15. A system for evaluating an expression, the system comprising: a processor; a memory in electrical communication with the processor; a debugger executing in the processor that performs an initial evaluation of the expression with a setting to enable automatic reevaluation of all expressions within a first time period of a corresponding first evaluation of the expressions, detects an action that causes a value of the expression to become stale, wherein the following actions cause the expression to become stale: evaluating another expression, switching threads, changing a process, switching call stack frames or when a user steps through a line of code in the debugger and, in response, setting the debugger to disable automatic reevaluation of selected expressions, decreases a time period during which reevaluation of expressions must be completed to a second time period shorter than the first time period, and commands an expression evaluator to evaluate the expression; and the expression evaluator evaluates the expression.
15. A system for evaluating an expression, the system comprising: a processor; a memory in electrical communication with the processor; a debugger executing in the processor that performs an initial evaluation of the expression with a setting to enable automatic reevaluation of all expressions within a first time period of a corresponding first evaluation of the expressions, detects an action that causes a value of the expression to become stale, wherein the following actions cause the expression to become stale: evaluating another expression, switching threads, changing a process, switching call stack frames or when a user steps through a line of code in the debugger and, in response, setting the debugger to disable automatic reevaluation of selected expressions, decreases a time period during which reevaluation of expressions must be completed to a second time period shorter than the first time period, and commands an expression evaluator to evaluate the expression; and the expression evaluator evaluates the expression. 17. The system of claim 15 , wherein the selected expressions are explicit and implicit expressions.