I ran a very simple experiment gauging the overhead of OCaml's new first-class modules.

let f x = x + 1

let _ =

let acc = ref 0 in

for i = 0 to 10000000 do

acc := !acc + (f 1)

done

module type S = sig val f : int -> int end

module M = struct let f x = x + 1 end

let package = (module M : S)

let _ =

let acc = ref 0 in

for i = 0 to 10000000 do

let module Local = (val package : S) in

acc := !acc + (Local.f 1)

done

let o = object method f x = x + 1 end

let _ =

let acc = ref 0 in

for i = 0 to 10000000 do

acc := !acc + o#f 1

done

# ocamlopt direct.ml -o direct

# ocamlopt first_class_module.ml -o first_class_module

# ocamlopt obj.ml -o obj

# time ./direct

real 0m0.008s

user 0m0.010s

# time ./first_class_module

real 0m0.012s

user 0m0.020s

# time ./obj

real 0m0.058s

user 0m0.060s

So, the overhead of first class modules is slight, especially when compared with method invocation.