- Beginners - Haskell.org

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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Re: [Haskell-beginners] if True than let...
by Bernhard Lehnert 25 Jun '09

25 Jun '09

> Since ifs are an expression in haskell, they return a value, All right, that explains it. Today I learned to think of that expression thing even in do blocks. Thank you to everyone! Cheers, Bernhard

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Re: [xmonad] xmonad vs. full screen
by Thomas Friedrich 25 Jun '09

25 Jun '09

Rickard Nilsson wrote: > Quoting Thomas Friedrich <info(a)suud.de>: >> 2) I would propose going back to the old behavior of handling >> full-screen-programs in xmonad. It really should be the *user* who >> decides how "full-screen" a full-screen-program shall be. Isn't this >> also more following the philosophy of xmonad? You as a user decide, >> what is happening on your screen? It might be not as intuitive, when >> switching to xmonad and you are used to other WMs, where full-screen >> means *full screen*. I really miss that functionality, now that its >> gone. What do you think? > > I think the old behaviour made much more sense. As it is now, it's not > possible to play a web flash movie in a separate, arbitrarily sized, > window. You either have to watch it fullscreen-floating, or integrated > in the web-page. > > @Thomas - Did you ever manage to revert Xmonad to the old behaviour? > No unfortunately I didn't. I changed my manageHook to myManageHookDef = scratchpadManageHook (W.RationalRect 0.25 0.375 0.5 0.35) <+> composeAll [ ... , isFullscreen --> doFullFloat , isDialog --> doCenterFloat ] This way, when I want to watch a Flash-Movie in full screen, the video window actually covers the full screen. However, the downside of it is that also xpdf and others start flouting when going to full screen. Which is stupid, as I always have to push them back into the tiling, when I want to have a "full screen" xpdf next to, say, emacs. Also you will have a little border around the flouting window, which doesn't look good in full screen. But I don't want to remove the borders from flouting windows, as I like to have a border, when a flouting window is not covering the whole screen. So as you see, the solution is far from optimal. Cheers, Thomas

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lhs2TeX
by Thomas Friedrich 24 Jun '09

24 Jun '09

Hi everyone, A Google-search wasn't successful, that's why I am asking here. I want to use the program lhs2TeX. In the manual it says there are different styles that you can use, and some examples how they look like. But they never say how to call them. If I like to see the verbatim-style or the tt-style. What do I include in my document? Cheers, Thomas

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HXT: Gathering text from Nodes
by aditya siram 23 Jun '09

23 Jun '09

Hi all, I'm having trouble using HXT to gather up text from all nodes with a certain name, and return it in a tuple with another node. So given 3 child nodes called "kid" containing "a", "b" and "c", and 1 child node called "name" containing "A", I want to return ("A", ["a","b","c"]). My current code at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=6187#a6187returns [("A","a")("B","b")("C","c")]. Any help is appreciated. -deech

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Re: [Haskell-beginners] Updating lists inside another list
by Lee Duhem 23 Jun '09

23 Jun '09

On Tue, Jun 23, 2009 at 11:29 AM, Joel Neely<joel.neely(a)gmail.com> wrote: > WRT the first case, I would have expected something like; > > addToInnerList [] a b = [(a,[b])] > > on the assumption that one "grows" a structure from an empty state. > Your thoughts? > I think it isn't what Aaron wants, that is "to add an element to the inner list of the pair with the head value of my choosing". Anyway, it is an option. lee

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Updating lists inside another list
by Aaron MacDonald 22 Jun '09

22 Jun '09

I have a list of pairs. Each pair has a value and a list. So, the structure looks like this: [ (a, [b, c]), (b, [a, c]) ] What I want to do is be able to add an element to the inner list of the pair with the head value of my choosing. So, for the above list: > addToInnerList [ (a, [b, c]), (b, [a, c]) ] a d [ (a, [b, c, d]), (b, [a, c]) ]

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