Depending on your needs and your comfort level with fancier types, the existential approach to ADTs might solve your problem. The following code is a demonstration you can cut-and-paste-and-run. This is example akin to upcasting in Java to an interface that lets you print things. That way you know how to print every object (or do whatever else it is you need to do) in the list. Beware: there is no safe downcasting (that's what Typeable would be for); that would likely be more than you need. There are other ways to do this with existentials (e.g. bounded existentials), but this is what came out of my head when I read your post. Existentials seems to be the "Haskellish" way to reduce a hetergenous list to a collection of objects with common operations. HList would be the Haskellish way for more static and flexible assurances. {-# OPTIONS -fglasgow-exts #-} module Test where data PrintPackage = forall a . PrintPackage a (a -> String) instance Show PrintPackage where show (PrintPackage val showMethod) = showMethod val list = [ PrintPackage 3 show , PrintPackage "string" show , PrintPackage 3.4 show ] main = print list Hope that helps more than it confuses, Nick On 11/17/06, Henning Thielemann wrote:

On Fri, 17 Nov 2006, Valentin Gjorgjioski wrote:

...

Is some kind of collection of object with different types in Haskell exist? Except the tuples, which have fixed length. I find this

* Tuples heterogeneous, lists homogeneous. * Tuples have a fixed length, or at least their length is encoded in their type. That is, two tuples with different lengths will have different types. * Tuples always finite.

But I need something which is heterogeneous and non-fixed length. I'm used do Java, and this switch to functional languages is very strange to me. So, to be clear, I need something like LinkedList<Object> in java.

Can you please help me or suggest me, what can I use in this case?

If the number of types to cover is fixed, then I suggest a data type like

data T = ConsInt Int | ConsString String | ConsChar Char

Since this is a very FAQ I wonder why I don't find the answer in any of the Haskell wikis. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

On 17.11.2006 19:08 Donn Cave wrote:

On Fri, 17 Nov 2006, Valentin Gjorgjioski wrote: ...

...

But I need something which is heterogeneous and non-fixed length. I'm used do Java, and this switch to functional languages is very strange to me. So, to be clear, I need something like LinkedList<Object> in java.

I looked in a couple of on-line tutorials, thinking I would quickly find something about this, but -- maybe in my haste I missed something. Objective CAML documentation gets right to it, I wonder if we're assuming prior exposure to strongly typed functional languages?

Anyway, you need to define a type that accounts for the types you want to support -

data Object = IntObject Int | StringObject String

objectString :: Object -> String objectString (IntObject v) = show v objectString (StringObject v) = v

main = mapM (putStrLn . objectString) [(IntObject 7), (StringObject "eight")]

Donn Cave, donn@drizzle.com

Thanks to all try to help me. I like the solution from Donn and Henning. I obviously not read enough. The solution is also in Yet Another Haskell Tutorial. Ok, not this much explicit, but its there. It says We have seen an example of the data type with one constructor: Pair. It is also possible (and extremely useful) to have multiple constructors(I'm shame that I even didn't know this). It's interesting that it's said EXTREMELY useful. And yes, it is! So, I think that Henning idea is good and this should go on some haskell wiki - s. Once again thanks to all for quick and nice solution.

i have cut-and-pasted the last few postings into the wiki: http://haskell.org/haskellwiki/Heterogenous_collections it'is very ad hoc, but i hope it helps those who bump into it a little. and it's easier to edit and improve than a mailing list thread. (-: matthias

On Mon, Nov 20, 2006 at 02:13:30PM +0100, Henning Thielemann wrote:

To: Matthias Fischmann Cc: haskell-cafe@haskell.org From: Henning Thielemann Date: Mon, 20 Nov 2006 14:13:30 +0100 (CET) Subject: Re: [Haskell-cafe] Collection of objects?

On Fri, 17 Nov 2006, Matthias Fischmann wrote:

...

i have cut-and-pasted the last few postings into the wiki:

http://haskell.org/haskellwiki/Heterogenous_collections

it'is very ad hoc, but i hope it helps those who bump into it a little. and it's easier to edit and improve than a mailing list thread. (-:

Thanks a lot for setting it up!

Maybe there should be a FAQ category? This page would certainly belong to it.

Sure, makes sense to me. (Not that it has to. ;) I just haven't done it because I don't know very much about wiki categories. I am also currently a little distracted. But hey, it's wiki! By all means do stuff to it. cheers, matthias

On Fri, 17 Nov 2006 20:04:22 +0100, Steve Schafer wrote:

myList = [(Integer 42), (Character 'a'), (Float 3.14159)]

You can leave out the parentheses, making even simpler: myList = [Integer 42, Character 'a', Float 3.14159] -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ -- Using Opera's revolutionary e-mail client: https://secure.bmtmicro.com/opera/buy-opera.html?AID=789433