On Fri, Jun 5, 2009 at 2:58 PM, MH <mhamro@gmail.com> wrote:

I actually meant

data Container a = Many a(Container a)

but here is what I don't understand (fyi, I am a beginner) how can you
construct  this container? I can do

I think I saw the above definition described as a coalgebra or codata:

http://blog.sigfpe.com/2007/07/data-and-codata.html

Basically, that list has to always be infinite.

 

let a = Many "somestring" - and I will get back a function but I can not do
let a = Many 'a' "somestring"  - because the second param is not
(Container a) type.
let a = Many 'a' (Container ['a','a']) - doesn't work either because
Container is a type not a constructor (right or I am missing
something?).

So I have two questions:
1. When I do
let b = Many "somestring" , I get
:t b
b :: Container[Char] -> Container[Char]
what is it and why I am allowed to pass just one parameter to Many
(and how can I use it)?

2. How can you construct that container?
data Container a = Many a(Container a)
let a = ????

Check that link above for answers to some of your questions.

I think that basically you want a different type:

data NonEmptyList a = JustOne a | MoreThanOne a (NonEmptyList a)