- Haskell-Cafe - Haskell.org

file line operation perhaps need loop
by Sun Yi Ming 21 Jul '05

21 Jul '05

Hello, I have two txt file,and i want to mix the two files line by line, e.g. $ cat url1.txt url1_1.line url1_2.line $ cat url2.txt url2_1.line url2_2.line and i want this file as result: $ cat aha.txt url1_1.line url2_1.line url1_2.line url2_2.line i first write this snippet of code: --- import System.IO mix :: [a] -> [a] -> [a] mix [] ys = ys mix xs [] = xs mix (x:xs) (y:ys) = [x,y] ++ mix xs ys f1 = do contents1 <- readFile "url1.txt" contents2 <- readFile "url2.txt" let urls1 = lines contents1 urls2 = lines contents2 urls = mix urls1 urls2 writeFile "aha.txt" (unlines urls) -- this works fine, but i think if the two file are very big, and the readFile will consume too many mem.so i need to read the file line by line but stunned by the loop in IO Monad: --- main = do h1 <- openFile "url1.txt" ReadMode h2 <- openFile "url2.txt" ReadMode line1 <- hGetLine h1 line2 <- hGetLine h2 print $ line1 : line2 : [] -- i don't howto do hClose h1 hClose h2 -- any ideas? thank you all. -- Sun Yi Ming

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RE: [Haskell-cafe] FFI and callbacks
by Bayley, Alistair 20 Jul '05

20 Jul '05

> From: John Goerzen [mailto:jgoerzen@complete.org] > > How would I make this sort of system play nice with Haskell threads? > > Also, the brief FFI docs that I've found don't really explain > callbacks > in FFI very well to me. Are there any other resources on > this anywhere? This might help: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign.Ptr.html# t%3AFunPtr ...and I can also show you some code which uses a simple callback. Or are you past that stage now, and more worried about interaction of callbacks and threads? Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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FFI and callbacks
by John Goerzen 20 Jul '05

20 Jul '05

Hi, I'm looking at packaging an event-driven console widget set (CDK) for Haskell using FFI. I know that other event-driven widget sets have Haskell bindings, but I'm not quite sure how to make everything play nice with forkIO. These systems generally have some sort of an opaque main loop, implemented in C. This loop would usually never return, or perhaps only return once the UI is destroyed. How would I make this sort of system play nice with Haskell threads? Also, the brief FFI docs that I've found don't really explain callbacks in FFI very well to me. Are there any other resources on this anywhere? Thanks, -- John

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Word32 to Int converions
by yin 20 Jul '05

20 Jul '05

hello, how do I convert an Word32 (or WordXYZ) to Int, or Integer, or Float, ...? The Int conversion is the priority. Thanks. Matej 'Yin' Gagyi

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RE: [Haskell-cafe] Strict and non-strict vs eager and lazy, was Confused about Cyclic struture
by Simon Marlow 20 Jul '05

20 Jul '05

On 18 July 2005 15:19, Bayley, Alistair wrote: > Not a taker (yet - where can I find information about non-lazy > implementation of non-strict languages? From Google so far: > speculative evaluation (Eager Haskell), call-by-name vs call-by-need.) > > Wikipedia frustratingly hints that "other evaluation strategies are > possible", but that's all it says: > http://en.wikipedia.org/wiki/Non-strict_programming_language While most of Haskell admits other evaluation strategies, lazy I/O pretty much requires lazy evaluation for lazy input streams. If the list returned by getContents is evaluated mulitple times, you don't want to read more data from the input each time, hence previous evaluations must be shared. Cheers, Simon "down with lazy I/O" Marlow

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Haskell, SDL, OpenGL
by yin 20 Jul '05

20 Jul '05

Hello, I'm trying to write a 3D renderer in Haskell using SDL bindings (Lemmih) and OpenGL bindings. When I try to compile my work, ghc runs collect2: /usr/libexec/gcc/i686-pc-linux-gnu/3.4.3-20050110/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o a.out -u GHCziBase_Izh_static_info -u GHCziBase_Czh_static_info -u GHCziFloat_Fzh_static_info -u GHCziFloat_Dzh_static_info -u GHCziPtr_Ptr_static_info -u GHCziWord_Wzh_static_info -u GHCziInt_I8zh_static_info -u GHCziInt_I16zh_static_info -u GHCziInt_I32zh_static_info -u GHCziInt_I64zh_static_info -u GHCziWord_W8zh_static_info -u GHCziWord_W16zh_static_info -u GHCziWord_W32zh_static_info -u GHCziWord_W64zh_static_info -u GHCziStable_StablePtr_static_info -u GHCziBase_Izh_con_info -u GHCziBase_Czh_con_info -u GHCziFloat_Fzh_con_info -u GHCziFloat_Dzh_con_info -u GHCziPtr_Ptr_con_info -u GHCziPtr_FunPtr_con_info -u GHCziStable_StablePtr_con_info -u GHCziBase_False_closure -u GHCziBase_True_closure -u GHCziPack_unpackCString_closure -u GHCziIOBase_stackOverflow_closure -u GHCziIOBase_heapOverflow_closure -u GHCziIOBase_NonTermination_closure -u GHCziIOBase_BlockedOnDeadMVar_closure -u GHCziIOBase_BlockedIndefinitely_closure -u GHCziIOBase_Deadlock_closure -u GHCziWeak_runFinalizzerBatch_closure -u __stginit_Prelude /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../crt1.o /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../crti.o /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/crtbegin.o -L/home/yin/devel/third/sdlh/SDLstable/img/lib/SDL-0.2.0 -L/usr/lib/ghc-6.4 -L/usr/lib -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110 -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110 -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../../i686-pc-linux-gnu/lib -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../.. Main.o -lHSSDL-0.2.0 \ /usr/lib \ -lSDL -lHSOpenGL -lHSOpenGL_cbits -lGLU -lGL -lm -lHSbase -lHSbase_cbits -lHSrts -lm -lgmp -ldl -lgcc -lgcc_eh -lc -lgcc -lgcc_eh /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/crtend.o /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../crtn.o Collect2 don't understand the /usr/lib parameter and fails with this message: /usr/bin/ld: /usr/lib: No such file: File format not recognized collect2: ld returned 1 exit status I just rerun collect2 by hand without pre /usr/lib parameter, and it works. What have I to do or where have to look in to find and resolve the problem? I use Gentoo 2005.0 and SDL binding from SDLStable brach installed by hand (I didn't use Portage). Is it the packages.conf file, where propably is the problem? Thanks for every answer... Matej 'Yin' Gagyi

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Named data type members
by yin 20 Jul '05

20 Jul '05

Hello, I've a data type: data SomeData = SomeData { int1 :: Int, int2 :: Int } class SomeClass where infix 1 `i_` i_ :: SomeData -> Int -> SomeData infix 1 `_i` _i :: SomeData -> Int -> SomeData instance SomeClass SomeData where (SomeData int1 int2) `i_` i = SomeData (int1 + i) int2 (SomeData int1 int2) `_i` i = SomeData int1 (int2 + i) 1. Possible optimalizations ...? 2. There are several data type members in 'SomeData' and I wish to access them in other way then "(SomeData m1 m2 m3 m4 m5 ... m100) `a` i = SomeData (m1 + i) m2 m3 ... m100" 3a. The 'SomeData' is something like c structure, which holds all application data. One instanciated variable is passed in more functions to each other: -- SomeData is now very complex initsomeData :: SomeData initsomeData = do var1 <- init... ... SomeData 0 0 0 1 1 True False (-1) var1 var2 ....... main = do p <- initAplication processEvents p -- First Function processEvents :: SomeData -> IO () processEvents p = do event <- pollEvent case event of Event1 -> do processEvents (SomeData v1 v2 v3 (v4*2) ... v100) Event2 -> do processEvents (SomeData v1 ... False ... v100) Event2' -> do processEvents (SomeData v1 ... True ... v100) EventQuit -> do return () NoMoreEvents -> do computeNext p _ -> processEvents p where (SomeData v1 v2 v3 v4 v5 ... v100) = p -- An time based function computeNextStep = do timeDelta <- getTimeDelta let i1' = compute1 i1 i2 i3 ... i50 i2' = compute2 i1 i2 i3 ... i50 ... if (condition) then -- need more computing computeNextStep (SomeData u1 u2 ... u50 i1 i2 ... i50) else do p' <- (SomeData u1 u2 ... u50 i1 i2 ... i50) draw p' processEvents p' where (SomeData u1 u2 ... u50 i1 i2 ... i50) = p -- ux - uninteresting (State variables, like left-key-down was last left-key related event: move left) -- ix - interesting for computing -- where x = 1, 2 ... 50 ... ... 3b. every funtion needs only a part of the data... can I build for every fintion a class with defined operators to manipulate with the data and then: * pass the variable in func1 known as class A to func2, which is infering a class B? Thank you all... Matej 'Yin' Gagyi

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How to variables
by yin 20 Jul '05

20 Jul '05

Hello all! I'm doing a 3D simulation. Now I need something like variables in imperative languages. My mainLoop check for new events and renders scene. To use input for controling camera position I need variables. An equivalent code in C: void main_loop() { int loop_num = 0; bool run = 1; SDLEvent e; while(run) { while(SDL_PollEvent(&e)) { if(e.type == SDL_KeyDown) { if(... == SDLK_Left) camera_pos_x--; } else if ... ... } drawScene(); loop_num++; } } How to implement camera_pos_x, y and z as variables, which chage values in run? This is only simplified example, but it's important to implement it in as most imperative form as possible. Thank fo all answers. Matej 'Yin' Gagyi

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RE: [Haskell-cafe] Proposal: deriving ShallowEq?
by Ralf Lammel 20 Jul '05

20 Jul '05

Ben, thanks for your clever questions. Let's look at a little GHC 6.4 demo. {-# OPTIONS -fglasgow-exts #-} import Data.Generics main = do print $ toConstr () == toConstr () print $ toConstr (5::Int) == toConstr () print $ toConstr (\(x::Int) -> x) == toConstr () print $ id `shallowEq` (id::Int -> Int) shallowEq :: Data a => a -> a -> Bool shallowEq x y = toConstr x == toConstr y This gives: *Main> main True False *** Exception: toConstr Some observations: a) We need to say what sort of Num this 5 is because otherwise instance selection will not know how to obtain the constructor for 5. In this sense, SYB is not different from normal type-class-based programming. Well, modulo default declarations; http://www.haskell.org/tutorial/numbers.html So this works (because of a default declaration) *Main> show 5 == show () False This one doesn't because there is no default declaration: *Main> toConstr 5 == toConstr () <interactive>:1:0: Ambiguous type variable `a' in the constraints: ... b) You are right. An independently polymorhic shallowEq is more expressive since it allows us to compare the top-level constructors of different specializations of the same type, e.g., Just Char vs. Just String. shallowEq' :: (Data a, Data b) => a -> b -> Bool shallowEq' x y = toConstr x == toConstr y You also spotted the issue that shallowEq' cannot operate on data of types a and b where either of these is a type scheme. The reason for that is that the instances of Data (and Typeable) involve constraints for the types of the children. As long as these children types are not fixed, toConstr cannot be computed since instance selection cannot be completed. In general, this is a good idea because of the types of gfoldl (and the gmap derivates), but for exercises like shallow equality this is in the way. I don't have a good solution to offer. (I don't see either that this is a show stopper in practice.) We may think of splitting up the API as to define several more precise classes ... c) Just for the record, SYB's toConstr throws for function types. Ralf > -----Original Message----- > From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe- > bounces(a)haskell.org] On Behalf Of Ben Lippmeier > Sent: Tuesday, July 19, 2005 8:56 PM > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Proposal: deriving ShallowEq? > > Ralf Lammel wrote: > > As Bulat points out, the GHC primitive dataToTag# > > indeed nicely solves the problem. Ben, just for > > completeness' sake; with SYB, you get such reflective > > information too (and others): > > > > shallowEq :: Data a => a -> a -> Bool > > shallowEq x y = toConstr x == toConstr y > > > > (dataToTag# returns Int, while toConstr comprises other things like the > > constructor name.) > > > > Ralf, > Yes, I ended up using the "propper" SYB approach instead, though I have > noticed that the reflection data types Constr and DataRep make no > mention of type variables or functions. > > For example, this works fine: > > getTag (Just 5) ==# getTag (Just{}) > > getTag (Just (\x -> x)) ==# getTag (Just{}) > > But this does not > > toConstr (Just 5) == toConstr (Just{}) > Ambiguous type variables. > > > toConstr (Just (\x -> x)) == toConstr (Just{}) > No instance for Data (t -> t) > > I appreciate the reasons why this is so, though I think it's interesting > to see the practical consequences. > > A toConstr version of shallowEq works ok so long as you provide a type > signature to constrain both arguments to be the same type, and one of > them is always fully constructed - which is fine for me at the moment. > > Ben. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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what does ' mean?
by Logesh Pillay 20 Jul '05

20 Jul '05

Dear list A v. newbie question. I found the following code on the web /-- Andrew Bromage / /-- If you're doing permutations, then I suppose you want this too: //-- (subLists might be a more suggestive name) /combinations *:: [a] -> [[a]] *combinations [] = [[]] combinations (x:xs) = combinations xs ++ [ x:xs' | xs' <- combinations xs ] What does ' (apostrophe) mean? I see no reference to it in Simon Thompson's book. Thanks Logesh

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