- Haskell-Cafe - Haskell.org

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

20 Jul '05

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.) Regards, Ralf > -----Original Message----- > From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe- > bounces(a)haskell.org] On Behalf Of Bulat Ziganshin > Sent: Tuesday, July 19, 2005 1:18 AM > To: Ben Lippmeier > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Proposal: deriving ShallowEq? > > Hello Ben, > > Tuesday, July 19, 2005, 11:01:32 AM, you wrote: > BL> I often find it useful to determine whether two objects are using the > BL> same constructor, without worrying about the constructors' arguments. > > BL> There is way to hack together a partial implementation of the > ShallowEq > BL> class within GHC, but it leaves much to be desired: > > BL> > instance Show a => ShallowEq a where > BL> > (=@=) a b > BL> > = (head $ words $ show a) == (head $ words $ show b) > > reading GHC sources is always very interesting :) > > that is from GHC/Base.hs : > > %********************************************************* > %* * > \subsection{@getTag@} > %* * > %********************************************************* > > Returns the 'tag' of a constructor application; this function is used > by the deriving code for Eq, Ord and Enum. > > The primitive dataToTag# requires an evaluated constructor application > as its argument, so we provide getTag as a wrapper that performs the > evaluation before calling dataToTag#. We could have dataToTag# > evaluate its argument, but we prefer to do it this way because (a) > dataToTag# can be an inline primop if it doesn't need to do any > evaluation, and (b) we want to expose the evaluation to the > simplifier, because it might be possible to eliminate the evaluation > in the case when the argument is already known to be evaluated. > > \begin{code} > {-# INLINE getTag #-} > getTag :: a -> Int# > getTag x = x `seq` dataToTag# x > \end{code} > > > > > -- > Best regards, > Bulat mailto:bulatz@HotPOP.com > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Error with Float
by Dinh Tien Tuan Anh 19 Jul '05

19 Jul '05

So there's no way to get exact stream that represents a fraction, such as: .5 = .10000000000000000.... .2 = .00110011001100110011............ ??? >From: Udo Stenzel <u.stenzel(a)web.de> >To: Dinh Tien Tuan Anh <tuananhbirm(a)hotmail.com> >Subject: Re: [Haskell-cafe] Error with Float >Date: Tue, 19 Jul 2005 20:51:14 +0200 > > > Value is: 0.800000000000001 > >What did you expect? Floating point numbers are inexact by their very >nature. Since 0.8, like many other numbers, is not representable >exactly in binary, (10 * 0.8 - 8) will certainly not be zero. > >Take a course in numerical mathematics some time. It helps. > > >Udo. >-- >HTML needs a <rant> tag. -- Alan Cox ><< signature.asc >> _________________________________________________________________ It's fast, it's easy and it's free. Get MSN Messenger 7.0 today! http://messenger.msn.co.uk

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Proposal: deriving ShallowEq?
by Ben Lippmeier 19 Jul '05

19 Jul '05

Hello, I often find it useful to determine whether two objects are using the same constructor, without worrying about the constructors' arguments. An example, using some arbitrary data type "Thingo": > class ShallowEq a where > shallowEq :: a -> a -> Bool > data Thingo a b > = TOne a > | TTwo a b Int Char Float > | TThree Int Char b b > (TOne 23) `shallowEq` TOne{} True > (TThree 5 'c' True False) `shallowEq` TTwo{} False -- Having some sort of generic shallowEq operator reduces the need for a host of predicates such as: (this one from Data.Maybe) > isJust x > = case x of > Just {} -> True > _ -> False .. which is an approach that is obviously going to be tedious when the size of the data type becomes large. -- There is way to hack together a partial implementation of the ShallowEq class within GHC, but it leaves much to be desired: > instance Show a => ShallowEq a where > (=@=) a b > = (head $ words $ show a) == (head $ words $ show b) Notice that in the example the term "TTwo{}" is a partially constructed record. The implementation relies on laziniess to avoid trying to show the missing fields (which would fail). -- Questions: 1) Does anyone know a better/existing way to implement ShallowEq that doesn't involve enumerating all the constructors in the data type? 2) If not, can anyone think of reasons why it wouldn't be a good idea for GHC to derive ShallowEq (by expanding said enumeration)?

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