> For now I have setteled for
>   type forest = Forest of forest StringMap.t

This is a very reasonable thing to do.  That, or compile with -rectypes.

> Can you give an example of why rectypes by default is dangerous?

Recursive types don't break type soundness and are handled fine by the
OCaml typechecker -- objects and variants use them in an essential way.

The "danger" is that they cause obviously wrong code to pass
type-checking and receive "impossible" recursive types, so you notice
the problem not at the point of definition of the bad code, but at
point of use.  A simplified example is this:

      let f x = x :: x

where the author of that code really intended

      let f x = x @ x

With -rectypes, the wrong definition (with ::) is accepted with type

val f : ('a list as 'a) -> 'a list = <fun>

and it's only when you try to apply f to a "normal" list that the
problem arises, with a hard-to-understand error message:

f [1;2;3];;
   ^
This expression has type int but is here used with type 'a list as 'a

- Xavier Leroy