June 2009 - Beginners

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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decorate-sort-undecorate in haskell
by Ivan Uemlianin 11 Jul '09

11 Jul '09

Dear All I'm learning Haskell from a background in Python, and I'm just looking at the sort and sortBy functions in Data.List. In Python, the decorate-sort-undecorate pattern is a popular alternative to using an explicit compare function. For example, to sort a list of lists by length: def sortByLength(xs): dec_xs = [(len(x), x) for x in xs] dec_xs.sort() undec_xs = [x[1] for x in dec_xs] return undec_xs This is often preferred to something like: xs.sort(lambda x,y: len(x) > len(y)) # just used lambda so it fits on one line I think the reasoning is that plain sort() is done at C speed, whereas sort(func) is done in Python, so, depending on the list I suppose, dsu can be worth the trouble. (Sorry for vagueness). Anyway, I was wondering if dsu is popular in Haskell, and whether or when it would make sense to use dsu rather than sortBy. Here's a verbose dsu I wrote myself: sortDSU decFunc a = undecorate (sort (decorate decFunc a)) decorate decFunc [] = [] decorate decFunc (x:xs) = ( ((decFunc x), x) : decorate decFunc xs ) undecorate [] = [] undecorate ( (_, y) :xs) = ( y : undecorate xs ) Here's a terser and perhaps more idiomatic (but I think equivalent) dsu which I then found in a comment at the Real World Haskell website: dsuSort decFunc a = map snd (sort (zip (map decFunc a) a)) I have tested both functions with length and sum in ghci. So, do Haskell programmers use the decorate-sort-undecorate pattern? If not, why not? If so, when? Thanks and best wishes Ivan -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan(a)llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================

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GHC compiling for different platforms
by Bernhard Lehnert 04 Jul '09

04 Jul '09

Hi folks! When I compile a program under windows - will the .exe run under any windows? Do I have to distribute only the .exe or more files? When I compile a program under a 64bit-Linux on an AMD-CPU, will the executable run under 32bit and/or Intel-CPU as well? Are there any, say, compiler flags, to improve portability? Thanks, Bernhard

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ghc profiler beginner
by José Prous 30 Jun '09

30 Jun '09

I never used the ghc profiler before, when I try to compile the program with: ghc -prof -auto -O2 -o Main --make Main.hs I get: Could not find module `Data.Binary.IEEE754': Perhaps you haven't installed the profiling libraries for package `data-binary-ieee754-0.3'? Use -v to see a list of the files searched for. Is there a way to tell the profiler to ignore the external libraries or I have to recompile all the libraries with profiling enabled? thanks

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High precision doubles
by Aaron MacDonald 29 Jun '09

29 Jun '09

Is there anything in Haskell that can represent doubles with higher precision than the regular kind? I'm working with formulas that generate sets of values, and values that I know should be zero are ending up as things like -7.105427357601002e-15 or -1.7763568394002505e-14, which is causing all kinds of butterfly effects. I've heard of Rational, but I need to also work with irrational numbers (namely, pi and anything that comes from cos and sin). What other options do I have? - Aaron

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Rigid type variables match error
by Darryn 28 Jun '09

28 Jun '09

Hi, I wonder if anyone can explain what is going on here and what to do about it. I'm fairly new to Haskell, so apologies in advance if my question seems naive. I've cut my code down to a minimum that reproduces the problem. I receive an error in GHCI in the following code, complaining that it cannot match the rigid type variables for the instance definition for Ainst for the function a3. Can anyone advise about what to do about it? ------------------------------------------------ class A a where a1 :: a a2 :: a -> a a3 :: (B b) => b -> a class B b where b1 :: Int -> b data (B b) => Ainst b = I | J (Ainst b) | K b instance (B b) => A (Ainst b) where a1 = I -- a2 :: (B b, A a) => a -> a a2 = J -- a3 :: (B b, A a) => b -> a a3 = K -- Error! -- a3 = K `asTypeOf` a3 -- Error even with this! data Binst = Val Int instance B Binst where b1 = Val ------------------------------------------------ Test5.hs:17:9: Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the instance declaration at Test5.hs:12:12 `b' is a rigid type variable bound by the type signature for `a3' at Test5.hs:5:13 Expected type: b -> Ainst b1 Inferred type: b -> Ainst b In the expression: K `asTypeOf` a3 In the definition of `a3': a3 = K `asTypeOf` a3 Failed, modules loaded: none.

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Re: [Haskell-cafe] What is an "expected type" ...
by Joe Fredette 28 Jun '09

28 Jun '09

When Haskell runs it's type checker, it tries to "guess" the type of each function. Thats why you can write: map (+1) and it knows that you're talking about a function of type: Num a => [a] -> [a] Another thing, called 'defaulting' resolves this, but you didn't ask about that, so I won't go into it. An expected type is one that you provide to the compiler in the form of a type signature, this can be used to specialize a general type (like the one I showed) or to resolve ambiguous types the compiler can't, or just for documentation/good practice. So when I write: foo :: Num a => [a] -> [a] foo ls = map (+1) ls The "expected type" for `foo` is `Num a => [a] -> [a]`. I imagine you're asking this because you got an error which said your expected type doesn't match your inferred type. That might, for instance, happen if I wrote: bar :: String bar = 'a' 'a' has type `Char`, since `String` is not `Char`, the type checker infers that 'a' has type char, but _expects_ it to be type String. Two solutions are as follows: --- Method 1 bar :: Char bar = 'a' --- Method 2 bar :: String bar = "a" Can you see why those two changes fix the problem? Also, just as a matter of process, I forwarded this to the haskell-beginners list, as I imagine type errors like these come up a lot, and someone probably has a better explanation over there. /Joe michael rice wrote: > as opposed to an "inferred type"? > > Michael > > > ------------------------------------------------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Gtk2Hs and Cairo on Windows
by Philippe D.-P. 26 Jun '09

26 Jun '09

Hello, I would like to use Haskell with Gtk2Hs/Cairo for a project, but I have trouble making it work on Windows. I've installed the lastest GHC + Gtk2Hs and I tried to copy/paste the code from examples on this tutorial http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/app1.xhtml. On linux; no problem. But for some reason I get a "not in scope: `eventSent`" error on Windows and I can't compile. How can I solve the problem and, more importantly, is it possible to write portable code with Gtk2Hs/Cairo ? Thank you

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if True than let...
by Bernhard Lehnert 26 Jun '09

26 Jun '09

Hi, I'm sorry because I am absolutely sure, this is bloody obvious to the knowing. Being a total beginner I'm stuck. In the main = do part I wrote: 1: if a == True then putStrLn "Yes!" else putStrLn "No." 2: if a == True then let b = "+" else let b = "-" Line #1 works perfectly well. Read line #2 as pseudocode and you'll see what I want to do. Read it in ghci and it produces " parse error on input `=' " I tried 'case of' but it doesn't work either. What am I doing wrong? Thank you for any help, Bernhard

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on
by Ivan Uemlianin 26 Jun '09

26 Jun '09

Dear All During some recent discussion about different ways of sorting a list by some comparison function, one of the methods suggested was like this: sortByFunc = sortBy (compare `on` func) Now, I can imagine how this works, and I don't want to discuss the efficiency of this implementation (I imagine it's not "Schwartzian", for example;). I know what the backticks do (allow infix use of a function). My question is: what is "on"? I'm afraid I haven't been able to find anything about this, no doubt because of all the false positives coming up in searches. Please could someone tell me its official title, or where I could find it in the docs? Thanks and best wishes Ivan -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan(a)llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================

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