There remains the problem of the global variable N, which Harry and Bruce might stumble across by accident. George has to have some handle to get at the statistics counter, and any handle George can use intentionally, Bruce and Harry can use accidentally. One thing that George can do is rename N to MAPCAR-STATISTICS-COUNTER, and warn Bruce and Harry not to use a global variable with that name. This is still better than the original situation — at least now Bruce and Harry need not change their programs, and it is George's responsibility to find a name which does not conflict. {Note Can George do better?}
In the case of RANDOM, where the state information is truly local in that no one wants to access it except its owner, we can combine the use of lexical scoping and of side effects to manipulate a completely hidden state variable. For example, suppose we want several independent pseudo-random number generators, initialized with different seeds. We can make a pseudo-random number generator generator as follows:
(DEFINE (RGEN SEED)
(LAMBDA () (PROGN (SETQ SEED
((LAMBDA (Z) (COND ((> Z 0) Z)
(T (+ Z -32768.))))
(* SEED 899.)))
SEED)))
Each call to RGEN delivers as its value a new pseudo-random number generator which is an instance of the prototype described by the LAMBDA-expression which is the body of RGEN. Each one has a state variable which is its seed. The state of each instance is distinct from that of every other instance. This gives one the power of the own variables of ALGOL 60 without any additional mechanism.
Side Effects in the Interpreter
In order to write a simple interpreter which implements the side effect SETQ, we will postulate the existence of two side effect operators which alter S-expressions:
(RPLACA X Y) and (RPLACD X Y)
return the value of X (which must not be atomic), but as a side effect alters X so that its car or cdr, respectively, is the value of Y. (The introduction of operators which modify S-expressions causes a number of nasty problems, which we will consider presently.) We will use these operators to alter the structure of the environment ENV. We modify EVAL to recognize the SETQ construct (see Figure 11). On seeing "SETQ" in the "operator position" of the expression, EVAL dispatches to EVSETQ after recursively evaluating the value to be assigned. EVSETQ uses LOOKUP to find the effective binding of the variable mentioned in the SETQ. If there is such a binding, RPLACA is used to change the value associated with the
