1. A computer-based method of accessing a closure variable defined in a function of a computer program from a lambda expression that is invoked from a computer program location outside the function while an instance of the function is active, the variable stored in a frame on a runtime stack, the frame corresponding to the function instance, comprising: a) in response to entering the function during program execution, storing a pointer to the variable in a data structure; b) in response to an instruction to retrieve a value of the variable, the instruction in the lambda expression, employing the variable pointer to locate the variable in the frame on the runtime stack and retrieve the variable value.