Here what I need to do:

module type FRAGMENT = sig
   type foo (* Closed polymorphic union type *)
   val do_something : foo -> whatever
end

module Merge_fragments (F1:FRAGMENT) (F2:FRAGMENT) : #FRAGMENT = struct
   type foo = [ F1.foo | F2.foo ]
   let do_something = function
     | #F1.foo as f1 -> F1.do_something f1
     | #F2.foo as f2 -> F2.do_something f2
end

Up to now I have used automatic code generation to actually write the ML 
file which implements the functor istantiation. Could such an extension 
be made to Ocaml type system?

Alex

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