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fixpoint operator for ULC with specific reduction property
by ben 04 Nov '10

04 Nov '10

Hello, I'm looking for a fixpoint operator Z for the untyped lambda calculus that has the property Z g --> g (Z g) for any g. The best-known fixpoint operator Y := \f. (\x. f (x x)) (\x. f (x x)) does not have this property, but only Y g --> T <-- g (Y g) for some T. Does anybody know such a Z I'm looking for? Greetings, ben ps. Please keep me in CC, since I'm not currently subscribed to the list.

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Case statements shorter
by Angel Alonso 02 Nov '10

02 Nov '10

Hi, I have the following piece of code: n = 3 case n of 3 -> "Something" 4 -> "Something" _ -> "Other" Is it possible to shorten the case statement into something like this?: n = 3 case n of 3 or 4 -> "Something" _ -> "Other" Thanks. -- Angel Alonso

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Re: [Haskell-beginners] Case statements shorter
by uu1101＠gmail.com 02 Nov '10

02 Nov '10

Nice distinction! Thank you for the correction. Il giorno 02/nov/2010, alle ore 09.11, Roel van Dijk <vandijk.roel(a)gmail.com > ha scritto: > On Mon, Nov 1, 2010 at 7:23 PM, uu1101(a)gmail.com <uu1101(a)gmail.com> > wrote: >> * _ is not a special keyword in haskell. It's a regular variable so >> will >> always match. It's usually used when you don't use it's value in >> the right >> hand side or when it adds no information. In this case its value >> will be n >> so there is no reason to introduce another name. > > This is incorrect. _ is a pattern not a variable. > > I can write > > f (_:_) = "bla" > > but not > > f (x:x) = "bla" > Conflicting definitions for `x' In the definition of `f' > > So _ is a pattern that always matches but is never bound to a value. > > Regards, > Roel

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How to Install Gtk2Hs on Windows?
by John Smith 01 Nov '10

01 Nov '10

I tried following the directions on http://jystic.com/2010/10/20/installing-gtk2hs-on-windows/, but when I run cabal install gtk, I get * Missing C libraries: z, cairo * Missing C libraries: gobject-2.0, glib-2.0, intl I seem to have everything asked for in the PATH. pkg-config reports that Gtk+ 2.16.2 is installed.

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error when unregistering a package
by Ashish Agarwal 28 Oct '10

28 Oct '10

I always have trouble with conflicting packages when I do a fresh install of the Haskell Platform on an Ubuntu machine. Last time, I unregistered lots of packages, and let the necessary ones get recompiled. That fixed things. However, this time I'm having trouble with the unregister step. For many packages I get an error about a file not existing, e.g. $ sudo ghc-pkg unregister QuickCheck ghc-pkg: /var/lib/ghc-6.12.1/package.conf.d/QuickCheck-2.1.0.3-d4ae7d01dc2504e1785e18cd555e91ed.conf: removeLink: does not exist (No such file or directory) However, for many other packages, it works fine. Anyone know how to unregister a package when you get this error?

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Help-me read file
by Gilmara Pompelli 28 Oct '10

28 Oct '10

Hello I have a file with 100 lists, with 100 ints. I have to read the file and apply the map and sort functions on lists. II did it to read file: learquivo :: FilePath -> IO ([[Int]]) learquivo s = do conteudo <- readFile s return (read conteudo) But now applying the sort, the error appears: <interactive>:1:5: Couldn't match expected type `[a]' against inferred type `IO [[Int]]' how can I fix? Thanks

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Best practice for catching pure errors in IO?
by John Smith 27 Oct '10

27 Oct '10

I have a pure value which may throw errors when it is evaluated in an IO monad. My code to handle it looks like this: temp <- try $ evaluate value case temp of Left (error::ErrorCall) -> scoldTheUserAndTryAgain Right correct -> doSomething Can this be done without the temporary variable, or with less plumbing? I tried lifting either, but GHC was fussing about ambiguous types, appending a signature to a pattern match is no longer allowed(?), and it didn't look that much cleaner.

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read
by Russ Abbott 27 Oct '10

27 Oct '10

Hi, I'm having a problem with the read function. I thought it took a string and converted it into a value (if there is a conversion). But it doesn't seem to work for me on GHCi Prelude> read "123" *** Exception: Prelude.read: no parse Prelude> read (show 123) *** Exception: Prelude.read: no parse There must be something trivial that I'm missing. Thanks. *-- Russ Abbott*

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Yampa
by Luca Ciciriello 27 Oct '10

27 Oct '10

Hi All. I've started to use Haskell regularly in my job in industry. My job is to develop software for UAVs (Unmanned Aerial Vehicle). In particular I want to use Yampa to handle the sensor gimbal of the vehicle. Is there someone with some kind of experience in the industrial use of Yampa? Where can I find (if exists...) documentation to guide me in using Yampa in the real world? Thanks for any answer. Luca.

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Need help with code duplication (types and typeclasses)
by Edward Turpin 26 Oct '10

26 Oct '10

I can't seem to get my head around types and typeclasses (as well as many other things in Haskell). Here's the following code I need help with. In module 1: //////////////////////// Start of Code ////////////////////////////// data NewtonRow = NewtonRow { iteration :: Integer, xn :: Double, fxn :: Double, errEst :: Double } instance Show NewtonRow where show row = show (iteration row) ++ "\t" ++ showEFloat (Just 8) (xn row) "\t" ++ showEFloat (Just 2) (fxn row) "\t" ++ showEFloat (Just 2) (errEst row) "\n" newtype NewtonTable = NewtonTable { getNewtonTable :: [NewtonRow] } instance Show NewtonTable where show table = "n\t" ++ "xn\t\t\t" ++ "fxn\t\t" ++ "xn - xn-1\n" ++ concatMap show (getNewtonTable table) instance Monoid NewtonTable where mempty = NewtonTable [] table1 `mappend` table2 = NewtonTable $ (getNewtonTable table1) ++ (getNewtonTable table2) cons :: NewtonRow -> NewtonTable -> NewtonTable row `cons` table = NewtonTable (row : getNewtonTable table) ////////////////////// End of Code //////////////////////////////// In module 2: ///////////////// Start of more Code /////////////////////////////// data BisectRow = BisectRow { iteration :: Integer, start :: Double, end :: Double, middle :: Double, deviation :: Double, fm :: Double } instance Show BisectRow where show row = show (iteration row) ++ "\t" ++ showEFloat (Just 4) (start row) "\t" ++ showEFloat (Just 4) (end row) "\t" ++ showEFloat (Just 4) (middle row) "\t" ++ showEFloat (Just 4) (deviation row) "\t" ++ showEFloat (Just 4) (fm row) "\n" newtype BisectTable = BisectTable { getBisectTable :: [BisectRow] } instance Show BisectTable where show table = "n\t" ++ "start\t\t" ++ "end\t\t" ++ "middle\t" ++ "error\t\t" ++ "f(middle)\n" ++ concatMap show (getBisectTable table) instance Monoid BisectTable where mempty = BisectTable [] table1 `mappend` table2 = BisectTable $ (getBisectTable table1) ++ (getBisectTable table2) cons :: BisectRow -> BisectTable -> BisectTable row `cons` table = BisectTable (row : getBisectTable table) ////////////////////// End of code ///////////////////////////////// There is obviously duplication going on, but I can't figure out how to get rid of it. "BisectTable" is essentially the same as "NewtonTable" except that it has a different number of columns, different column headers (in the "show" functions), and different row elements. "BisectRow" and "NewtonRow" are also somewhat similar. And the "cons" functions in each module pretty much do the same thing. I'm just trying to learn Haskell, so as far as I know I might be doing ass-backwards. Can anybody give any suggestions?

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