February 2010 - Beginners

defining expression
by John Moore 07 Feb '10

07 Feb '10

Hi, This gives me the explaination that it expecting a Expression but it been giving a IO b can some explain whats going on please. import Maybe data Expression = Val Double | Add Expression Expression | Subtract Expression Expression | Multiply Expression Expression | Divide Expression Expression | Var String | Let String Expression Expression deriving Show demo1 = (Add(Multiply(Divide(Subtract(Val 25)(Val 5))(Val 10))(Val 7))(Val 30)) type Dict =[(String,Expression)] emptyDict :: Dict emptyDict = [] addEntry :: String->Expression ->Dict -> Dict addEntry n e d = (n,e): d lookupEntry :: String -> Dict -> Maybe Expression lookupEntry n [] = Nothing lookupEntry n (x:xs) = if (n == k) then (Just v) else lookupEntry n xs where (k,v) = x evalStep :: Dict -> Expression -> Expression evalStep d(Val x)= (Val x) evalStep d(Add x y) = case x of (Val a) -> case y of (Val b) -> Val (a+b) left -> Add x (evalStep d y) right -> Add (evalStep d x)y evalStep d(Subtract x y) = case x of (Val a) -> case y of (Val b) -> Val (a-b) left -> Subtract x (evalStep d y) right -> Subtract (evalStep d x)y evalStep d(Multiply x y) = case x of (Val a) -> case y of (Val b) -> Val (a*b) left -> Multiply x (evalStep d y) right -> Multiply (evalStep d x)y evalStep d (Divide x y) = case x of (Val a) -> case y of (Val b) -> Val (a/b) left -> Divide x (evalStep d y) right -> Divide (evalStep d x)y evalStep d (Let n e1 e2) = do putStrLn ("Adding definition for "++n) v <- evalStep d (addEntry n e1 d) e2 putStrLn ("Removing definition for "++n) return v evaluate :: Dict-> [Expression] -> Expression -> IO () evaluate d(x:xs) e = do putStrLn (show e) putStrLn "Do another step (y/n) or rollback (r)? :" c <- getLine case c of "y" -> let e'= (evalStep d e)in evaluate d (e:x:xs) e'-- build up history "r" -> case (x:xs) of (x:xs)-> evaluate d xs x []-> do { putStrLn "Empty" ;evaluate d(x:xs) e } "n" -> putStrLn $ "Ok you said no :" ++ c John

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Installing hmatrix on OS X
by Duncan Mortimer 07 Feb '10

07 Feb '10

Hi all, I'm trying to install the easyVision library on OS X, following the notes at http://mit.edu/harold/Public/easyVisionNotes.html. However, despite a few hours web trawling and hacking away with various cabal options, I can't get "hmatrix" to install. I'm using OS X 10.5.8 with GHC 6.10.4. I installed gsl using macports, as suggested in the notes above, but cabal doesn't seem to be able to find it. The error I'm getting is: [13:22:35 dmorti hmatrix-0.8.2.0]$ cabal install hmatrix Resolving dependencies... [1 of 1] Compiling Main ( /var/folders/05/05Ltg9vxE+a1hR422nbuE++++TI/-Tmp-/hmatrix-0.8.2.05810/hmatrix-0.8.2.0/Setup.lhs, /var/folders/05/05Ltg9vxE+a1hR422nbuE++++TI/-Tmp-/hmatrix-0.8.2.05810/hmatrix-0.8.2.0/dist/setup/Main.o ) Linking /var/folders/05/05Ltg9vxE+a1hR422nbuE++++TI/-Tmp-/hmatrix-0.8.2.05810/hmatrix-0.8.2.0/dist/setup/setup ... Configuring hmatrix-0.8.2.0... Checking foreign libraries... FAIL *** Sorry, I can't link GSL. *** Please make sure that the appropriate -dev packages are installed. *** You can also specify the required libraries using *** cabal install hmatrix --configure-option=link:lib1,lib2,lib3,etc. setup: Package hmatrix-0.8.2.0 can't be built on this system. cabal: Error: some packages failed to install: hmatrix-0.8.2.0 failed during the building phase. The exception was: exit: ExitFailure 1 It appears that others have also had this error, but have been able to fix it up by including "--extra-lib-dirs" and "--extra-include-dirs" options, or using "ld-options: -framework vecLib". I've tried both of these approaches, but still get the same errors. Can anyone offer any suggestions? Thanks very much for your help, Duncan Mortimer

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How to initialize a C struct with FFI
by Marco De Oliveira 06 Feb '10

06 Feb '10

Hi, I try to initialize a C struct using the FFI API and ghc stays blocked when I execute my code (and I got sometimes the error "out of memory"). I do not understand, I do not find any mistake in my code: {-# OPTIONS -XTypeSynonymInstances #-} import Foreign.C.Types import Foreign.Marshal.Alloc import Foreign.Ptr import Foreign.Storable type CStruct = (CULong, CULong) instance Storable CStruct where sizeOf _ = 2*sizeOf (undefined::CULong) alignment _ = alignment (undefined::CULong) test = alloca (\pStruct -> do poke pStruct ( (fromIntegral 1)::CULong, (fromIntegral 1)::CULong ) ) Thanks for your help. Marco

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Re: [Haskell-beginners] Just clarifying the "pred" and "succ" functions in Haskell
by Brandon S. Allbery KF8NH 06 Feb '10

06 Feb '10

Andy Elvey wrote: > So, I may look at doing what I would call "lpred" and lsucc" - the > predecessor and successor of a list element. I'm somewhat surprised > that (from what I can tell) Haskell doesn't seem to have those two > functions for a list. I may be wrong.... Again, lists don't work that way; a list in Haskell is a single immutable object, you can pull items from it using (!!), head, etc., but you can't have a pointer into the "middle" of a list. You can have a sublist (for example, `tail ["foo", "bar", "baz"]' = `["bar", "baz"]') --- but you can't get from there to the "foo", as it isn't part of that new list. (This despite the fact that what `tail' gives you is going to be shared in actual storage with the original list. You don't have a backpointer into that original list to follow.) This is actually a feature of the Haskell model: it's usually easier to reason about these kinds of structures, where you can treat any operation as producing a new object and the Haskell compiler takes care of optimizations for you (shared representations, possibly recognizing that it can quietly do an update-in-place because you can't possibly reach the old value, etc.). You may want to take a look at the Seq type defined in the Data.Seq module, though; you have a `view' (what you can think of as a cursor) on a given Seq which can be moved back and forth. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

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Typeclass problems
by Patrick LeBoutillier 06 Feb '10

06 Feb '10

Hi all, I've written a small type class that is meant to behave a bit like Show: class Show a => Echo a where echo :: a -> String echoList :: [a] -> String echoListSep :: a -> String echo = show echoListSep _ = " " echoList [] = "" echoList [x] = echo x echoList (x:xs) = echo x ++ echoListSep x ++ echoList xs instance Echo Char where echo c = [c] echoListSep _ = "" instance Echo a => Echo [a] where echo = echoList instance Echo Int where instance Echo Integer where instance Echo Double where instance Echo Float where gen = map (const 1) f xs = map echo $ gen xs However the code doesn't compile on the last line. But it does compile if I replace 'echo' with 'show'. What's missing to make my typeclass behave like Show and accept input of any type (besides all the missing instances...)? I looked at the code for Show but I didn't see anything that looked "magical"... Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Just clarifying the "pred" and "succ" functions in Haskell
by Andy Elvey 06 Feb '10

06 Feb '10

Hi all - I'm doing a public-domain package of "functional" programs" in C, and that involves trying to replicate (in C) a number of Haskell functions. The two that I'm looking at now are "pred" and "succ". I've seen the examples on zvon.org, which give "pred 5" as being 4, and "pred B" as being "A". Ok, that's fine, no problem. However, is my understanding correct that this can be extended to lists (arrays in C) so that (for example) for a list ["foo", "bar", "baz"] , "pred "bar" " would give you "foo" , and "succ "bar" " would give you "baz"? Thanks in advance - bye for now - - Andy

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Re: Fast tree manipulation
by Gabi 05 Feb '10

05 Feb '10

Hi Heinrich, I didn't see any mature zipper libraries to efficiently manipulate trees. Can you point me to any mature libs ? Aside from that, there are many reasons why I would represent trees using sequence. The major one is that many GA operations like 2 point crossover and mutation are trivial if we represent the genome in vectors or sequence instead of trees. The downside is the cost of the translation of sequences back to trees. -- Regards, Gabi http://bugspy.net

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Simulation of interconnect network
by Roger King 05 Feb '10

05 Feb '10

I am building a simulator for an interconnect network for a multiprocessor computer. I would like to develop it in Haskell as an opportunity to learn Haskell. The network will have a number of routers with input ports and output ports and crossbars between them. I would like to simulate the protocol. This would be an event driven simulator. It would be at a high level, leaving out many details. I would like it to be fast and be able to run it on several processors. I would like to know if you have any advice. Has anyone done this before? Are there any discrete event simulators written in Haskell? R

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show vs. Data.ByteString.Char8.unpack for ByteString
by Hong Yang 04 Feb '10

04 Feb '10

Hi, What is the difference between show and unpack when the function arguments are of ByteString type? I thought they are the same, but they are not interchangeable at compile time. Thanks, Hong

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Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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