Page:AIM-453.djvu/9

 This doesn't work. The occurrence of "FOO" does not refer to the atomic symbol FOO as a constant; it is treated as a variable, just as "X" is.

The essential problem is that we want to be able to write any S-expression as a constant in a program, but some S-expressions must be used to represent other things, such as variables and procedure invocations. To solve this problem we invent a new notation: (QUOTE x) in a program represents the constant S-expression x. {Note QUOTE Mapping} Thus we can write our test as "(EQ X (QUOTE FOO))". Similarly,

constructs a list from the values of Y and Z, and compares the result to the value of X, while

compares the value of X to the constant S-expression "(LIST Y Z)". Because the QUOTE construction is used so frequently in LISP, we use an abbreviated notation: "'FOO" is equivalent to "(QUOTE FOO)". This is only a notational convenience; the two notations denote the same S-expression. (S-expressions are not character strings, but data objects with a certain structure. We use character strings to notate S-expressions on paper, but we can use other notations as well, such as little boxes and arrows. We can and do allow several different character strings to denote the same S-expression.)

An Interpreter for LISP Recursion Equations

We now have enough machinery to begin our examination of the genetic history of LISP. We first present a complete interpreter for LISP recursion equations. The language interpreted is a dialect of LISP which allows no free variables except for names of primitive or defined procedures, and no definitions of procedures within other procedures.

The driver loop reads in definitions of procedures of the form:

and saves them. It can also read in requests to apply some defined procedure to some arguments (or, more generally, to evaluate any expression), in which case it prints the resulting value. An expression may consist of variable references, constants (numbers and quoted S-expressions), procedure calls, and conditional expressions (COND). The defined procedures may refer to each other and to initially supplied primitive procedures (such as CAR, CONS, etc.). Definitions may contain "forward references", as long as all necessary definitions are present at the time of a request for a computation. The interpreter itself is presented here as a set of such definitions, and so is meta-circular.

The language is intended to be evaluated in applicative order;