MH writes:

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

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.

Right, so you need to pass a 'Container a' for that parameter.

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?).

It doesn't work because the Container parameter must be of the same type. The parameters here are Char (due to the 'a' as the first parameter to Many), and [Char] (due to the ['a','a'] given as parameter to the Container parameter. These are not the same, so it fails.

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)?

This is just partial application. Pass one more parameter to build the whole thing.

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

Well you need a 'Container a' to do it. So you can do for instance: let a = Container 'x' a or let b = Conainer 'y' undefined -k -- If I haven't seen further, it is by standing in the footprints of giants

On Fri, Jun 5, 2009 at 4:13 PM, Jason Dagit wrote:

On Fri, Jun 5, 2009 at 2:58 PM, MH 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)

I am still not sure why so much attention is being given to the recursive version. I find the most elegant and useful to be this simple one: data NonEmpty a = NonEmpty a [a] With the obligatory conversions: toList (NonEmpty x xs) = x:xs fromList [] = Nothing fromList (x:xs) = Just (NonEmpty x xs) Luke