I think immutable could benefit from a Value Range Propagation-like uniqueness logic:
string a; char[] b; string c = a ~ b; // result of ~ is always unique string z = c.dup; // also any kind of .dup is definitely unique char[] q = z.dup; // also okay -- assign to a non-immutable The lines w/ dup or ~ could be considered legal, since these ops produce definitely unique values. A handful of specific actions known to produce unique values would produce temporaries with a 'uniqueness' property, which allows them to go either way toward mutable or immutable. This list includes (malloc, new, ~, .dup) but maybe others. Once an expression is assigned to a variable it becomes immutable or mutable as per that variable's traits. As an extension variables could be labeled as 'unique' which essentially means they contain a value that is not shared. Unique would only be legal in cases where escape analysis can easily prove that the value does not get copied into both immutable and mutable variables (see below about portability). You could label a function as producing unique values -- only legal if the value returned is definitely unique. Again, if the language cannot do escape analysis then the user will be required to do a defensive dup. Portability note: For portability reasons it is better to have well defined escape analysis rules that can be implemented by any compiler -- so a smart compiler might actually know there is no escape but still be required to forbid a particular expression since it breaks the rules. Kevin
