2. The Source Language - SCHEME
The basic language processed by RABBIT is a subset of the SCHEME language as described in [SCHEME] and [Revised Report], the primary restrictions being that the first argument to ASET must be quoted and that the multiprocessing primitives are not accommodated. This subset is summarized here.
SCHEME is essentially a lexically scoped ("full funarg") dialect of LISP. Interpreted programs are represented by S-expressions in the usual manner. Numbers represent themselves. Atomic symbols are used as identifiers (with the conventional exception of T and NIL, which are conceptually treated as constants). All other constructs are represented as lists.
In order to distinguish the various other constructs, SCHEME follows the
usual convention that a list whose car is one of a set of distinguished atomic
symbols is treated as directed by a rule associated with that symbol. All other
lists (those with non-atomic cars, or with undistinguished atoms in their cars)
are combinations, or function calls. All subforms of the list are uniformly
evaluated in an unspecified order, and then the value of the first (the function)
is applied to the values of all the others (the arguments). Notice that the
function position is evaluated in the same way as the argument positions (unlike
most other LISP systems). (In order to be able to refer to MacLISP functions,
global identifiers evaluate to a special kind of functional object if they have
definitions as MacLISP functions of the EXPR, SUBR, or LSUBR varieties. Thus
"(PLUS 1 2)" evaluates to 3 because the values of the subforms are <functional object for PLUS>, 1, and 2; and applying the first to the other two causes
invocation of the MacLISP primitive PLUS.)
The atomic symbols which distinguish special constructs are as follows:
- LAMBDA
- This denotes a function. A form
(LAMBDA (var1 var2 ... varn) body)
