- Beginners - Haskell.org

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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Re: Majority Element
by Keith Sheppard 22 Feb '09

22 Feb '09

I think this gets you what you want: majority nums = find (\x -> fromIntegral (length x) > (fromIntegral (length nums)) / 2) (group(sort(nums))) It returns a "Maybe" list containing all majority elements. Something that avoids the sort would be faster though. -Keith > Message: 8 > Date: Sun, 22 Feb 2009 01:15:30 +0000 > From: "Matthew J. Williams" <matthewjwilliams1(a)googlemail.com> > Subject: [Haskell-beginners] Majority Element > To: beginners(a)haskell.org > Message-ID: <49a0a726.278e420a.6b19.ffffe361(a)mx.google.com> > Content-Type: text/plain; charset="us-ascii"; format=flowed > > Dear listers > > What is a majority element in an array? > > Sincerely > Matthew J. Williams > > > > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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Baffled by Parsec
by Colin Paul Adams 22 Feb '09

22 Feb '09

The tutorials I've found don't tell me what I want to know :-( I have lines that begin with an integer, then white-space, and then further structure to the end-of-line. So for a starter, could you show me how to write a parser for the integer (returning an Int), and how to combine it with a parser for the rest of the line (take that as written - it isn't, but I'll try to do that myself given such a kick-start). Thanks. -- Colin Adams Preston Lancashire

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Majority Element
by Matthew J. Williams 21 Feb '09

21 Feb '09

Dear listers What is a majority element in an array? Sincerely Matthew J. Williams

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Help with monads (I think...)
by Patrick LeBoutillier 21 Feb '09

21 Feb '09

Hi all, I'm trying to implement the following simple Perl program in Haskell: my $nb_tests = 0 ; sub ok { my $bool = shift ; $nb_tests++ ; print STDOUT ($bool ? "ok" : "nok") . " $nb_tests\n" ; } ok(0) ; ok(1) ; The output is: nok 1 ok 2 I'm pretty much a Haskell newbie, but I know a bit about monads (and have been reading "Real World Haskell"), and I think I need to put the ok function must live inside some kind of state monad. My problem is that I also would like the ok function to perform some IO (as shown above, print). How is a case like this handled? Can my function live in 2 monads? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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flip lookup in ghci
by Matt R 20 Feb '09

20 Feb '09

So what's this all about then? How come a has become ()? ghci> :t lookup lookup :: (Eq a) => a -> [(a, b)] -> Maybe b ghci> :t flip lookup flip lookup :: (Eq a) => [(a, b)] -> a -> Maybe b ghci> let lookupIn = (flip lookup :: (Eq a) => [(a, b)] -> a -> Maybe b ) ghci> :t lookupIn lookupIn :: [((), b)] -> () -> Maybe b

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