Strange behavior of IO in Ocaml. It firstly reads value for scanf, and after that printf all stuff (even those, which it should print before scanf). test_rw2 contains even simpler example.

test_rw.ml

type ast 	= Cons of ast * ast
			| Write
			| Read

open Printf
open Scanf


let myVar = ref 0
let set_myVar x =
	myVar := x

let rec interpret ast = match ast with
	| Cons(s1, s2) 	->
		interpret s1;
		interpret s2;
	| Write			-> printf "myVar = %d\n" !myVar
	| Read			->
		begin
			printf "? myVar = ";
			scanf "%d" (set_myVar);
		end

let _ = interpret (Cons(Write,Cons(Read,Write)))

test_rw2.ml

open Printf
open Scanf

let myVar = ref 0
let set_myVar x =
	myVar := x

let _ = 
	begin
		printf "myVar = %d\n" !myVar;
		printf "? myVar = ";
		scanf "%d" (set_myVar);
		printf "myVar = %d\n" !myVar;
	end