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Steele and Sussman
67
The Art of the Interpreter
{Value Quibble} Page 8
"Did he ever return?
No, he never returned,
And his fate is still unlearned..."
— The Man Who Never Returned
(Charlie on the MTA)
We said "EVAL's purpose is to determine the values of expressions". But what is the value of the expression (DRIVER)? It is certainly not an illegal or useless expression to evaluate, yet it has no value. The purpose of the expression is to cause a certain process to be evolved; it is an "infinite loop", which never returns. This process includes side effects (READ and PRINT) through which it interacts with the user. This situation arises because the system of interest is broken into two parts with independent state: the computer and the user. We will have more to say about this later.
