August 2011 - Beginners

New style exceptions...
by Manfred Lotz 10 Aug '11

10 Aug '11

Hi there, Exception handling in RWH is called old-style and deprecated. I didn't find a tutorial telling how to do the new style. Here I created two examples both dealing with an exception when creating a directory. I would like to know if the way I did is ok or not. Improvements resp. corrections are most welcome First way: <--------------snip------------------------------------> {-# LANGUAGE ScopedTypeVariables #-} module Main where import System.Exit import System.Directory import Control.Exception import Prelude hiding ( catch, readFile ) main :: IO () main = do handle (\(e :: SomeException) -> print $ show e) $ do createDirectoryIfMissing True "/tmr/a/b/c/d" _ <- exitWith $ ExitFailure 1 return () print "Directory created successfully" <--------------snap------------------------------------> Second way: <--------------snip------------------------------------> module Main where import System.Exit import System.Directory import Control.Exception import Prelude hiding ( catch, readFile ) main :: IO () main = do catch (createDirectoryIfMissing True "/tmr/a/b/c/d") exhandler print "Directory created successfully" where exhandler :: SomeException -> IO () exhandler e = do print e _ <- exitWith $ ExitFailure 1 return () <--------------snap------------------------------------> -- Tnanks, Manfred

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What is the use of Data.List.Key merge
by informationen 09 Aug '11

09 Aug '11

Hi, what is the merge function from the Data.List.Key module useful for? Could you give me some concrete examples? Maybe one could add these examples to the module description? Kind regards Chris

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Re: [Haskell-beginners] making function problem (chapter 6 of Programming in Haskell)
by Roelof Wobben 09 Aug '11

09 Aug '11

---------------------------------------- > Subject: RE: [Haskell-beginners] making function problem (chapter 6 of Programming in Haskell) > From: ds(a)iai.uni-bonn.de > To: rwobben(a)hotmail.com > Date: Tue, 9 Aug 2011 15:33:19 +0200 > > On Tue, 2011-08-09 at 13:15 +0000, Roelof Wobben wrote: > > Lets make a try > > > > > > > > ( ^) :: Int -> Int -> Int > > > > ^ x 0 = 1 > > > > ^ x y = x + x ^ (y-1) > > > nearly perfekt, only syntax for (^) is still not ok. You would have > noticed when compiling. > The "^" is by default infix (like "+"), because its a symbol and not > starting with a letter (the exact specification you can look up in your > book I think, or in the Haskell report). > If you want to use is prefix, like a "normal" function, then you have to > put it in brackets, ie. write (^). > So either write > > (^) x 0 = 1 > (^) x y = x * (x ^ (y-1)) > > or (I think this works as well) > > x ^ 0 = 1 > x ^ y = x * (x ^ (y-1)) > > Please notice also the extra brackets around x ^ (y-1) in my definition. > You may not know the precedence of * and ^. > If you eliminate the brackets, make sure you gave the right priority to > ^, that the program does not calculate (x * x) ^ (y-1). > > And, last but not least a second typo: why do you use "+" in your > definition of the power? > because of this 4 * 3 = 4 + 4 + 4 so the forumala on the second run would be 4 + 4* 3 Roelof

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making function problem (chapter 6 of Programming in Haskell)
by Roelof Wobben 09 Aug '11

09 Aug '11

Hello, After a short holiday I now studying chapter 6 of this book. For the first exercise I have to make the function for ^ for postitive numbers. So I did these steps. Step 1 : Defining the type ^ :: [Int] -> Int Step 2 : enumarate the cases ^ [] = ^ [n, 0] = ^ [n, ns] = Step 3 : Define the simple cases ^ [] = [] ^ [n.0] = 1 Step 4 : Define the other cases ^[n.ns] = n * ^ns Step 5 : generalize and simplify : ^[] = [] ^[-, 0] = 1 So the whole function would be : ^ :: [Int] -> Int ^[] = [] ^[-, 0] = 1 ^ [n,ns] = n * ^ns But the answer in the book only says : m ^ 0 = 1 m (n+1) = m * m ^n so no type defenition and a whole different solution to the last rule. So my question is : Is what I did wrong ? Roelof

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having trouble with helloworld style program
by Sunil S Nandihalli 08 Aug '11

08 Aug '11

Hello everybody, All I want to do in the following code is to read an integer from stdin and print it back to stdout. Can somebody help me fix my code snippet below.. Thanks, Sunil module Main where import Prelude stringToInt::String->Int stringToInt str = read str main = do x<-getLine y<-stringToInt x print y

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Conciseness question
by Manfred Lotz 08 Aug '11

08 Aug '11

Hi all, In Lua I could do something like this: -- initialize empty table P = {} P.a = "bla1" P.b = "bla2" and so on. Now I can refer to each value by name, and I also can easily iterate over the table P. How can I do something similar in Haskell. Note: I do want only write each variable one time (or two times if I count the type definition). I thought about: data P = P { a :: String, b :: String } Then I have one definition pval = P { a = "bla1", b = "bla2" } Now I could refer to each val easily, e.g. a pval. However, I don't see that I could iterate over the members of pval. It there a way to do what I want without defining a list like this? pls p = [ (a p), (b p)] Perhaps a comletely different way? -- Manfred

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Parse string with optional entries [Parsec]
by Stefan.Lukasser-EE＠infineon.com 08 Aug '11

08 Aug '11

Hi all, I'm trying to parse a string in one of the following forms using Parsec: "1\t1\t123\t456" -> desired output: [Just 1, Just 1, Just 123, Just 456] or "1\t\1\t\t456" -> desired output: [Just 1, Just 1, Nothing, Just 456] i.e., the input is a list of numbers, separated by '\t' characters, with the possibility of missing entries. I'm having troubles with the backtracking in case of missing numbers. Can anybody give me a hint? Thanks in advance, Stefan

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Re: [Haskell-beginners] [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by KC 08 Aug '11

08 Aug '11

> As pointed out earlier in this list, the name of the class and the methods > are inconsistent. > Monoid refers to a general algebraic structure, whereas > mempty and mappend refer to certain instances like lists. > Thank you for articulating what I was trying to get at. -- -- Regards, KC

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Identifying general patterns / typeclass instances
by Tim Cowlishaw 08 Aug '11

08 Aug '11

Hi there all, I've come up with the following sketch for a simple agent-based simulation program in Haskell. However, I'd be interested in knowing whether there are any obvious instances of more general typeclasses hidden in there that I haven't recognised! https://gist.github.com/1129216 Some explanation: A simulation is comprised of a starting state, and a set of agents which take in a state and output an updated state. The particular simulation I'm working on has very specific timing semantics in that, for each 'step' of the simulation, all the agents must see the same state (so they cannot simply be composed and applied sequentially), and the resulting states must be applied to create the next state in a well-defined order (hence the monoid restriction on the SimState type). For this reason, I'm having trouble coming up with a monad instance (which would seem to me to imply that the 'agent' functions would be applied sequentially). However, are there any other more general typeclasses that might offer useful abstractions in this case? In addition, the 'building up' of a simulation from an initial state also looks to me like an anamorphism of some kind - is this correct? Many thanks, Tim

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Where to put an experimental package?
by . 07 Aug '11

07 Aug '11

Hi all, I have written a small module with some basic algorithms for graphical models. I intend this for experimentation and want to extend it as I go. It is probably not the most beautiful code, and most probably also not the most efficient, but I also want to change that as I learn more. Now .. if I wanted to put that online so maybe other interested people can find it, where would you go to? Is hackage a good place for such things? Or is hackage for more mature projects only? Thanks a lot, Christian

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