1. A computer-implemented method for evaluating a program instruction within the stack evaluator, the method comprising: generating a call frame associated with the program instruction within a call stack, wherein the call frame includes a first expression specified by the program instruction; generating a child frame associated with the first expression within the call stack; receiving a modification to the child frame that sets the value of the first expression within the child frame; and processing the call frame based at least in part on the value of the first expression within the child frame.