Evaluating expressions in a software environment

Code for a program is received by a compiler (or other entity). The code includes an expression for a variable and a marker that specifies when the expressions should be evaluated during execution of the program. Example markers can indicate that an expression should be evaluated immediately, once or always. Additional functionality is provided to the code in order to evaluate the expression at the specified time during execution of the program. If the expression is to be evaluated always, then the expression will likely be dependent on a item that can change. Functionality is provided that evaluates the expression when the item changes so that the variable can be updated.