList' 10000 (0.46 secs, 3791240 bytes) *Main> run fromListL getList' 100000 (44.71 secs, 38515056 bytes) Both options seem about the same to me. The foldl' version is somewhat better in the worst case key ordering (though it changes which ordering that is). I could offer both as alternatives, but I think it'd be better to work on a version that is less succeptable to variation for the order of keys. -- Live well, ~wren -- Live well, ~wren

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Fwd: [Haskell-cafe] Auto-deriving Control.Exception.Exception
by Antoine Latter 05 Jan '09

05 Jan '09

Forwarding response to cafe ---------- Forwarded message ---------- From: Antoine Latter <aslatter(a)gmail.com> Date: Sun, Jan 4, 2009 at 2:13 PM Subject: Re: [Haskell-cafe] Auto-deriving Control.Exception.Exception To: "eyal.lotem(a)gmail.com" <eyal.lotem(a)gmail.com> On Sun, Jan 4, 2009 at 11:50 AM, eyal.lotem(a)gmail.com <eyal.lotem(a)gmail.com> wrote: > If this is valid: > > import qualified Control.Exception as Exc > > genericFromException :: Typeable a => Exc.SomeException -> Maybe a > genericFromException (Exc.SomeException e) = cast e > instance Exc.Exception SomeType where fromException = > genericFromException > > then why not have an auto-deriving rule for Exception (fromException = > cast . unSomeException)? If you just say: > instance Exception YourType where and leave the instance body empty, it will use the default definitions of 'fromException' and 'toException', which do the right thing by default. At least, I'm pretty sure that's what happens. The Haddock docs have lost their links to the source code. -Antoine

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Haskell, successing crossplatform API standart
by Belka 05 Jan '09

05 Jan '09

Hello! (1) Is anybody aware of SOA approach being supported in Haskell? Found HAIFA package (SOAP, WEB services), but it seems to be a RIP project (with it's last updated in 2006) - trying to install it is a total mess (for me, a newby). (2) Please, perhaps experienced developers could suggest any better crossplatform (crosslanguage) API standart supported in Haskell? For me the main priority is orientation on "being supported everywhere in far future"... Thanks in advance -- View this message in context: http://www.nabble.com/Haskell%2C-successing-crossplatform-API-standart-tp20… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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WriterT [w] IO is not lazy in reading [w]
by Paolino 05 Jan '09

05 Jan '09

As someone suggested me, I can read the logs from Writer and WriterT as computation goes by, if the monoid for the Writer is lazy readable. This has been true until I tried to put the IO inside WriterT > {-# LANGUAGE FlexibleContexts #-} > import Control.Monad.Writer > k :: (MonadWriter [Int] m) => m [Int] > k = let f x = tell [x] >> f (x + 1) in f 0 > works :: [Int] > works = snd $ runWriter k > hangs :: IO [Int] > hangs = snd `liftM` runWriterT k > main = take 20 `liftM` hangs >>= print The main hangs both interpreted and compiled on ghc 6.10.1. The issue is not exposing with IO alone as main = print "test" >> main is a working program. Thanks for explanations. paolino

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ANN: bytestring-trie 0.1.0
by wren ng thornton 05 Jan '09

05 Jan '09

-------------------------------------------- -- Announcing: bytestring-trie 0.1.0 (beta) -------------------------------------------- An efficient finite map from (byte)strings to values. The implementation is based on big-endian patricia trees, like Data.IntMap. We first trie on the Word8 elements of a Data.ByteString, sharing string prefixes where possible, and then trie on the big-endian bit representation of those elements. Patricia trees have efficient algorithms for union and other merging operations, but they're also quick for lookups and insertions. -------------------------------------------- -- Future Extensions -------------------------------------------- * I've done spot checking to make sure it works, but haven't constructed an extensive test suite. Does anyone know of a good data set already out there for testing corner cases? I'm sure other dictionary writers have come up with one. * A Word8 is much smaller than the architecture's natural Word size. Therefore it'd be more efficient for the trie on bits to read off a whole Word at a time. This work is in progress, but I need help from people with 64-bit and big-endian machines in order to verify the code works on those architectures. * Using ByteStrings allows for trieing on any packed byte representation of a value, but they're not Strings. Integration with Data.ByteString.Char8, utf8-string, and utf8-light should happen. * The current implementation also only accepts strict ByteStrings. When chopping up strings to share prefixes we share the smaller string. For very long strings when many deletions are expected, this can still hold onto more memory than necessary. Accepting lazy ByteStrings or adding heuristics for when to copy prefixes instead of sharing will fix this. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie Darcs: http://code.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://code.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring-trie/ -- Live well, ~wren

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Auto-deriving Control.Exception.Exception
by eyal.lotem＠gmail.com 05 Jan '09

05 Jan '09

If this is valid: import qualified Control.Exception as Exc genericFromException :: Typeable a => Exc.SomeException -> Maybe a genericFromException (Exc.SomeException e) = cast e instance Exc.Exception SomeType where fromException = genericFromException then why not have an auto-deriving rule for Exception (fromException = cast . unSomeException)?

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teaching functional programming at work
by Warren Harris 05 Jan '09

05 Jan '09

I am seeking suggestions from the haskell cafe for teaching functional programming concepts to colleagues at work. I'm currently working on a project using ocaml and functional programming techniques, and am a lone ranger at my workplace when it comes to this sort of thing (we are primarily a python shop). However there seems to be a growing curiosity in functional programming, and I think there's a lot to be learned from from it whether one chooses a functional language for their next big project or not. But I'm a practitioner, not an academic or lecturer, and although I find the idea of helping my colleagues understand these concepts to be an exciting prospect, I'm not really sure where to start in terms of materials or overall direction. My sense is to form a study group (perhaps going through Real World Haskell together), but I'm a little afraid of assembling people together for a "now what?" experience. So my first question is whether this is even a good idea? Some things I think would be useful to get across are: - fp is more than just an exercise in avoiding assignment statements -- why a smart programmer should care? - reading knowledge of ocaml and/or haskell (even syntax, precedence and infix operators can be an initial stumbling block) - core concepts: type classes, proper tail recursion, higher-order functions, folds, combinators, monads, monad transformers, arrows - evidence that concise expression can lead to less bugs / better maintainability (hard to prove, but perhaps a sense can be conveyed) - relationship between types / theorems / programs / proofs and why this is important for the future of software Regarding this last point, my own sense has been that by leveraging a powerful type system such as found in ocaml or haskell has allowed me to achieve things I never would have been able to without it (or at least would have ended up with a much less elegant implementation and much more debugging) but many seem to find types to be either an impedance to getting things done quickly, or at best irrelevant. I know this is for the most part a religious war, but I would like my colleagues to arrive at their opinion from an informed perspective, which means learning that there's more to typeful programming than what is presented by java or c++. Finally, any comments on how to make the learning experience fun, engaging, and a positive experience would be greatly appreciated. Warren

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ANN: wxHaskell 0.11.1
by Jeremy O'Donoghue 04 Jan '09

04 Jan '09

The wxHaskell development team is pleased to announce the release of wxHaskell 0.11.1, a Haskell binding for the wxWidgets GUI library. The Haskell support is built on a reasonably complete C language binding, which could be used as the basis for wxWidgets support on other languages/platforms which do not have easy mechanisms for linking with C++ code. The main highlights of wxHaskell 0.11.1 are: - Support for XRC resource files, allowing GUI design using a visual tool. Note that this is currently not type safe, and programs will crash if a widget is not cast to the correct type on loading. - Support for wxWidgets 2.8.x. Support for wxWidgets 2.4.2 is now dropped and wxHaskell will not compile against versions of wxWidgets prior to 2.6. This means that exploratory development using GHCi is no longer possible. Workaround is to continue to use older wxHaskell versions. - Support for GHC 6.10 - Preliminary support for Cabal / Hackage The full list of changes is provided at the end of this mail. Binary packages are available from the wxHaskell download site at http://sourceforge.net/project/showfiles.php?group_id=73133, for the following platforms: - Windows - OS X (Intel platform only) - Source code .tar.gz and .zip - Documentation (cross-platform) The wxHaskell libraries (wxcore and wx) are also available from Hackage (http://hackage.haskell.org) About wxHaskell --------------- wxHaskell is a Haskell binding to the wxWidgets GUI library for recent versions of the Glasgow Haskell Compiler. It provides a native look and feel on Windows, OS X and Linux, and a medium level programming interface. The main project page for wxHaskell is at http://wxhaskell.sourceforge.net. The latest source code for wxHaskell can always be obtained from http://darcs.haskell.org/wxhaskell. There are developer (wxhaskell-devel(a)lists.sourceforge.net and user (wxhaskell-users(a)lists.sourceforge.net) mailing lists, and a wiki page at http://haskell.org/haskellwiki/WxHaskell which can provide more information to those interested. The C language binding for wxHaskell was derived from an original C language binding created for the Eiffel programming language by the ELJ project (http://elj.sourceforge.net) Non backward compatible changes: - Preliminary Cabal / Hackage support - Added "--global" argument to configure script - Added "--user" argument to configure script - Changed wxhaskell official web page to Haskell wiki at http://haskell.org/haskellwiki/WxHaskell - Changed official darcs repository to code.haskell.org - Adapted the wxHaskell C-layer to work with wxWidgets 2.8 - Adapted some part of wxcore API to be able to refer to wxWidgets 2.8 documentation for wxcore functions - Added "TopLevelWindow", which mainly removes some functionality from "Frame" - Changed "--with-stc" argument to "--with-contrib" - Removed "Wave" type synonym Backward compatible additions: - Added support for using XRC resource files to load most controls and menus attached to frames. - Added sample file showing how to use XRC support to attach command handlers to menu items - Added sample file showing how to use properties with many controls. - Added "--enable-optimization" argument to configure script - Added "--O*" argument to configure script - Added "--enable-library-profiling" argument to configure script - Added "--p" argument to configure script - Added "-fvia-C" argument to configure script. And moved this compilation flag to configure script - Added Image / ByteString conversion functions - Adapted the configuration to work with GHC 6.10. - Changed "Var" type synonym from "IORef" to stm's "TVar" for thread safety - Changed "imageGetPixelArray" and "imageCreateFromPixelArray" to be more flexible. - Changed "Point", "Size", "Vector" and "Rect" to be type synonym. - Added "wxcMilliSleep". Now, "wxcAppUSleep" is deprecated. Use "wxcMilliSleep" instead - Added very experimental wxGraphicsContext support - Added wxSVGFileDC functions Bugfixes: - Applied DEPRECATED pragma to old deprecated functions. Just documented in Haddock before. - Fixed "processExecAsync" hangs Windows GUI-only program - Fixed Windows binary install problem when path with spaces (bug 1400488) - Probably fixed the applicattion failed to initialize properly when using Windows binary. -- Jeremy O'Donoghue jeremy.odonoghue(a)gmail.com -- Jeremy O'Donoghue jeremy.odonoghue(a)gmail.com

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Connect to DBMS - what's the cost
by Belka 04 Jan '09

04 Jan '09

Hello, community people! Is anybody aware, what aproximately is the cost for the acquiring connection to DB (and also disconnecting from it)? I guess that may differ from DBMS to DBMS, so I mostly am interested in PostgreSQL case, but for other DBMS it's also good to know. Actually, since all Haskell libraries, that provide interfaces to DBMS, use foreign functions, - the question should be focused on commonly used C libraries. Thanks in advance! -- View this message in context: http://www.nabble.com/Connect-to-DBMS---what%27s-the-cost-tp21275600p212756… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: about the concatenation on a tree
by Achim Schneider 04 Jan '09

04 Jan '09

"Max cs" <max.cs.2009(a)googlemail.com> wrote: > hi all, not sure if there is someone still working during holiday > like me : ) > > I got a little problem in implementing some operations on tree. > > suppose we have a tree date type defined: > > data Tree a = Leaf a | Branch (Tree a) (Tree a) > > I want to do a concatenation on these tree just like the concat on > list. Anyone has idea on it? or there are some existing > implementation? > The semantics of tree concat vary, depending on what type of tree you want to have. In the simplest case, it's unbalanced, so you can just do concat :: Tree a -> Tree a -> Tree a concat x y = Branch x y if, on the other hand, you want to keep the tree balanced, or sorted, or whatever, things get more involved. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.

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How do we decide on the new logo?
by Fritz Ruehr 04 Jan '09

04 Jan '09

Now that the new year is upon us, I suppose we must decide how to decide* on the new logo for the Haskell site. I'm not sure what the protocol and procedure for voting and tallying should be, but see below for a suggestion. (* a higher-order decision, very appropriate) If you haven't seen the page on the Wiki with proposed new logos, be sure to go there and take a look (but recall that submissions are now closed): <http://haskell.org/haskellwiki/Haskell_logos/New_logo_ideas> I want to thank Don for proposing the contest and everyone who contributed logo designs or modifications. As the designer of the last "official" logo, I think this new crop is terrific, with many suggestions that are more professional-looking and sleeker than the old one: I will have a hard time deciding which one to vote for. Without starting a war on the theory of voting systems, perhaps we should use a system which allows for a certain amount of secondary (etc.) preference to be expressed? (Uh-oh, here come Control.Monad.Voting.HareSTV and Control.Monad.Voting.BordaCount and a hundred other variations, complete with back-tracking and trampolined continuations and ... .) Once we have a winning design, we could perhaps award the designer(s) with a T-shirt or some similar item. We have a small amount of "CafeBucks" (or whatever) accrued in the CafePress account, despite all efforts to avoid profit (rather like avoiding success at all costs). In the past, these funds have been used to purchase courtesy shirts for a few Haskell luminaries, at the discretion of the store proprietor. (Currently I think there are about $60 available.) -- Fritz (Ruehr)

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[ANN] Haskell web server + wiki: salvia-0.0.4 + orchid-0.0.6
by Sebastiaan Visser 04 Jan '09

04 Jan '09

Happy new year, you all! I'm pleased to announce three new packages on Hackage: * salvia-0.0.4: A lightweight modular web server framework. * orchid-0.0.6: A(nother) Wiki written in Haskell, currently using Darcs as a versioning back-end and Salvia as the application server. Orchid ships as a library that can be installed as a server module for the Salvia framework. * orchid-demo-0.0.4: A simple demo application using Salvia and Orchid to serve an example darcs repository. An online demo of the wiki can be found at [1]. The front-end is a typical Ajax application and heavily relies on the availability of JavaScript. A more plain (REST like) web interface to system can be found at [2]. You can signup, login and edit/create some pages. Feel free to play around, it's just a demo. Orchid has an abstract notion of a versioning back-end, theoretically allowing for multiple versioning systems. Currently only a Darcs back- end is available, but it shouldn't be too much work to implement a liaison for Git, Subversion, Mercurial etc. The Wiki uses its own experimental document system with printers to HTML, LaTeX, PDF via LaTeX and some others in the making. The system makes us of the UUAGC [3] attribute grammar system, but is not required in order to build the package. The document system is plug-in based allowing easy addition of new structures. There are currently plug-ins for inline LaTeX formulae, HSColour'ed Haskell code snippets and a table of contents. Some others in the making. Be aware, this is an early release to allow people the play around with the packages and enable me to see how this all behaves in the wild. The tools are not yet finished and I cannot guarantee any form of stability. But, in my experience, they seem to `just work'. Most of the time. I had some trouble getting all dependencies right on systems other than my own. Using `cabal install' seems to behave differently from `./ Setup install' in the package directory, but this may vary from system to system. It is known to work at least on Macosx and Linux with GHC 6.8, 6.10 or even 6.11. Pdflatex is needed for PDF generation, dvipng for inline formulas. Have fun and best wishes for this new year! -- Sebastiaan [1] http://funct.org/wiki/ [2] http://funct.org/wiki/data/Index.html [3] http://www.cs.uu.nl/wiki/HUT/AttributeGrammarSystem

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Haskell Weekly News: Issue 99 - January 3, 2009
by Brent Yorgey 04 Jan '09

04 Jan '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090103 Issue 99 - January 03, 2009 --------------------------------------------------------------------------- Welcome to issue 99 of HWN, a newsletter covering developments in the [1]Haskell community. Happy new year to all! May 2009 be a year full of joy, family, friends, professional success, much Haskell hacking, and a minimal number of rabid weasels. Just in case. Announcements #haskell IRC channel reaches 600 users. Don Stewart [2]announced that 7 years after its inception, under the guiding hand of Shae Erisson (aka shapr), the [3]#haskell IRC channel on freenode has reached 600 concurrent users! citeproc-hs-0.2. andrea rossato [4]announced the release of [5]citeproc-hs-0.2, a Haskell implementation of the Citation Style Language, which adds a Bibtex like citation and bibliographic formatting and generation facility to [6]pandoc. This version adds support for citation collapsing, a wrapper around [7]hs-bibutils, and some [8]API documentation. hs-bibutils-0.1. andrea rossato [9]announced the first release of [10]hs-bibutils, Haskell bindings to Chris Putnam's [11]bibutils. Bibutils is a library and a set of bibliographic utilities to interconvert between various bibliography database formats using a common MODS-format XML intermediate. Haskell koans. Gwern Branwen [12]issued an RFK (Request for [13]Koans), following the success of his CFH (Call for [14]Haiku). [ANN] Haskell web server + wiki: salvia-0.0.4 + orchid-0.0.6. Sebastiaan Visser [15]announced the release of three new packages: [16]salvia, a lightweight modular web server framework; [17]orchid, a(nother) wiki written in Haskell, using Darcs as a versioning back-end and Salvia as the application server; and [18]orchid-demo, a simple demo application using Salvia and Orchid to serve an example darcs repository. You can play around with an [19]online demo. gitit-0.4.1, recaptcha-0.1. John MacFarlane [20]announced the release of [21]gitit-0.4.1, a wiki program that stores pages in a git repository. This release adds support for (optional) captchas, using the reCAPTCHA service. The reCAPTCHA code has been packaged as a separate library on Hackage, [22]recaptcha. monte-carlo-0.2, gsl-random-0.2.3. Patrick Perry [23]announced the release of a new version of the [24]monte-carlo package. The new version includes a more general type class, MonadMC, which allows all the functions to work in both MC and MCT monads; functions to sample from discrete distributions, and functions to sample subsets. There is also a [25]quick tutorial. Reading group for Programming Collective Intelligence. Creighton Hogg [26]announced that he would like to start a small group for the O'Reilly book [27]Programming Collective Intelligence, to work through translating some of the examples to Haskell. Email Creighton if you are interested in participating. Maintaining laziness. Henning Thielemann [28]announced that he has written a [29]tutorial on how to make functions lazy and how to test whether they are actually lazy. Request for feedback: Understanding Haskell Monads. Ertugrul Soeylemez [30]requested feedback on a new [31]monad tutorial. Discussion How do we decide on the new logo?. Fritz Ruehr began a [32]discussion of how to go about choosing a winner of the [33]Great 2009 Haskell Logo Contest. Weigh in if you care! Jobs Two Positions as Associate Professor in Software Engineering at Chalmers University. Koen Claessen [34]announced the availability of [35]two positions as Associate Professor at Chalmers University in Gothenburg, Sweden, within the division of Software Engineering and Technology at the department of Computer Science and Engineering. The application deadline is January 12, 2009. Blog noise [36]Haskell news from the [37]blogosphere. * GHC / OpenSPARC Project: [38]Bootstrapping. * Dan Piponi (sigfpe): [39]Rewriting Monadic Expressions with Template Haskell. * GHC / OpenSPARC Project: [40]Fighting dependencies. * GHC / OpenSPARC Project: [41]A new year and a new project. * Alson Kemp: [42]2009: The Year Of Hackage. * Patrick Perry: [43]Monte Carlo Poker Odds. * Joachim Breitner: [44]Handling explicit and implicit recursion in Haskell data. * Luke Palmer: [45]Domain Convergence. * Eric Kow (kowey): [46]riot is almost a Haskell mail client. * John Goerzen (CosmicRay): [47]Real World Haskell update. * Alson Kemp: [48]A Plea For "cabal install". * Alson Kemp: [49]Cyptol on Slashdot. Quotes of the Week * lilac: <bohdan> how do I see the number of reductions required to calculate something? <lilac> bohdan: the usual method is to ask Cale to reduce it by hand :) * conal: If it's purely functional, how do you *do* anything? You don't ;-) * ddarius: The opposite ends of CS meet in the Haskell world. * EvilTerran: forcedYet :: a -> Bool; forcedYet x = x `seq` True -- :P * bmh: I dream in folds. One day I'll dream in monads. * sclv: dreaming is a monad. About the Haskell Weekly News New editions are posted to [50]the Haskell mailing list as well as to [51]the Haskell Sequence and [52]Planet Haskell. [53]RSS is also available, and headlines appear on [54]haskell.org. To help create new editions of this newsletter, please see the information on [55]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [56]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49824 3. http://haskell.org/haskellwiki/IRC_channel 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16709 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/citeproc-hs 6. http://johnmacfarlane.net/pandoc/ 7. http://code.haskell.org/hs-bibutils/ 8. http://code.haskell.org/citeproc-hs/docs/ 9. http://article.gmane.org/gmane.comp.lang.haskell.general/16708 10. http://code.haskell.org/hs-bibutils/ 11. http://www.scripps.edu/~cdputnam/software/bibutils/ 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49774 13. http://haskell.org/haskellwiki/Koans 14. http://haskell.org/haskellwiki/Haiku 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49759 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/salvia 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/orchid 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/orchid-demo 19. http://funct.org/wiki/] 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49747 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gitit 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/recaptcha 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49739 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monte-carlo 25. http://quantile95.com/2008/12/31/monte-carlo-poker-odds/ 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49716 27. http://tinyurl.com/7x4rga 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49655 29. http://www.haskell.org/haskellwiki/Maintaining_laziness 30. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49546 31. http://ertes.de/articles/monads.html 32. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/49833 33. http://haskell.org/haskellwiki/Haskell_logos/New_logo_ideas 34. http://article.gmane.org/gmane.comp.lang.haskell.general/16712 35. http://www.chalmers.se/cse/EN/news/vacancies/positions/two-positions-as 36. http://planet.haskell.org/ 37. http://haskell.org/haskellwiki/Blog_articles 38. http://ghcsparc.blogspot.com/2009/01/building.html 39. http://sigfpe.blogspot.com/2009/01/rewriting-monadic-expressions-with.html 40. http://ghcsparc.blogspot.com/2009/01/in-in-morning.html 41. http://ghcsparc.blogspot.com/2008/12/alright.html 42. http://www.alsonkemp.com/haskell/2009-the-year-of-hackage/ 43. http://quantile95.com/2008/12/31/monte-carlo-poker-odds/ 44. https://www.joachim-breitner.de/blog/archives/316-Handling-explicit-and-imp… 45. http://lukepalmer.wordpress.com/2008/12/31/domain-convergence/ 46. http://koweycode.blogspot.com/2008/12/riot-is-almost-haskell-mail-client.ht… 47. http://changelog.complete.org/archives/861-real-world-haskell-update-2 48. http://www.alsonkemp.com/haskell/a-plea-for-cabal-install/ 49. http://www.alsonkemp.com/haskell/cyptol-on-slashdot/ 50. http://www.haskell.org/mailman/listinfo/haskell 51. http://sequence.complete.org/ 52. http://planet.haskell.org/ 53. http://sequence.complete.org/node/feed 54. http://haskell.org/ 55. http://haskell.org/haskellwiki/HWN 56. http://code.haskell.org/~byorgey/code/hwn/

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Re: [Haskell-cafe] How to check object's identity?
by Xie Hanjian 03 Jan '09

03 Jan '09

Thanks guys :-) * Nicolas Pouillard <nicolas.pouillard(a)gmail.com> [2009-01-03 16:39:59 +0100]: > Excerpts from Xie Hanjian's message of Sat Jan 03 16:28:30 +0100 2009: > > Hi, > > > > I tried this in ghci: > > >Prelude> 1:2:[] == 1:2:[] > > True > > > > Does this mean (:) return the same object on same input, or > > (==) is not for identity checking? If the later is true, how > > can I check two object is the *same* object? > > Here (==) is the structural equality. There is generally no exposed function > to check for identity of values. This would expose too much of the memory > model and the compilation process to give a specification to such a function. > > -- > Nicolas Pouillard -- jan=callcc{|jan|jan};jan.call(jan)

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Re: Updating doubly linked lists
by oleg＠okmij.org 03 Jan '09

03 Jan '09

Stephan Guenther wrote: > Is it possible to change a particular node of the doubly linked list? > That is to say, that would like to have a function: > update :: DList a -> a -> DList a > where > update node newValue > returns a list where only the value at the node which is passed in is > set to the new Value and all other values are the same. All this of > course in a pure way, that is without using (M/T/TM)Vars or IORefs. It is possible to do all of this, and more: - no rebuilding of the whole list on updates to the list - the update operation takes constant time (for lists longer than 32 elements on 32-bit platform) - both cyclic and terminated lists can be handled, uniformly - no monads used or mentioned - let alone no IORef, STRef, TVars, etc. The algorithm is essentially imperative (and so permits identity checking and in-place `updates') but implemented purely functionally. No destructive updates are ever used. Therefore, all the changes can be undone and re-done, and the code is MT-safe. The code is easily generalizable to 2D. Here are the tests > testl = fromList [1..5] > testl_s = takeDL 11 testl *FL> testl_s [5,1,2,3,4,5,1,2,3,4,5] > testl1 = update (-1) testl > testl1_s = takeDL 11 testl1 *FL> testl1_s [-1,1,2,3,4,-1,1,2,3,4,-1] > testl2 = update (-2) . move_right' . move_right' $ testl1 > testl2_s = takeDL 11 testl2 *FL> testl2_s [-2,3,4,-1,1,-2,3,4,-1,1,-2] > -- Old testl is still available > testl3 = update (-2) . move_right' . move_right' $ testl > testl3_s = takeDL 11 testl3 *FL> testl3_s [-2,3,4,5,1,-2,3,4,5,1,-2] It is not for nothing Haskell is called the best imperative language. One can implement imperative algorithms just as they are -- purely functionally, without any monads or other categorical notions. module FL where import qualified Data.IntMap as IM -- Representation of the double-linked list type Ref = Int -- positive, we shall treat 0 specially data Node a = Node{node_val :: a, node_left :: Ref, node_right :: Ref} data DList a = DList{dl_counter :: Ref, -- to generate new Refs dl_current :: Ref, -- current node dl_mem :: IM.IntMap (Node a)} -- main `memory' -- Operations on the DList a empty :: DList a empty = DList{dl_counter = 1, dl_current = 0, dl_mem = IM.empty} -- In a well-formed list, dl_current must point to a valid node -- All operations below preserve well-formedness well_formed :: DList a -> Bool well_formed dl | IM.null (dl_mem dl) = dl_current dl == 0 well_formed dl = IM.member (dl_current dl) (dl_mem dl) is_empty :: DList a -> Bool is_empty dl = IM.null (dl_mem dl) -- auxiliary function get_curr_node :: DList a -> Node a get_curr_node DList{dl_current=curr,dl_mem=mem} = maybe (error "not well-formed") id $ IM.lookup curr mem -- The insert operation below makes a cyclic list -- The other operations don't care -- Insert to the right of the current element, if any -- Return the DL where the inserted node is the current one insert_right :: a -> DList a -> DList a insert_right x dl | is_empty dl = let ref = dl_counter dl -- the following makes the list cyclic node = Node{node_val = x, node_left = ref, node_right = ref} in DList{dl_counter = succ ref, dl_current = ref, dl_mem = IM.insert ref node (dl_mem dl)} insert_right x dl@DList{dl_counter = ref, dl_current = curr, dl_mem = mem} = DList{dl_counter = succ ref, dl_current = ref, dl_mem = IM.insert ref new_node $ IM.insert curr curr_node' mem} where curr_node = get_curr_node dl curr_node'= curr_node{node_right = ref} new_node = Node{node_val = x, node_left = curr, node_right = node_right curr_node} get_curr :: DList a -> a get_curr = node_val . get_curr_node move_right :: DList a -> Maybe (DList a) move_right dl = if next == 0 then Nothing else Just (dl{dl_current=next}) where next = node_right $ get_curr_node dl -- If no right, just stay inplace move_right' :: DList a -> DList a move_right' dl = maybe dl id $ move_right dl fromList :: [a] -> DList a fromList = foldl (flip insert_right) FL.empty takeDL :: Int -> DList a -> [a] takeDL 0 _ = [] takeDL n dl | is_empty dl = [] takeDL n dl = get_curr dl : (maybe [] (takeDL (pred n)) $ move_right dl) -- Update the current node update :: a -> DList a -> DList a update x dl@(DList{dl_current = curr, dl_mem = mem}) = dl{dl_mem = IM.insert curr (curr_node{node_val = x}) mem} where curr_node = get_curr_node dl testl = fromList [1..5] testl_s = takeDL 11 testl testl1 = update (-1) testl testl1_s = takeDL 11 testl1 testl2 = update (-2) . move_right' . move_right' $ testl1 testl2_s = takeDL 11 testl2 -- Old testl is still available testl3 = update (-2) . move_right' . move_right' $ testl testl3_s = takeDL 11 testl3

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What are side effects in Haskell?
by Hans van Thiel 03 Jan '09

03 Jan '09

Hello All, I just saw somewhere that one of the purposes of monads is to capture side effects. I understand what a side effect is in C, for example. Say you want to switch the contents of two variables. Then you need a third temporary variable to store an intermediate result. If this is global, then it will be changed by the operation. But what is a side effect in an fp language like Haskell? As a follow up, I understand why an IO action like getLine or getChar is not a function; its results can be different for different calls. But why not have something like getChar c or getLine str? The types of c or str are pretty clear, aren't they? Just something that's been puzzling me for some time now...thanks. Regards, Hans van Thiel

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Re: [Haskell-cafe] Don't make 'show*' functions
by Donn Cave 03 Jan '09

03 Jan '09

Quoth brian <brianchina60221(a)gmail.com>: | On Fri, Dec 26, 2008 at 1:55 PM, Jeff Heard <jefferson.r.heard(a)gmail.com> wrote: |> Off the top of my head, I would say that the traditional meaning of |> Show could be changed to Serial, where serial encompasses both Read |> and Show -- possibly we could find a more efficient read function, |> several have been proposed. Then a separate class could be made for |> HumanReadable (or Loggable) where the point would bet that we write |> something that can be read by humans without conforming to a |> particular grammar that Haskell could read back in. | | +1. 'Show' sounds like it's for what Java 'toString' and Ruby 'to_s', | etc., do, and that's not right. To be fair, this seems to be a natural ambiguity with string representation. Python kind of blundered into it around 2.0, when someone decided to "fix" the __repr__ function for floating point to display the best decimal value, where it had until then used a rounded value. So users were confounded to see for example that 1.0 - 0.1 is 0.90000000000000002, instead of 0.9, but that does make some sense if __repr__ is to be used for string marshalling. Python has two string representation functions, __str__ and __repr__. The documentation of these two functions is a long standing problem that will probably never be resolved. If as proposed above there had also been a third function that is strictly about marshalling, that might have made it a little easier to focus on the difference betweeen str and repr, where unfortunately the documentation has confused the issue with ambiguous notions like friendliness. For me, "str" is about conversion to string data, where a data type contains or represents string data. Conversion implies loss. For example, an HTML or RFC822 header would convert to its "wire" data. But that's at odds with expectations for numeric types, for example - I'd make str(65) = 'A', but of course it's actually '65'. "repr" portrays the value as a Python object - what you would see for example from the interactive interpreter. Since Python objects are commonly "in the IO monad", there are all kinds of cases where the repr value can't fully marshal the object. Donn

4 4

Parsec : Problems with operator precedence
by Erik de Castro Lopo 03 Jan '09

03 Jan '09

Hi all, I'm using Text.ParserCombinators.Parsec.Expr to parse expressions for a Javascript like language. This language has C-like logical operators ('&&' and '||') and bitwise operators ('&' and '|'). Furthermore, the language definition states that bitwise operators have a higher precedence than the logical ones. I therefore have the following (trimmed): import qualified Text.ParserCombinators.Parsec.Expr as E opTable :: [[ E.Operator Char st Expression ]] opTable = [ -- Operators listed from highest precedence to lowest precedence. {- snip, snip -} [ binaryOp "&" BinOpBinAnd E.AssocLeft ], [ binaryOp "^" BinOpBinXor E.AssocLeft ], [ binaryOp "|" BinOpBinOr E.AssocLeft ], [ binaryOp "&&" BinOpLogAnd E.AssocLeft ], [ binaryOp "||" BinOpLogOr E.AssocLeft ] ] binaryOp :: String -> (SourcePos -> a -> a -> a) -> E.Assoc -> E.Operator Char st a binaryOp name con assoc = E.Infix (reservedOp name >> getPosition >>= return . con) assoc but I still get the following parse error: unexpected "|" expecting end of "|" or term on the line: if (name == null || value == null) If I change the above from a logical to a bitwise OR, the parser accepts it quite happily. Any clues as to what I'm doing wrong here? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- BSD: A psychoactive drug, popular in the 80s, probably developed at UC Berkeley or thereabouts. Similar in many ways to the prescription-only medication called "System V", but infinitely more useful. (Or, at least, more fun.) The full chemical name is "Berkeley Standard Distribution".

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Gitit - Encoding
by Arnaud Bailly 03 Jan '09

03 Jan '09

Hello, I have started using Gitit and I am very happy with it and eager to start hacking. I am running into a practical problem: characters encoding. When I edit pages using accented characters (I am french), the accents get mangled when the page come back from server. The raw files are incorrectly encoded. Where Shall I look for fixing this issue ? Thanks ps: the wiki is live at http://www.notre-ecole.org -- Arnaud Bailly, PhD OQube - Software Engineering web> http://www.oqube.com

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Minor quibble
by Louis Wasserman 03 Jan '09

03 Jan '09

Do CORE pragmas inhibit optimizations going between the code inside and outside of the pragma? This seems the case with certain code of mine, but it's not even alluded to in the GHC documentation. ;_; Louis Wasserman wasserman.louis(a)gmail.com

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Beginner question
by Benjamin Bach 03 Jan '09

03 Jan '09

Supposing I have the following code: > module Main(main) where > main = putStr (show []) I will get these errors from GHC and Hugs respectively: > Main.hs:2:16: > Ambiguous type variable `a' in the constraint: > `Show a' arising from a use of `show' at Main.hs:2:16-22 > Probable fix: add a type signature that fixes these type variable(s) > ERROR "src/Main.hs":2 - Unresolved top-level overloading > *** Binding : main > *** Outstanding context : Show b But if I change my code to the following, it will compile. > module Main(main) where > main = putStr (show [1]) I have no problems typing in "putStr (show [])" in Hugs... runs fine. So what's wrong? I've really tried hard to think of an explanation, but with no luck.. Thanks, Benjamin

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HDBC-ODBC - MS-Access backend problems with inserts
by Günther Schmidt 02 Jan '09

02 Jan '09

Hi, I'm using HDBC extensively in my app and it works very well. Most of the time my app uses an Sqlite database with HDBC and no problems there. The only problem I have is when I use it against an Access database backend in which I want to do inserts. Creating a connection works fine, also running queries. It only budges when I want to do inserts. *RedcomBatch.Export> handleSqlError $ run dbc "insert into mytable values (?)" [toSql "dummytext"] *** Exception: user error (SQL error: SqlError {seState = "[\"HY104\"]", seNativ eError = -1, seErrorMsg = "bindparameter 1: [\"98: [Microsoft][ODBC Microsoft Ac cess Driver]Ung\\252ltiger Genauigkeitswert. \"]"}) *RedcomBatch.Export> Any ideas why? Günther The German part of the error message reads "invalid precision value"

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Re: [Haskell-cafe] representation on persistent store question
by Antoine Latter 02 Jan '09

02 Jan '09

On Jan 1, 2009 11:50pm, "Galchin, Vasili" <vigalchin(a)gmail.com> wrote: > it is a bioinformatics standard .. . I am writing on this newsgroup in order to try to be objective to get a "correct" and elegant answer .. in any case I am helping on the bioinformatics code (you can see on Hackage). I am trying to finish the 2Bit file format code ... it seems to me that bioinformatics as an area is not clearly defined .... e.g. it is unclear to me whether "offset" is a marshalled/serialized concept or or unmarshalled/unserialized concept ..... this distinction is very important .... I will have to think about more myself! > > > Regards, Vasili > Here's some code using Data.Binary to store data as offsets into a byte array. I haven't tested it too much, so it may have bugs. Maybe there's some inspiration in there. -Antoine >>>> import Data.Binary import Data.Binary.Get import Data.Binary.Put import Data.ByteString.Lazy (ByteString) import qualified Data.ByteString.Lazy as B data TestStruct = TestStruct { property1 :: ByteString , property2 :: ByteString , property3 :: ByteString } deriving Show {- The serialized format looks like (all big-endian): * first offset into data block (Word32) * second offset into data block (Word32) * third offset into data block (Word32) * length of bnary data block (Word32) * binary data block (Arbitrary binary data) -} instance Binary TestStruct where put struct = let data1 = property1 struct data2 = property2 struct data3 = property3 struct dataBlock = data1 `B.append` data2 `B.append` data3 offset1 = 0 offset2 = offset1 + B.length data1 offset3 = offset2 + B.length data2 in do putWord32be $ fromIntegral offset1 putWord32be $ fromIntegral offset2 putWord32be $ fromIntegral offset3 putWord32be $ fromIntegral $ B.length dataBlock putLazyByteString dataBlock get = do offset1 <- getWord32be offset2 <- getWord32be offset3 <- getWord32be dataBlockLength <- getWord32be dataBlock <- B.drop (fromIntegral offset1) `fmap` getLazyByteString (fromIntegral dataBlockLength) let (data1, rest1) = B.splitAt (fromIntegral $ offset2 - offset1) dataBlock (data2, rest2) = B.splitAt (fromIntegral $ offset3 - offset2 - offset1) rest1 data3 = rest2 return $ TestStruct data1 data2 data3 <<<<<

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representation on persistent store question
by Galchin, Vasili 02 Jan '09

02 Jan '09

Hello, Say I have several data structures that are marshalled(using Binary class) and written out linearly on persistence store. I want to calculate the offsets in bytes of these various data structures in a functional language way. What is the "suggested" (elegant) way .... ? Regards, Vasili

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The Psychology of Computer Programming
by Achim Schneider 02 Jan '09

02 Jan '09

http://books.google.com/books?id=tiMPAAAACAAJ&hl=de (by Gerald M. Weinberg) I suggest you procure a copy and read it for a) the lulz (a lot of them) b) insight into your own hack mode c) insight into how to hack recruiting and team lead d) ways to make Haskell even more subversive -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.

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Stupid question #852: Strict monad
by Brian Hurt 02 Jan '09

02 Jan '09

First off, let me apologize for this ongoing series of stupid newbie questions. Haskell just recently went from that theoretically interesting language I really need to learn some day to a language I actually kinda understand and can write real code in (thanks to Real World Haskell). Of course, this gives rise to a whole bunch of "wait- why is it this way?" sort of questions. So today's question is: why isn't there a Strict monad? Something like: data Strict a = X a instance Monad Strict where ( >>= ) (X m) f = let x = f m in x `seq` (X x) return a = a `seq` (X a) (although this code doesn't compile for reasons I'm not clear on- I keep getting: temp.hs:4:0: Occurs check: cannot construct the infinite type: b = Strict b When trying to generalise the type inferred for `>>=' Signature type: forall a b1. Strict a -> (a -> Strict b1) -> Strict b1 Type to generalise: forall a b1. Strict a -> (a -> Strict b1) -> Strict b1 In the instance declaration for `Monad Strict' as a type error. Feel free to jump in and tell me what I'm doing wrong.) Obviously, there would also be a StrictT monad transformer. The point here is that there are some monads (State being the obvious one) that come in both strict and lazy variants- the two variants could be eliminated, and the strict variant would just become a StrictT (State x) a. Or maybe a StateT x Strict a. Hmm. Which raises the interesting question of what, if any, semantic difference there is between a StrictT (State x) a and a StateT x Strict a. In any case, the idea came when I was thinking about monads being "code regions"- that we can talk about such and such a function being "in" the IO monad or STM monad. The idea was that one could define a "strict" code region you could put code in. Brian

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[HUMOR] Haskell koans
by Gwern Branwen 02 Jan '09

02 Jan '09

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 In the spirit of my previous email on haikus (which turned out fairly well), I'd like to issue a RFK (Request For Koans) to everyone! You can find what I've accumulated already on http://haskell.org/haskellwiki/Koans . There are a few there, but half of them are mine. There is, however, a fairly complete external links section covering not just vaguely-Haskellish koans, but also FP and programming in general. So feel free to chip in with any koans or links to koans, if you know of any. I'm sure I must've missed a few just for #haskell alone. Example: gwern> a young novice spoke to #haskell, asking to be taught about the ways of the Maybe monad. 'What have you brought me to see?' asked the #haskeller. 'Nothing', replied the noob. 'Then take it away!'" - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkldNUkACgkQvpDo5Pfl1oJk4wCeMSsuFMI89oEHTdDCewkHQh5U CvYAn3xfgjbiwt9P9Z2+v8vnRbzztBKH =E7Pd -----END PGP SIGNATURE-----

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hackage, gtk2hs
by Cetin Sert 02 Jan '09

02 Jan '09

Hi, A package I want to upload only builds with the unreleased gtk2hs version from the darcs repository and not the latest released version 0.9.13 or any lesser. But the repository version seems to not have changed, so if I say in my cabal file: gtk >= 0.9.14 it does not even build with the gtk2hs version from the repos, on the other hand if I say: gtk >= 0.9.13 people using 0.9.13 may be surprised to see the package does not build. What should I do o__O? Wait for the next gtk2hs release before releasing my package? Or is there a way to notify _potential users/those interested_ of the need to use the latest gtk2hs they can get from the darcs repos. Best Regards, Cetin Sert

3 2

definition of data
by Max.cs 01 Jan '09

01 Jan '09

hi all, I want to define a data type Tree a that can either be a or Branch (Tree a) (Tree a)? I tried data Tree a = a | Branch (Tree a) (Tree a) deriving Show but it seems not accpetable in haskell ? any way I could achieve this ? Thanks max

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ANN: gitit-0.4.1, recaptcha-0.1
by John MacFarlane 01 Jan '09

01 Jan '09

I'm pleased to announce the release of gitit-0.4.1, which I've just uploaded to HackageDB. Gitit is a wiki program that stores pages in a git repostory. Gitit now has support for (optional) captchas, using the reCAPTCHA service. I've packaged up the reCAPTCHA code as a separate library on HackageDB, recaptcha. Upgrading from older versions of gitit: The format of gitit's user database has changed. (gitit now generates a new random salt for each user, instead of using a single static salt for all users.) Unfortunately, this means that current users of gitit (yes, all seven of you) will have to delete your gitit-users file and have your users create new accounts. Sorry about the breaking change, but better now than later. When you upgrade, you'll also need to delete the _local directory, since changes have been made to the data structure that holds the application state. This shouldn't have any ill effects, since everything of lasting importance (users, pages) is stored elsewhere. If you use a configuration file, or if you want to start using reCAPTCHA (which requires a configuration file), you will also have to add fields for the captcha system. See data/SampleConfig.hs for the format. John

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Re: [Haskell-cafe] bottom case in proof by induction
by raeck＠msn.com 01 Jan '09

01 Jan '09

I am afraid I am still confused. > foo [] = ... > foo (x:xs) = ... > There is an implied: > foo _|_ = _|_ > The right side cannot be anything but _|_. If it could, then that would imply we could solve the halting problem: in a proof, how I could say the right side must be _|_ without defining foo _|_ = _|_ ? and in the case of > bad () = _|_ > bad _|_ = () mean not every function with a _|_ input will issue a _|_ output, so we have to say what result will be issued by a _|_ input in the definitions of the functions if we want to prove the equvalence between them? However, in the case of map f _|_ , I do believe the result will be _|_ since it can not be anything else, but how I could prove this? any clue? ps, the definition of map does not mention anything about _|_ . Thanks Raeck From: Luke Palmer Sent: Wednesday, December 31, 2008 10:43 PM To: Max.cs ; raeck(a)msn.com Subject: Re: [Haskell-cafe] bottom case in proof by induction On Wed, Dec 31, 2008 at 3:28 PM, Max.cs <max.cs.2009(a)googlemail.com> wrote: thanks Luke, so you mean the _|_ is necessary only when I have defined the pattern _|_ such as foo [] = [] foo _|_ = _|_ foo (x:xs) = x( foo xs ) -- consider non-termination case That is illegal Haskell. But another way of putting that is that whenever you do any pattern matching, eg.: foo [] = ... foo (x:xs) = ... There is an implied: foo _|_ = _|_ The right side cannot be anything but _|_. If it could, then that would imply we could solve the halting problem: halts () = True halts _|_ = False Because _|_ is the denotation of a program which never halts. To do it a bit more domain-theoretically, I'll first cite the result that every function has a fixed point. That is, for every f, there is a function fix f, where fix f = f (fix f). (The fix function is actually available in Haskell from the module Data.Function). Then let's consider this bad function: bad () = _|_ -- you can't write _|_ in Haskell, but "undefined" or "let x = x in x" mean the same thing bad _|_ = () Then what is fix f? Well, it either terminates or it doesn't, right? I.e. fix f = () or fix f = _|_. Taking into account that fix f = f (fix f): If it does: fix f = () = f () = _|_, a contradiction. If it doesn't: fix f = _|_ = f _|_ = (), another contradiction. >From a mathematical perspective, that's why you can't pattern match on _|_. >From an operational perspective, it's just that _|_ means "never terminates", and we can't determine that, because we would try to run it "until it doesn't terminate", which is meaningless... Luke

4 3

bottom case in proof by induction
by raeck＠msn.com 01 Jan '09

01 Jan '09

Dear all, Happy New Year! I am learning the Induction Proof over Haskell, I saw some proofs for the equivalence of two functions will have a case called 'bottom' but some of them do no have. What kind of situation we should also include the bottom case to the proof? How about the functions do not have the 'bottom' input such as: foo [] = [] foo (x:xs) = x : (foo xs) thank you, Raeck

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ANN: monte-carlo-0.2, gsl-random-0.2.3
by Patrick Perry 01 Jan '09

01 Jan '09

I've released a new version of the monte-carlo packages for haskell. Here are the highlights for monte-carlo: Changes in 0.2: * More general type class, MonadMC, which allows all the functions to work in both MC and MCT monads. * Functions to sample from discrete distributions. * Functions to sample subsets For a quick tutorial, see my blog post at http://quantile95.com/2008/12/31/monte-carlo-poker-odds/ Happy New Year, everyone! Patrick

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Re: [Haskell-cafe] about the concatenation on a tree
by Henk-Jan van Tuyl 01 Jan '09

01 Jan '09

On Wed, 31 Dec 2008 17:19:09 +0100, Max cs <max.cs.2009(a)googlemail.com> wrote: > Hi Henk-Jan van Tuyl, > > Thank you very much for your reply! > > I think the concatenation should be different to thhe > > treeConcat :: Tree a -> Tree a -> Tree a > > the above is a combination of two trees instead of a concatenation, so > I think the type of treeConcat should be: > > treeConcat :: Tree (Tree a) -> Tree a > > instead. How do you think? : ) I tried to implement it .. but it seems > confusing.. to me > > Thanks > > Max Hello Max, The function treeConcat :: Tree (Tree a) -> Tree a cannot be created, as it has an infinite type; you can however, define a function that replaces leafs with trees, for example treeConcat' in the following module, that replaces all leaves that contains a one with a given tree: > module TreeConcat where > data Tree a = Leaf a | Branch (Tree a) (Tree a) > deriving Show > treeConcat' :: Num a => Tree a -> Tree a -> Tree a > treeConcat' (Leaf 1) tree = tree > treeConcat' (Leaf x) _ = Leaf x > treeConcat' (Branch x y) tree = Branch (treeConcat' x tree) (treeConcat' > y tree) > main :: IO () > main = print $ treeConcat' (Branch (Leaf 1) (Leaf 2)) (Branch (Leaf 3) > (Leaf 4)) This displays: Branch (Branch (Leaf 3) (Leaf 4)) (Leaf 2) If this doen't help you either, I need to know more about what you are trying to do. Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ -- > > > On Wed, Dec 31, 2008 at 3:33 PM, Henk-Jan van Tuyl > <hjgtuyl(a)chello.nl>wrote: > >> Hi Max, >> >> A simple way to do this: >> >> module TreeConcat where >>> >> >> data Tree a = Leaf a | Branch (Tree a) (Tree a) >>> deriving Show >>> >> >> treeConcat :: Tree a -> Tree a -> Tree atreeConcat xs ys = Branch xs ys >>> >> >> main :: IO () >>> main = print $ treeConcat (Leaf 1) (Leaf 2) >>> >> >> But perhaps you want a certain ordering? Have a look at: >> >> http://hackage.haskell.org/packages/archive/AvlTree/4.2/doc/html/Data-Tree-… >> >> >> -- >> Regards, >> Henk-Jan van Tuyl >> >> >> -- >> http://functor.bamikanarie.com >> http://Van.Tuyl.eu/ >> -- >> >> >> --

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about the concatenation on a tree
by Max cs 01 Jan '09

01 Jan '09

hi all, not sure if there is someone still working during holiday like me : ) I got a little problem in implementing some operations on tree. suppose we have a tree date type defined: data Tree a = Leaf a | Branch (Tree a) (Tree a) I want to do a concatenation on these tree just like the concat on list. Anyone has idea on it? or there are some existing implementation? Thank you and Happy New Year! regards, Max

3 2