Unexpected behaviour of strings initialized with quotes

["Arthur Chargueraud" <cours_caml@france-ioi.org>]

I am surprized by a difference of behaviour between the strings
"bbbb" and (String.make 4 'b').
I have been programming in Caml for a while, and I've always
assumed that the two expression would be rather equivalent.

The idea is that when writing:  let f() = "bbbb"
Any call to function f() will return the same string (ie same pointer).
This is of course not the case when writing:  let f() = String.make 4 'b'

Below is a program where this behaviour is causing a problem:

   code:
         for i = 0 to 3 do
            let s = "bbbb" in
            s.[i] <- 'a';
            Printf.printf "string s is now %s\n" s;
         done;

   outputs:
         string s is now abbb
         string s is now aabb
         string s is now aaab
         string s is now aaaa

What it means is that this code is just equivalent to:

         let s = "bbbb" in
         for i = 0 to 3 do
            s.[i] <- 'a';
            Printf.printf "string s is now %s\n" s;
         done;

which I find really unexpected. I was waiting for:

         string s is now abbb
         string s is now babb
         string s is now bbab
         string s is now bbba

which is what happens when using (String.make 4 'b') inside the loop.


This is not a real problem, since it is not usual to modify
strings initialized with quotes, but I am just wandering
about the reason of such a behaviour...


Arthur Chargueraud