December 2011 - Beginners

error logging
by Dennis Raddle 20 Dec '11

20 Dec '11

Okay, I want to write code in the Either monad in order to throw errors with logging. I did this a while ago but can't find my code and I forget what I did. Note that I'm using ghc 6.12.3, which I installed via the haskell platform. First question is: do I use Control.Monad.Error or Control.Monad.Either? Second question is: I want to have each monadic function within the stack of functions that is currently running catch the error and add some context to the log. For instance I need to have a function called, let's say, "catchAndAnnotate" that functions like the following. I'm just making this up-- how is it really done? func1 :: SomeType -> Either String SomeType func1 x = ("running in func1 with argument " ++ show x) `catchAndAnnotate` (do y <- somework x when (isBad y) (throwError "oops") return y) somework :: SomeType -> Either String SomeType somework x = ("running in somework with argument ++ show x) `catchAndAnnotate` (whatever x) whatever :: SomeType -> Either String SomeType whatever = ...

2 1

llvm haskell bindings
by Philippe Sismondi 19 Dec '11

19 Dec '11

I am running OS X 10.6.8 with Xcode 4. I have the Haskell Platform installed, ghc v. 7.0.3. I cannot get the llvm haskell bindings to install correctly using cabal. I get this at the tail end of messages from cabal: checking llvm-c/Core.h usability... yes checking llvm-c/Core.h presence... yes checking for llvm-c/Core.h... yes checking llvm/Support/DynamicLibrary.h usability... yes checking llvm/Support/DynamicLibrary.h presence... yes checking for llvm/Support/DynamicLibrary.h... yes checking for LLVMModuleCreateWithName in -lLLVMCore... no checking for LLVMModuleCreateWithName in -lLLVMCore... no configure: error: could not find LLVM C bindings cabal: Error: some packages failed to install: llvm-0.10.0.1 failed during the configure step. The exception was: ExitFailure 1 Has anyone had any luck getting this set up? Googling leads me to some rather terse comments that the Apple llvm installation does not include the required llvm libraries. I hate to install a separate set of llvm stuff with macports, or the llvm repository, because I fear that I might run into all kinds of puzzling conflicts with the Apple stuff. Ideas? - P -

2 1

FFI export lazy list of string
by Alexander.Vladislav.Popov 19 Dec '11

19 Dec '11

Hi, Haskellers. Advise me please, how I can export lazy and potentially infinite list of string from Haskell program. I think I must call it iteratively: the first call initiate some structure and other calls iterate over it, something like pair of function `find_first' and `find_next'. And how to marshall this structure between programs. Or think in a wrong way? Does any example exist how I can make it? Alexander Popov

2 4

Map instances in the list-tries package
by Matthew Moppett 19 Dec '11

19 Dec '11

I want to use the list-tries package for a little hobby project, but my naive idea about how it should work is obviously wrong: module TryPat where import Data.ListTrie.Patricia.Map a = singleton "harry" 99 yields the error: No instance for (Data.ListTrie.Base.Map.Map map0 Char) arising from a use of `singleton' Possible fix: add an instance declaration for (Data.ListTrie.Base.Map.Map map0 Char) In the expression: singleton "harry" 99 In an equation for `a': a = singleton "harry" 99 Now, there is some stuff about this in the docs: The data types are parametrized over the map type they use internally to store the child nodes: this allows extending them to support different kinds of key types or increasing efficiency. Child maps are required to be instances of the Map class in Data.ListTrie.Base.Map<http://hackage.haskell.org/packages/archive/list-tries/0.4.1/doc/html/Data-…>. Some operations additionally require an OrdMap instance. But frankly, I don't understand it, especially the type signatures. Any hints on how I might go about making an instance for Map map0 Char?

2 2

Thanks
by CEO'Riley 19 Dec '11

19 Dec '11

Michael, Benjamin, thanks much. Regards, CEO'Riley Charles E. O'Riley Jr.

1 0

Re: [Haskell-beginners] Installing packages ?
by Michael Xavier 19 Dec '11

19 Dec '11

I would begin by looking at the docs for that package: http://hackage.haskell.org/package/StatisticalMethods-0.0.0.1 In particular, the Modules section. Find the functions you want from the modules that contain them and import them into your program. IE if you wanted to work with a confusion matrix, whatever that is, and your program looks like: module MyModule where ..... You instead do: module MyModule where import Statistics.ConfusionMatrix ... do stuff with the functions in that module ... You may also find this handy: http://haskell.org/haskellwiki/Import_modules_properly and this tutorial: http://learnyouahaskell.com/modules Hope that helps On Sun, Dec 18, 2011 at 9:39 PM, CEO'Riley <ceoriley(a)gmail.com> wrote: > The package was StatisticalMethods. Unfortunately, I’m new at this and > very anal. When you say import it into the project I’m using, I assume you > mean via the Haskell program (WinGHC). If that’s correct, then what nexts? > **** > > ** ** > > ** ** > > ** ** > > Regards,**** > > CEO’Riley**** > > Charles E. O’Riley Jr.**** > > ** ** > > *From:* Michael Xavier [mailto:nemesisdesign@gmail.com] > *Sent:* Sunday, December 18, 2011 11:37 PM > *To:* ceoriley(a)gmail.com > *Cc:* beginners(a)haskell.org > *Subject:* Re: [Haskell-beginners] Installing packages ?**** > > ** ** > > Import it into a project you're using.**** > > ** ** > > What package was it?**** > > On Sun, Dec 18, 2011 at 9:19 PM, CEO'Riley <ceoriley(a)gmail.com> wrote:**** > > **** > > Hello all,**** > > **** > > I recently installed a statistical package using cabal on my Windows > machine. I need to know what to do next in order to get access to the > statistical functionality. I’ve looked but have not been able to fine the > answer. Thanks.**** > > **** > > **** > > **** > > Regards,**** > > CEO’Riley**** > > Charles E. O’Riley Jr.**** > > **** > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners**** > > > > **** > > ** ** > > -- > Michael Xavier > http://www.michaelxavier.net**** > > LinkedIn <http://www.linkedin.com/pub/michael-xavier/13/b02/a26>**** > > ** ** > -- Michael Xavier http://www.michaelxavier.net LinkedIn <http://www.linkedin.com/pub/michael-xavier/13/b02/a26>

1 0

Installing packages ?
by CEO'Riley 19 Dec '11

19 Dec '11

Hello all, I recently installed a statistical package using cabal on my Windows machine. I need to know what to do next in order to get access to the statistical functionality. I've looked but have not been able to fine the answer. Thanks. Regards, CEO'Riley Charles E. O'Riley Jr.

2 1

How to remove some duplication from this code?
by Peter Hall 14 Dec '11

14 Dec '11

I'm following this article about GADTs [http://en.wikibooks.org/wiki/Haskell/GADT] and it suggests, as an exercise, to handle invalid trees at runtime, without GADTs, when evaluating a simple arithmetic syntax tree. My attempt is below. It seems to work, but I could do with some feedback, as it isn't quite satisfactory. It feels like I should be able to remove some of the duplicated code in the eval function, and also in evalIntExpr and evalBoolExpr, which are identical except for having Left and Right reversed. Thanks, Peter -------- Arithmetic.hs module Arithmetic where import Data.Maybe data Expr = I Int | B Bool | Add Expr Expr | Mult Expr Expr | Eq Expr Expr eval :: Expr -> Maybe (Either Bool Int) eval (B b) = return $ Left b eval (I i) = return $ Right i eval (Mult e1 e2) = do a1 <- evalIntExpr e1 a2 <- evalIntExpr e2 return $ Right $ a1 * a2 eval (Add e1 e2) = do a1 <- evalIntExpr e1 a2 <- evalIntExpr e2 return $ Right $ a1 + a2 eval (Eq e1 e2) = do a1 <- evalIntExpr e1 a2 <- evalIntExpr e2 return $ Left $ a1 == a2 evalIntExpr :: Expr -> Maybe Int evalIntExpr e = eval e >>= unwrap where unwrap (Right x) = Just x unwrap (Left x) = Nothing evalBoolExpr :: Expr -> Maybe Bool evalBoolExpr e = eval e >>= unwrap where unwrap (Left x) = Just x unwrap (Right x) = Nothing ------- Main.hs module Main ( main ) where import Arithmetic import Data.Maybe import Data.Either test :: Expr test = Eq (Mult (Add (I 1) (I 2) ) (I 5) ) (I 15) main :: IO () main = do putStrLn $ case eval test of Nothing -> "Invalid expression" Just (Left x) -> show x Just (Right x) -> show x

2 1

Read package name, version from .cabal file
by Shakthi Kannan 14 Dec '11

14 Dec '11

Hi, I am trying to read a .cabal find and print the package name and version. Here is the code snippet: === Code === getName :: FilePath -> IO () getName cabal = do gdesc <- readPackageDescription normal cabal let desc = flattenPackageDescription gdesc final = pkgName $ package desc putStrLn final === END === === Output === Couldn't match expected type `[Char]' with actual type `PackageName' === END === I see that the method 'package' from: http://www.haskell.org/cabal/release/cabal-latest/doc/API/Cabal/Distributio… allows me to return the PackageIdentifier. Using pkgName returns a PackageName. http://www.haskell.org/cabal/release/cabal-latest/doc/API/Cabal/Distributio… But, how can I obtain the string value from it? Appreciate any help in this regard. Thanks! SK -- Shakthi Kannan http://www.shakthimaan.com

2 2

What's the difference between those two solution?
by Haisheng Wu 13 Dec '11

13 Dec '11

Hello, I'm trying to solve Euler problem 104 with the solution "My Solution" below but it takes quite long time therefore I quite. Then I turn to haskell wiki for better solution which work well but I can not figure out why it is better than mine. I'm wondering whether more function call decrease the performance. Could you please help a little? Thank you. *-- | My Solution * main = print $ snd $ head $ dropWhile (\(x,y) -> (not . bothNinePandigit "123456789") x) (zip fibs [1..]) bothNinePandigit digits n = isFirstNinePandigit digits n && isLastNinePandigit digits n isLastNinePandigit digits n = digits == sort (lastDigits 9 n) isFirstNinePandigit digits n = digits == sort (firstDigits 9 n) firstDigits k n = take k (show n) lastDigits k n = show (n `mod` 10^k) fibs = 0 : 1 : zipWith (+) fibs (tail fibs) *-- | From Haskell Wiki * fibs = 1 : 1 : zipWith (+) fibs (tail fibs) isFibPan n = let a = n `mod` 1000000000 b = sort (show a) c = sort $ take 9 $ show n in b == "123456789" && c == "123456789" ex_104 = snd $ head $ dropWhile (\(x,y) -> (not . isFibPan) x) (zip fibs [1..])

2 2