- Haskell-Cafe - Haskell.org

rewrite rules to specialize function according to type class?
by Patrick Bahr 15 Feb '11

15 Feb '11

Hi all, I am trying to get a GHC rewrite rule that specialises a function according to the type of the argument of the function. Does anybody know whether it is possible to do that not with a concrete type but rather a type class? Consider the following example: > class A a where > toInt :: a -> Int > {-# NOINLINE toInt #-} > class B a where > toInt' :: a -> Int The idea is to use the method of type class A unless the type is also an instance of type class B. Let's say that Bool is an instance of both A and B: > instance A Bool where > toInt True = 1 > toInt False = 0 > instance B Bool where > toInt' True = 0 > toInt' False = 1 Now we add a rule that says that if the argument to "toInt" happens to be an instance of type class B as well, use the method "toInt'" instead: > {-# RULES > "toInt" forall (x :: B a => a) . toInt x = toInt' x > #-} Unfortunately, this does not work (neither with GHC 6.12 or GHC 7.0). Expression "toInt True" gets evaluated to "1". If the rewrite rule is written with a concrete type it works as expected: > {-# RULES > "toInt" forall (x :: Bool) . toInt x = toInt' x > #-} Now "toInt True" is evaluated to "0". Am I doing something wrong or is it not possible for GHC to dispatch a rule according to type class constraints? Thanks, Patrick

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problems with HDBC
by Kurt Norwood 15 Feb '11

15 Feb '11

Hi, I am getting some very cryptic error messages while trying to compile some simple code using hdbc. I am able to run the following lines in ghci without any problems. I am using ghc 6.12.1 on Ubuntu 10.10. Thanks in advance, Kurt ################## # Contents of the file ################## import Database.HDBC.Sqlite3 import Database.HDBC main = dosomething dosomething :: IO () dosomething = do conn <- connectSqlite3 "testdb" run conn "create table test (id integer)" [] run conn "insert into test (id) values (6)" [] commit conn disconnect conn ############## # Error message ############## > ghc testsql.hs testsql.o: In function `s1eF_info': (.text+0x2e): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1eG_info': (.text+0x6e): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1eJ_info': (.text+0x16e): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1eM_info': (.text+0x272): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1ho_info': (.text+0x379): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnection_connectSqlite3_closure' testsql.o: In function `s1ho_info': (.text+0x489): undefined reference to `__stginit_HDBCzm2zi2zi6zi1_DatabaseziHDBC_' testsql.o: In function `s1ho_info': (.text+0x493): undefined reference to `__stginit_HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3_' testsql.o: In function `s1eF_info': (.text+0x36): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_disconnect_info' testsql.o: In function `s1eG_info': (.text+0x76): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_commit_info' testsql.o: In function `s1eJ_info': (.text+0x176): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_run_info' testsql.o: In function `s1eM_info': (.text+0x27a): undefined reference to `HDBCzm2zi2zi6zi1_DatabaseziHDBCziTypes_run_info' testsql.o: In function `s1eN_srt': (.data+0x8): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnectionImpl_zdfIConnectionConnection_closure' testsql.o: In function `s1ho_srt': (.data+0x18): undefined reference to `HDBCzmsqlite3zm2zi3zi1zi0_DatabaseziHDBCziSqlite3ziConnection_connectSqlite3_closure' collect2: ld returned 1 exit status

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Fwd: ANN: vector-buffer package 0.1
by Alexander McPhail 15 Feb '11

15 Feb '11

Hi list, Could someone explain why the error pointed out by Luke occurred? From: Luke Palmer <lrpalmer(a)gmail.com> I think this would be a very good question for the list. Don't worry, they're nice helpful folks. On Mon, Feb 14, 2011 at 10:10 PM, Alexander McPhail <haskell.vivian.mcphail(a)gmail.com> wrote: > Hi Roman, > > > Can you explain why the following code doesn't work? 'unsafePerformIO' is > used all the time in hmatrix. For example, adding two vectors, we create a > new Vector then use that as the target for an FFI C call. > > Is the difference because I'm using mutable Storable Vectors? I would have > thought that unsafeFreeze (at the end) would make sure the problem reported > by Luke wouldn't occur. > > Is there a problem with laziness in the let binding of Luke's example? > > I note that in Data.Vector.Storable there is a pure 'convert' function, > which is essentially what I am trying to emulate. > > -- | convert to a vector > toVector :: Storable a => Buffer a -> (V.Vector a) > toVector (B o n v) = unsafePerformIO $ do > w <- M.new n > i <- readIORef o > M.unsafeWith v $ \p -> > M.unsafeWith w $ \q -> do > let n' = n-i > copyArray q (p `advancePtr` i) n' > if i /= 0 > then copyArray (q `advancePtr` n') p i > else return () > V.unsafeFreeze w > {-# INLINE toVector #-} > > > Vivian > > From: Luke Palmer <lrpalmer(a)gmail.com> > > This interface is an outlaw. > > main = do > buf <- newBuffer 10 :: IO (Buffer Int) > pushNextElement buf 1 > let v1 = V.toList (toVector buf) > v2 = V.toList (toVector buf) > print v1 > pushNextElement buf 2 > print v2 > > Despite v1 and v2 being defined to equal the exact same thing, this > program prints two distinct lines. > > toVector depends on the current state of the buffer. If this is to be > a law-abiding interface, toVector must return a value in IO: > > toVector :: (Storable a) => Buffer a -> IO (Vector a) > > Luke >

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ANN: vector-buffer package 0.1
by Alexander McPhail 15 Feb '11

15 Feb '11

Hi, I have uploaded a small package, vector-buffer, to hackage. It provides a buffer that can be turned into a Data.Vector.Storable. The mapM* functions map from the oldest element, not the first. Similarly for the derived Vector. Feature requests etc. welcome. Vivian

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lhs2tex and line numbers
by Mitar 15 Feb '11

15 Feb '11

Hi! How could I convince lhs2tex to add in poly mode line numbers before each code line in code block? Mitar

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Re: [Haskell-cafe] Proving correctness
by kevin＠froglingo.com 14 Feb '11

14 Feb '11

On 14.02.2011, at 03:03 PM, Pavel Perikov [perikov(a)gmail.com] wrote: Sorely, Haskell can't prove logic with it. No predicates on values, guarantee that proof is not _|_. Haskell makes bug free software affordable, that's true. But it's not a proof assistant. Being equivalent to a class of total recursive functions, the EP (Enterprise-Participant) data model can assist users to enter arbitrary and meaningful expressions only. It is a new addition to programming languages. Check out here: http://www.froglingo.com/FroglingoPL.pdf Kevin

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Problems with (projects|community).haskell.org
by Erik de Castro Lopo 14 Feb '11

14 Feb '11

Hi all, Still a couple of problems with these servers. Firstly, community.haskell.org shows the default Apache "It works" page. It would be nice to have something better there. Secondly the mailman web interface on projects.haskell.org [0] is giving a Service Temporarily Unavailable message (and has been for a couple of days). Cheers, Erik [0] http://projects.haskell.org/cgi-bin/mailman/admindb/haskell-llvm -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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ghci command line execution
by Thomas Davie 14 Feb '11

14 Feb '11

A while ago I remember someone showing me some tool, I *think* ghci that allowed you to pass it a function of type String -> String as an input, and have it simply run that function on stdin (presumably using interact) to achieve useful things like this... $ cat myFile.txt | ghci -e 'unlines . zipWith (\x y -> show x ++ " " ++ y) [1..] . lines' Does anyone know what it was that I saw being used here? Thanks Tom Davie

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Why is there no "splitSeperator" function in Data.List
by Robert Clausecker 14 Feb '11

14 Feb '11

Is there any reason, that one can't find a function that splits a list at a seperator in the standard library? I imagined something like this: splitSeperator :: Eq a => a -> [a] -> [[a]] splitSeperator ',' "foo,bar,baz" --> ["foo","bar","baz"] Or something similar? This is needed so often, even if I can implement it in one line, is there any reason why it's not in the libs? Yours, Robert Clausecker

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Re: [Haskell-cafe] ANN: FunGEn-0.3 simple 2D game engine released
by Simon Michael 14 Feb '11

14 Feb '11

Hi Andre, good to hear from you. On Feb 13, 2011, at 5:27 PM, Andre Wilson Brotto Furtado wrote: > This is great, thanks Simon. I'm currently not involved in any > haskell projects anymore, but please do keep the ball rolling for > FunGEn. > > I'm currenty exploring how domain-specific development and software > product lines can be streamlined for game development. You can check > it out at http://sharpludus.codeplex.com. Volunteers to port such a > work to Haskell are welcome. Cool. Thanks, -Simon

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