Personally I would use binary trees. One could also define a datatype like this one:

datatype 'a nestl = A of 'a * 'a nestl | B of 'a nestl list * 'a nestl | NIL;

datatype 'a nestl = A of 'a * 'a nestl | B of 'a nestl list * 'a nestl | NIL

A (3, B ([A(2,NIL)], A(4, A (5, NIL))));

val it = A (3, B ([A (2, NIL)], A (4, A (5, NIL)))): int nestl

That's not very elegant though.

Larry Paulson

On 11 Oct 2016, at 15:28, David Topham <dtopham at ohlone.edu> wrote:

I doubt if I am the first person to ask this question, but I have not seen an "answer" in any of the ML books I have read yet...

Is there a way to represent a "set of sets" so that a function can process a query such as " is [2] an element of [ 3 , [2], 4, 5]?"

I know we can easily do a query such as " is 2 an element of [ 3 , 2, 4, 5]?"

I know the problem is that lists cannot have different types, but this is a common, normal query in mathematics so someone must have figured out a way to do it perhaps?

-Dave

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