April 2010 - Beginners

Re: Space leak in a fold even after the usual fixes
by Travis Erdman 13 Apr '10

13 Apr '10

> I take it that size of the array does not depend on arg ? > Apparently, despite your attempts to force evaluation, the array still > contains many unevaluated expressions. Documentation for the vector > pacakge reveals that Data.Vector is a *boxed* array, i.e. the entries > are only evaluated on demand, so that's where the unevaluated > expressions linger around. > Unless you need the extra laziness - which you probably don't, given how > you've structured your algorithm - you might want to switch to > Data.Vector.Unboxed or Data.Vector.Storable. Correct. The array size doesn't depend on arg (arg only controls how much of the input list to process. I did not realize that the boxed arrays would have this extra-lazy behavior. The reason I chose boxed is because I needed to store these very large trees (custom data type) that I referenced in an earlier question. Since D.V.Unboxed and Storable don't appear to support non-standard data types, is there another package that might better fit my needs? Data.IntMap perhaps? I originally chose an array over a map because I need fast random access to individual elements (and cheap modification). Thanks again, Travis Erdman

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Space leak in a fold even after the usual fixes
by Travis Erdman 12 Apr '10

12 Apr '10

The driver of my algorithm looks like this: foldl' processfn nullarray (take arg infinitelist) where processfn takes an array and the next set of inputs, processes, and delivers a new updated array (using the Data.Vector library). Apparently, I have a space leak ... the "maximum residency" varies linearly with the size of "arg" supplied, garbage collection consumes ~75% of CPU time, and, if the arg is too big, the whole thing crashes with an out of memory error. The algorithm should operate in constant space. As you can see, I'm using a strict left-fold and also have made the accumulating array strict in the processfn definition with a bang pattern. So, I'm sort of at a loss as to how to resolve this. The help provided on this list has been outstanding, thanks to all of you; hope you have something left in the tank for this one!

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hmatrix, and Float
by MAN 12 Apr '10

12 Apr '10

I noticed that hmatrix defines the auxiliary type 'Element', and provides instances for Double and Complex, but not Float. Then Vector and Matrix are defined in terms of Element... is there any reason why Float is intentionally left out. I'd like to use Numeric.LinearAlgebra with both single and double precision so as to compare results and times... could I?

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When to use ByteString rather than [Char] ... ?
by James Fisher 11 Apr '10

11 Apr '10

Hi, After working through a few Haskell tutorials, I've come across numerous recommendations to use the Data.ByteString library rather than standard [Char], for reasons of "performance". I'm having trouble swallowing this -- presumably the standard String is default for good reasons. Nothing has answered this question: in what case is it better to use [Char]? Could anyone point me to a good resource showing the differences between how [Char] and ByteString are implemented, and giving good a heuristic for me to decide which is better in any one case? Best, James Fisher

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Doubt in let expression
by Lakshmi Narasimhan 11 Apr '10

11 Apr '10

Hello I am going through Walder's "Monads for Functional Programming" document. On page 9, there is a piece of code that provides the definition of return and bind for the State Monad I have pasted two variations of the code. The first one matches the definition given in the book. The second one is what I came up with. -- State Monad variation -- Retain the eval function as such -- Modify M , unit, * definitions type M a = State->(a, State) type State = Int unit :: a -> M a unit a = (\e->(a,e)) star :: M a -> (a -> M b) -> M b m `star` k = (\x -> let (a, aState) = m x in let (b, bState) = k a aState in (b, bState)) -- State Monad variation -- Retain the eval function as such -- Modify M , unit, * definitions type M a = State->(a, State) type State = Int unit :: a -> M a unit a = (\e->(a,e)) star :: M a -> (a -> M b) -> M b m `star` k = (\x -> let (a, aState) = m x (b, bState) = k a aState in (b, bState)) In the first definition of star, there are two let statements, while I found that one let statement would do the work anyway, as done in the second defintion of start I am not able to understand if there is any semantic implication in this syntatic variation of let. Please help. Thanks -- Regards Lakshmi Narasimhan T V

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haskellDB DBDirect on Oracle question
by MH 09 Apr '10

09 Apr '10

I am trying to use dbdirect-hdbc-odbc to generate database description modules and I am getting the following error: dbdirect-hdbc-odbc: Prelude.(!!): index too large I am running the following command, where "test" is the name of the Oracle schema: dbdirect-hdbc-odbc "test" "dsn=ORACLE_DSN;UID=userName;PWD=password" Do you know what could be the problem and how can I work around it? Thanks, MH

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type class for custom numeric types
by Ashish Agarwal 09 Apr '10

09 Apr '10

I'd like to define an abstract type T, and some how make the following work: > 3 :: T -- allow integer literals to be read as values of type T > 100 :: T error: value out of bounds for type T -- place some restrictions on which integers are valid T values To get the first feature, is my only option to make T an instance of the Num class? If so, I'd have to make fromIntegral return error for out of bounds values. Is there any problem in doing that? The only law I find mentioned for instances of Num is: abs x * signum x == x. Is this the complete requirement?

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private types
by Ashish Agarwal 08 Apr '10

08 Apr '10

Is there a Haskell analogue to OCaml's private types? In OCaml, you can do this (converting to pseudo-Haskell syntax): type T = private A Int Then, A cannot be used directly to construct values of type T. A 3 Error: Cannot create values of the private type T You must provide another constructor, makeT, which takes an Int and returns a T. This allows you to do additional checks on the Int argument. However, you can still deconstruct values of type T. f (A x) = x + 2 This works even when f is defined in a different module. The benefit is that you can restrict the allowed values of T, but still have the convenience of using existing operations on these values once constructed. The solution that comes to mind is to make T abstract and an instance of some appropriate class, but there is no class that lets you pattern match on arbitrary variant types.

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Encoding strings
by legajid 08 Apr '10

08 Apr '10

Hi, i experiment problems writing files. Reading an access 97 database and displaying data in a wxhaskell grid, everything is correct. When i write data in a text file, some characters are translated : "é" becomes \233. How to correct this ? I looked for hSetEncoding but didn't find it in System.IO, nor in GHC.IO.Handle that doesn't exist on my system. I tried hSetBinaryMode, without success. If this helps, I run ghc 6.10.4 on windows XP. Thanks for an idea, Didier.

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Re: [Haskell-beginners] Map on Nested Lists
by Tim Sears 08 Apr '10

08 Apr '10

If you don't mind choosing a different data type (always a good question to ask yourself in haskell) you could simply use your own nested lists.... data Nested a = One a | Nest [Nested a] deriving Show instance Functor Nested where fmap f (One x) = One (f x) fmap f (Nest xs) = Nest (map (fmap f) xs) mapN :: (a -> b) -> Nested a -> Nested b mapN = fmap test = fmap (+1) $ Nest [One 1, Nest [ One 2, One 3], One 4 , Nest [ Nest [One 6]] ] --yields test = Nest [One 2,Nest [One 3,One 4],One 5,Nest [Nest [One 7]]] On Apr 8, 2010, at 9:05 AM, beginners-request(a)haskell.org wrote: > Re: [Haskell-beginners] Map on Nested Lists

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