Simon Helsen had given an explication about weak types and the reason their
existence, but there is still something i can't understand : if we do :

let toto y =
 let id x = x in
id id y ;;

this will be of type 'a ->'a
What is the difference with this :

let toto = let id x = x in id id ;;

I thought caml will understand that there is no difference !

why does caml make this difference ?