Maybe this will be interesting to somebody - alternative way to define and traverse tree without the help of union types (i call this general tree traversing :-)):
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let rec traverseTree node level =
if node.GetType().Name = "_Cons" then
if node.GetType().ToString().Contains("[System.Object]") then
(unbox node:(obj)list)
|> List.iter (fun x -> traverseTree x (level+1))
elif node.GetType().ToString().Contains("[System.Int32]") then
(unbox node:(int)list)
|> List.iter (fun x -> traverseTree x (level+1))
elif node.GetType().Name = "Int32" then
let un = (unbox node:int)
printfn "%A" ("Level "^string(level)^" Value "^string(un))
else
printfn "%A" "Unexpected data type!"

let tree = box([
box([
box([1]);
box([2;3;4])
]);
box([
5
])
])

traverseTree tree 1

So solution to compose general tree - is to box every list into object.

And in traverse phase - we simply unbox node and check its type - if it

is object - we continue recursively scan tree, if not - we reached

terminal node and need to do something with it :-)

This method can be easily extended for supporting traversing

dynamically more data types - such as tuples/Maps and etc. The only

problem is - really ugly definition of tree with the need to box each

and every list ... So maybe such box`ing can be done automatically by NET, so that LIST`s should be able to accept arbitrary number of lists of arbitrary depth (without manual boxing), instead of now being shown

message "This expression has type 'a list but is here used with type

int" :-)
So I`ll wait for such feature in lists in upcoming F# versions :-):-)