- Beginners - Haskell.org

Haskell: The Craft of Functional Programming 3rd ed is out.
by KC 17 Jun '11

17 Jun '11

http://www.haskellcraft.com/craft3e/Home.html -- -- Regards, KC

1 0

Is it possible to implement groupBy using folds?
by Zhi-Qiang Lei 16 Jun '11

16 Jun '11

Hi, The source code of groupBy shows that groupBy does not go through every element. Is it really possible to implement it using folds? Thanks. -- | The 'groupBy' function is the non-overloaded version of 'group'. groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs where (ys,zs) = span (eq x) xs Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

2 1

A neat, simple example of making code more efficient
by Costello, Roger L. 16 Jun '11

16 Jun '11

Hi Folks, This book [1] has a neat, simple example of making code more efficient. Suppose you want to create a function, quad, that raises a number to the 4th power. Example: quad 2 = 2 * 2 * 2 * 2 = 16 The function could be implemented like this: quad a = a * a * a * a That implementation requires three multiplications, which can be pretty expensive, especially with arbitrary numbers. Let's see if we can make it more efficient, by finding an implementation that uses less multiplications. By the associative law we can group the expression: (a * a) * (a * a) Each subexpression performs a square operation: (square a) * (square a) Which is itself a square operation: square (square a) That implementation requires only two multiplications. Let's take an example to see why: quad 2 = square (square 2) = square (2 * 2) -- First multiplication = square (4) -- Reduction = 4 * 4 -- Second multiplication = 16 -- Reduction to the answer Thus, by a little rework the number of multiplications is reduced from three to two. That is a significant efficiency improvement. Cool, aye? /Roger [1] http://www.amazon.com/Introduction-Functional-Programming-using-Haskell/dp/…

3 2

Haskell User Group in Stockholm
by Obscaenvs 16 Jun '11

16 Jun '11

OK, this *might* be slightly off-topic, but I couldn't find a more suiting list to post to: Any body here living in Stockholm interested in starting a user group? /Fredrik, old-as-hell graduate mathematics student

2 1

Options for creating a multiple select form via Yesod
by Michael Litchard 16 Jun '11

16 Jun '11

I'm trying to create a multiple select form, as illustrated on the following: http://api.jquery.com/selected-selector/ Here's the options I see possible: (1) Write a jQuery widget. (2) Use plain javascript via Julius (3) Use the low-level functions in Yesod.Form to write a widget (4) Use a pre-existing function that does what I need, but am not aware of this functionality (1) has appeal as it looks like something small I can contribute to the project. It will take me some extra time to figure out the details. But, I had a look at the other jQuery widgets and they seem to provide an approachable model to follow. (2) This looks like the most straight-forward approach. I'm just learning javascript so would have to figure out how to capture values in Haskell from the form. (3) This looks like the most difficult way. I don't think I know enough about the low-level functions in Yesod.Form to be able to accomplish this in a timely manner. (4) This is the best scenario. There's already a way to do this right now, and I just haven't identified it. If this is the case, I would appreciate being pointed in the right direction. Until informed otherwise, I'm evaluating options 1 and 2. All feedback welcomed. Thanks to all who made Yesod possible.

3 17

merge two handles
by Manfred Lotz 15 Jun '11

15 Jun '11

Hi all, I have two handles where I get stdout resp. stderr from a command output. Is it possible to merge those two handles so that I get a new input handle in a way that data can be read from the new handle whenever it is available from either of the two original handles? -- Manfred

3 10

How does the Deriving being implemented in Haskell?
by Haisheng Wu 15 Jun '11

15 Jun '11

Hi guys, What I understanding `deriving` is Haskell automatically make a type to be a instance of a particular typeclass. If that's the case, the question is How does Haskell do that underline? Take code below as sample, the type Day is now an instance of Bounded. data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday deriving (Bounded, Enum) Therefore, *minBound :: Day* will produce *Monday*. Just be curious how it is being implemented. Thanks. Simon

4 3

Example needed
by Zhi-Qiang Lei 15 Jun '11

15 Jun '11

Dear all, I wonder why haskell do not take foldl' as default foldl if foldl is not used in production often. I was told that because foldl is lazy evaluated. I know what the advantage of lazy evaluation: do not evaluate the expression which is not needed to, and can work on infinite list. Can you give me a example about how foldl can work on an infinite list? Thanks. Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

4 3

GHCI: C Library Load Path
by Christopher Howard 15 Jun '11

15 Jun '11

Is it possible to adjust from where GHCI loads C libraries? And the order of where it looks? I have this weird situation where I need GHCI to prefer one directory of .so files over another. Specifically, I need this in interactive mode (GHCI), not regular compiling (GHC). Also, I'm not writing code that uses C libraries, just using a module that loads one, so I'm not knowledgeable in the former subject. I could go into more details as to why I need this, but it would probably make this e-mail about four times longer, and I don't want to bore anyone unnecessarily. -- frigidcode.com theologia.indicium.us

1 0

Profiling install error
by blackcat＠pro-ns.net 14 Jun '11

14 Jun '11

I'm trying to install the profiling version of the base libraries on Fedora 15. I set the profiling options in ~/.cabal/config then tried to reinstall but I get %cabal install base --reinstall Resolving dependencies... cabal: internal error: impossible Is there anything I can do now?

3 3