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Indenting
by John Moore 05 Dec '09

05 Dec '09

Hi, I'm a bit stuck. I want to indent my answer by using a count function to indent my answer. Say for example I want to show the levels in a simple arithmethic expression. like 3*(4+5) I want to be able to print out : 4+5 = 9 3*9 = 27 (notice the indentation as I go through an expression. I tried this but it a mess, I tried to use prettyprint but this seemed very complicated. Any suggestions greatly appreciated type Indent = (Int) spacing :: Int -> Indent spacing n = Nothing spacing n (x:xs) = if (n==1) then (Just n) else spacing n xs where (n > 1) John

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"reusable" data in Haskell
by John Velman 04 Dec '09

04 Dec '09

In prolog, one can create a database of facts and predicates, which, depending on the prolog (AFAIK) can be 'compiled' and loaded quickly. How does one do something comparable in Haskell? I guess I have in mind something like a function that accesses the data with a key. And ideally, the data should itself be able to contain functions. I know there is a hash table library, and interfaces to databases like postgresql. If using hash table, is there a way to 'pickle' the hash table, and unpickle it? I'd be happy to have some pointers to someplace that this has been done (or some documentation on how to do it. Thanks, John Velman

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Eq a => [a]->[a]
by I. J. Kennedy 04 Dec '09

04 Dec '09

I am looking for a function f::Eq a => [a]->[a] that takes a list and returns the longest initial segment of the list for which all the elements are distinct. For example f [2,3,6,4,3,5] = [2,3,6,4]. I didn't see anything that matched using Hoogle, but I thought this might be a common enough operation that this function might exists somewhere in the standard packages.

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partitionM
by Tom Tobin 03 Dec '09

03 Dec '09

While working on some filesystem traversal code, I found myself wanting to use the 'partition' function from Data.List, but with the function 'doesDirectoryExist' in System.Directory (with type Filepath -> IO Bool). I noticed that there was no 'partitionM' in Control.Monad, so I set out to write one. Here's what I ended up with: > import Control.Monad (foldM) > partitionMHelper :: Monad m => (a -> m Bool) -> ([a],[a]) -> a -> m ([a],[a]) > partitionMHelper p (ts,fs) x = do > b <- p x > return (if b then (x:ts,fs) else (ts,x:fs)) > partitionM :: (Monad m) => (a -> m Bool) -> [a] -> m ([a], [a]) > partitionM p xs = foldM (partitionMHelper p) ([],[]) $ reverse xs This works for my trivial cases, but can fail with extremely large lists (not to mention being unable to take an infinite list and work properly when passed to something like 'take 5'). Is there a way to change the function to avoid having to traverse to the end of the list (via reverse) just to get the output in the proper order? I started trying to write a "foldrM", but haven't gotten anywhere useful yet. Can someone point me in the right direction?

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fail: wxhaskell with cabal
by Josh Bronson 03 Dec '09

03 Dec '09

Greetings Haskellers, Happy Thanksgiving! I'm using Ubuntu 9.10. I've performed the following steps: # apt-get install ghc6 I then successfully did a local install of the Haskell platform in ~/mine/install Now I am trying to get wxhaskell to install: $ cabal install wx [snip] Configuring wxcore-0.12.1.2... setup: Missing dependencies on foreign libraries: * Missing C libraries: wx_baseu-2.8, wx_baseu_net-2.8, wx_baseu_xml-2.8 This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. cabal: Error: some packages failed to install: wx-0.12.1.2 depends on wxcore-0.12.1.2 which failed to install. wxcore-0.12.1.2 failed during the configure step. The exception was: exit: ExitFailure 1 $ ls /usr/lib/libwx* /usr/lib/libwx_baseu-2.8.so /usr/lib/libwx_baseu_net-2.8.so.0.6.0 /usr/lib/libwx_baseu-2.8.so.0 /usr/lib/libwx_baseu_xml-2.8.so /usr/lib/libwx_baseu-2.8.so.0.6.0 /usr/lib/libwx_baseu_xml-2.8.so.0 /usr/lib/libwx_baseu_net-2.8.so /usr/lib/libwx_baseu_xml-2.8.so.0.6.0 /usr/lib/libwx_baseu_net-2.8.so.0 It looks like the libraries are there to me! any ideas?

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From Functional Dependencies to Type Families
by Adolfo Builes 03 Dec '09

03 Dec '09

> {-# OPTIONS -fglasgow-exts #-} > > module VarArg where > import Data.FiniteMap -- for an example below > > class BuildList a r | r-> a where > build' :: [a] -> a -> r > > instance BuildList a [a] where > build' l x = reverse$ x:l > > instance BuildList a r => BuildList a (a->r) where > build' l x y = build'(x:l) y > > --build :: forall r a. (BuildList a r) => a -> r > build x = build' [] x I'm trying to replace the code below to work with type families, I started out replacing the definition of class with : class BuildList r where type Build r build' :: [Build r] -> Build r -> r follow by the instance for [a] resulting in instance BuildList [a] where type Build [a] = a build' l x = reverse $ x:l Until here, everything is working, and I'm able to do > build' [2,3,4] 1 :: [Integer] > [4,3,2,1] then I move on to the next instance (a -> r) with instance BuildList r => BuildList (a-> r) where type Build (a -> r) = a build' l x = \ y -> build'(x:l) y But I get the following error Couldn't match expected type `Build r' against inferred type `a' `a' is a rigid type variable bound by the instance declaration at /home/adolfo/foo.hs:347:35 In the first argument of `(:)', namely `x' In the first argument of `build'', namely `(x : l)' In the expression: build' (x : l) y then I try with : instance BuildList r => BuildList (a-> r) where type Build (a -> r) = Build r build' l x = \ y -> build'(x:l) y And I get Couldn't match expected type `a' against inferred type `Build r' `a' is a rigid type variable bound by the instance declaration at /home/adolfo/foo.hs:347:35 Expected type: [Build r] Inferred type: [Build (a -> r)] In the second argument of `(:)', namely `l' In the first argument of `build'', namely `(x : l)' I have been trying to figure out, which type should it be, but I haven't found the correct one, any ideas ? Thanks - Adolfo Builes

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how to define records containing finite length lists.
by Srikanth K 02 Dec '09

02 Dec '09

Hi, I want to define a record say "Student". The 'C' equvivalent for the same could be struct Person { char name[10] int age } The closest I can find for doing such a thing in haskell appears to be data Person = Person{ name :: [char] age :: Int } I have not yet been able to find a suitable way to specify the constraint on the length of list(name in this case). So can someone let me know how we can impose this length constratint on the list and derive a new type. If standard haskell doesn't give this flexibility, are there any extensions in ghc to achieve the same. - Srikanth

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Typeclass question
by Patrick LeBoutillier 01 Dec '09

01 Dec '09

Hi all, I want to make all types that are instances of Integral instances of another type class, i.e: {-# LANGUAGE FlexibleInstances #-} class Intable a where toInt :: a -> Int instance (Integral a) => Intable a where toInt l = fromIntegral l However this gives the following error that I don't understand: q.hs:6:0: Constraint is no smaller than the instance head in the constraint: Integral a (Use -XUndecidableInstances to permit this) In the instance declaration for `Intable a' Can anyone offer insight on thie error? Also, is there perhaps a different way to do what I want to do? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Tips for a FFI-wrapped C library?
by Patrick LeBoutillier 01 Dec '09

01 Dec '09

Hi all, I have wrapped, using FFI, a C-library I am writing that calls into different programming languages. I put everything in the IO monad. So far it works as expected but I find the resulting usage code somewhat ugly (especially the bits musing 'sequence'). I tried using finalizers to delete the resources but it couldn't respect my dependencies. Can anyone offer any advice on making it feel more natural in Haskell? Note: The FFI code is available here: http://svn.solucorp.qc.ca/repos/solucorp/mlp/trunk/lib/haskell/ import qualified MLP import qualified MLP.Error import qualified MLP.Context import qualified MLP.Language import qualified MLP.Runtime import qualified MLP.Value import qualified MLP.Object main = do ctx <- MLP.Context.new -- create a context MLP.Context.with ctx $ do lang <- MLP.Language.init ctx "perl" -- init a language, i.e. perl rt <- MLP.Runtime.new ctx lang -- create a runtime for the language MLP.Runtime.loadLibrary ctx rt "calc" -- load a library, i.e. calc.pm val <- MLP.Object.new ctx rt "calc" [] "" -- create a calc object, i.e. my $obj = new calc() ; obj <- MLP.Value.takeObject val -- extract the object from the value MLP.Value.delete ctx val args_add <- sequence [MLP.Value.newLong 2, MLP.Value.newLong 3] args_mul <- sequence [MLP.Value.newDouble 3.5, MLP.Value.newDouble 4.6] val_add <- MLP.Object.callMethod ctx rt obj "add" args_add "" sequence $ map (MLP.Value.delete ctx) args_add val_mul <- MLP.Object.callMethod ctx rt obj "mul" args_mul "" sequence $ map (MLP.Value.delete ctx) args_mul MLP.Object.delete ctx obj MLP.Value.getLong val_add >>= putStrLn . show >> MLP.Value.delete ctx val_add MLP.Value.getDouble val_mul >>= putStrLn . show >> MLP.Value.delete ctx val_mul MLP.Runtime.delete ctx rt MLP.Language.delete ctx lang Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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recommended way to get haskell 6.10 installed on ubuntu hardy?
by Joe Van Dyk 30 Nov '09

30 Nov '09

There's no ubuntu package for 6.10 (only 6.8), and the haskell platform says you need 6.10 installed to build the source package. -- Joe Van Dyk http://fixieconsulting.com

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