February 2009 - Beginners

Array and IArray
by Francesco Bochicchio 26 Feb '09

26 Feb '09

Hello all, I'm reading stuff about the different types of arrays in Haskell ... One of the things that I don't understand If I do: import Data.Array arr = listArray (1,10) [1..] the type of arr is Array. My question is: is this array an instance of the type class defined in Data.Array.IArray? Or there is another kind of immutable boxed array somewhere in haskell libraries? The documentation makes a reference to 'the Array type exported by Data.Array.IArray' but if I do: import Data.Array.IArray arr = listArray (1,10) [1..] I get a couple of compiler errors Ciao & thanks in advance ----- FB

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Meaning of '!' in record defintion? UNPACK?
by Erik de Castro Lopo 26 Feb '09

26 Feb '09

Hi all, I'm looking at some code that defines a record: data Position = Position { posOffset :: {-# UNPACK #-} !Int , posRow :: {-# UNPACK #-} !Int , posColumn :: {-# UNPACK #-} !Int } What does the the exclamation mark mean? And UNPACK? Thanks in advance. Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- Microsoft is finally bringing all of its Windows operating system families under one roof. It will combine all of the features of CE, stability and support of ME and the speed of NT. It will be called Windows CEMENT...

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how to instance a class
by Daneel Yaitskov 26 Feb '09

26 Feb '09

Hi, I study opengl and decided to improve the Vertex3 type. data Vertex3 a => Vertex3 a! a! a! a vertex and a vector are same. It is often need to multiple a vector at a scalar or to calculate a scalar production of two vectors. I have realized the Num class for the Vertex3 type: instance (Num a) => Num (Vertex3 a) where (Vertex3 x y z) * (Vertex3 x' y' z') = (Vertex3 (x*x') (y*y') (z*z')) (Vertex3 x y z) + (Vertex3 x' y' z') = (Vertex3 (x+x') (y+y') (z+z')) (Vertex3 x y z) - (Vertex3 x' y' z') = (Vertex3 (x-x') (y-y') (z-z')) abs (Vertex3 x y z) = (Vertex3 (abs x) (abs y) (abs z)) signum (Vertex3 x y z) = (Vertex3 (signum x) (signum y) (signum z)) negate (Vertex3 x y z) = (Vertex3 (-x) (-y) (-z)) instance (Fractional a) => Fractional (Vertex3 a) where (Vertex3 x y z) / (Vertex3 x' y' z') = (Vertex3 (x/x') (y/y') (z/z')) -------------- But I can't instance my own class: class (Num a) => SVect a where (***) :: Num b => a -> b -> a instance (Num t) => SVect (Vertex3 t) where (Vertex3 x y z) *** c = Vertex3 (c*x) (c*y) (c*z) GHC posts about the error: surface.hs:107:36: Couldn't match expected type `b' against inferred type `t' `b' is a rigid type variable bound by the type signature for `***' at surface.hs:103:14 `t' is a rigid type variable bound by the instance declaration at surface.hs:105:14 In the second argument of `(*)', namely `x' In the first argument of `Vertex3', namely `(c * x)' In the expression: Vertex3 (c * x) (c * y) (c * z) ------------------------ I understand the "c" argument must have type as x,y and z, but I don't know what need to do. Daneel Yaitskov.

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Help with TAP implemation in haskell
by Patrick LeBoutillier 24 Feb '09

24 Feb '09

Hi, As a learning exercise, I'm trying to make a Haskell module (my first) that implements a TAP library (http://testanything.org/) The library basically provides some functions to perform unit tests and keeps track of some state. It also performs quite a bit of IO as it goes a long. As a first step I'm loosely basing my code on a Java implementation. You can find the code here: http://svn.solucorp.qc.ca/repos/solucorp/JTap/trunk/JTap.java. I've setup my TapState data structure and a monad transformer to be able to keep state and do IO within the same functions: data TapState = TapState { planSet :: Bool, noPlan :: Bool, skipAll :: Bool, testDied :: Bool, expectedTests :: Int, executedTests :: Int, failedTests :: Int } deriving (Show) type TAP a = StateT TapState IO a In the Java version there is a function called cleanup that is called after all the tests are performed to determine the return code and print some diagnostics: private int cleanup(){ int rc = 0 ; if (! plan_set){ diag("Looks like your test died before it could output anything.") ; return rc ; } if (test_died){ diag("Looks like your test died just after " + executed_tests + ".") ; return rc ; } if ((! skip_all)&&(no_plan)){ print_plan(executed_tests) ; } if ((! no_plan)&&(expected_tests < executed_tests)) { diag("Looks like you planned " + expected_tests + " test" + (expected_tests > 1 ? "s" : "") + " but ran " + (executed_tests - expected_tests) + " extra.") ; rc = -1 ; } if ((! no_plan)&&(expected_tests > executed_tests)) { diag("Looks like you planned " + expected_tests + " test" + (expected_tests > 1 ? "s" : "") + " but only ran " + executed_tests + ".") ; } if (failed_tests > 0){ diag("Looks like you failed " + failed_tests + " test" + (failed_tests > 1 ? "s" : "") + " of " + executed_tests + ".") ; } return rc ; } I'm having problems implementing the equivalent of this function in haskell. Inside a do block, is there a way to terminate the function immediately and return a result ("return" in the imperative sense, not the Haskell sense)? If not, must one really use deeply nested if/then/else statements to treat these special cases? All I could come up was this, which I find quite ugly: _cleanup :: Int -> TAP Int _cleanup rc = do ts <- get if (not $ planSet ts) then do diag "Looks like your test died before it could output anything." return rc else if (testDied ts) then do diag $ "Looks like your test died just after " ++ (show $ executedTests ts) return rc else ... Thanks a lot, Patrick Note: If it helps, you can find my Haskell code here: http://svn.solucorp.qc.ca/repos/solucorp/JTap/trunk/tap.hs -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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data constructor with IO type
by Daneel Yaitskov 24 Feb '09

24 Feb '09

Hi All, I'd like to know does it exist a method which allow to construct the TestB type without a temporary variable such as the TestA constructor. --- module Main where import Monad import Control.Concurrent data TestA = TestA (MVar Bool) xGetA (TestA x) = x data TestB = TestB { xGetB :: MVar Bool } --- main = do tmp <- newMVar False let t = TestB { xGetB = tmp } in do vt <- takeMVar t putStrLn ("HELLO" ++ show vt) --- main = do t <- liftM TestA (newMVar False) vt <- takeMVar (xGetA t) putStrLn ("HELLO" ++ show vt) --- Daneel Yaitskov

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ghc installation on mac osx 10.4.11(intel), Xcode 2.4?
by 7stud 24 Feb '09

24 Feb '09

Hi, I just bought Real World Haskell, and I'm trying to get the ghc compiler set up. The installer for the latest version of ghc, 6.10.1, is for Leopard(osx 10.5). What should I install for osx 10.4.11(Tiger)? Also, a question about "Beware of enumerating floating point numbers" on p. 11. The book doesn't explain what it means to enumerate floating point numbers without a step, so when it says to beware of the non-intuitive behavior of: ghci> [1.0..1.8] how would a reader know what the intuitive behavior is? Is the step .1, .01, .001? Is there a default step? Who knows? On p. 10 there is an error. Below the last code example on the page, it says: "In the latter case, the list is quite sensibly missing the endpoint of the enumeration, because it isn't an element of the series we defined." But the last line of the code example is this: ghci> [10,9..1] [10, 9, 8, 7, 6, 5, 4, 3, 2, 1] which quite clearly includes both endpoints.

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the ($) function (was desugaring an example from RWH)
by Michael Easter 23 Feb '09

23 Feb '09

re: desugaring. Thanks Andrew, that worked great! I have another, easier question from the same code in RWH: bogusTransfer qty fromBal toBal = do fromQty <- atomically $ readTVar fromBal [snip] Can someone please explain the ($) function in English? From the type signature, it seems to be an "apply function", but I can't quite explain when we use it. Clearly, it is used throughout RWH but I haven't found a good explanation. My guess is that it is when we want an "execution" of a function rather than a mere reference to it. Is that accurate? thanks again Michael -- ---------------------- Michael Easter http://codetojoy.blogspot.com: Putting the thrill back in blog http://youtube.com/ocitv -> Fun people doing serious software engineering

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desugaring an example from RWH
by Michael Easter 23 Feb '09

23 Feb '09

Folks, I have an example of the do-syntax that I would like to desugar. It is an example from Chapter 28 in RWH. I think I understand the ideas behind the following STM example, but I would like to present it to others in a desugared way: tryBogusSale = do players@(alice:bob:_) <- atomically populateWorld atomically $ alwaysSucceeds =<< consistentBalance players bogusSale Wand 5 alice bob I'm having problems especially with the 2nd line in the do block (i.e. the predicate for alwaysSucceeds). I would like to try and express this in terms of only bind (->) and return. thanks! Michael -- ---------------------- Michael Easter http://codetojoy.blogspot.com: Putting the thrill back in blog http://youtube.com/ocitv -> Fun people doing serious software engineering

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Getting into nested monad transformers
by Arthur Chan 23 Feb '09

23 Feb '09

Hey all, I've been trying to access the inner state "s" for this type: newtype TestThingey s a = TestThingey { runTrans :: ReaderT Int (StateT String (StateT s IO)) a } deriving (Monad, MonadIO, MonadState String, MonadReader Int) It doesn't seem to be doable. I could make it into a regular type declaration, but then I lose the GeneralizedNewtypeDeriving. Is this common? Or do people just avoid needing to use "lift"? -Arthur

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A better way?
by Keith Sheppard 23 Feb '09

23 Feb '09

Hello, I'm new to haskell and still getting used to working with lazy evaluation. I created a little function to calculate column widths for a 2D list of strings (a table) by iterating through the list and accumulating a max value for each column. My initial implementation ran out memory for tables with many rows because of lazy evaluation and here is how I dealt with it: {- | for a table, calculate the max width in characters for each column -} maxTableColumnWidths :: [[String]] -> [Int] maxTableColumnWidths [] = [] maxTableColumnWidths table = maxTableColumnWidthsInternal table [] maxTableColumnWidthsInternal :: [[String]] -> [Int] -> [Int] maxTableColumnWidthsInternal [] prevMaxValues = prevMaxValues maxTableColumnWidthsInternal (row:tableTail) prevMaxValues | seqList prevMaxValues = undefined | otherwise = maxTableColumnWidthsInternal tableTail (maxRowFieldWidths row prevMaxValues) -- this little function is for making the list strict... otherwise -- we run out of memory seqList [] = False seqList (head:tail) | head `seq` False = undefined | otherwise = seqList tail maxRowFieldWidths :: [String] -> [Int] -> [Int] maxRowFieldWidths row prevMaxValues = let colLengths = map length row lengthOfRow = length row lengthOfPrevMax = length prevMaxValues maxPrefixList = zipWith max colLengths prevMaxValues in if lengthOfRow == lengthOfPrevMax then maxPrefixList else if lengthOfRow > lengthOfPrevMax then maxPrefixList ++ (drop lengthOfPrevMax colLengths) else maxPrefixList ++ (drop lengthOfRow prevMaxValues) This works but it isn't very pretty (maybe also inefficient?). Is there a better way to deal with this kind of memory issue? Thanks! Keith

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