Possibly I'm being dumb here, but wouldn't it be better to use
a union type, eg:

type num = Int of int | Float of float

type list_of_nums = num list

let print_num = function
    Int i -> print_endline (string_of_int i)
  | Float f -> print_endline (string_of_float f)

let print_nums = List.iter print_num

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://freshmeat.net/users/rwmj
Merjis Ltd. http://www.merjis.com/ - all your business data are belong to you.
C2LIB is a library of basic Perl/STL-like types for C. Vectors, hashes,
trees, string funcs, pool allocator: http://www.annexia.org/freeware/c2lib/

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