Page:Scheme - An interpreter for extended lambda calculus.djvu/3

Sussman and Steele

December 22, 1975

2

The SCHEME Reference Manual

one to count the cdr, as follows:

(DEFINE COUNT
    (LAMBDA (L)
        (LABELS ((COUNTCAR
                  (LAMBDA (L)
                      (IF (ATOM L) 1
                          (+ (COUNTCAR (CAR L))
                             (COUNTCDR (CDR L))))))
                 (COUNTCDR
                  (LAMBDA (L)
                      (IF (ATOM L)
                          (IF (NULL L) 0 1)
                          (+ (COUNTCAR (CAR L))
                             (COUNTCDR (CDR L)))))))
                (COUNTCDR L))))         ;Note: COUNTCDR is defined here.

ASET

This is the side effect primitive. It is analogous to the LISP function SET. For example, to define a cell [Smith and Hewitt], we may use ASET as follows:

(DEFINE CONS-CELL
    (LAMBDA (CONTENTS)
        (LABELS ((THE-CELL
                  (LAMBDA (MSG)
                      (IF (EQ MSG 'CONTENTS?) CONTENTS
                          (IF (EQ MSG 'CELL?) 'YES
                              (IF (EQ (CAR MSG) '<-)
                                  (BLOCK (ASET 'CONTENTS (CADR MSG))
                                         THE-CELL)
                                  (ERROR '|UNRECOGNIZED MESSAGE - CELL|
                                          MSG
                                          'WRNG-TYPE-ARG)))))))
                THE-CELL)))

Those of you who may complain about the lack of ASETQ are invited to write (ASET' foo bar) instead of (ASET 'foo bar).

EVALUATE

This is similar to the LISP function EVAL. It evaluates its argument, and then evaluates the resulting s-expression as SCHEME code.

CATCH

This is the "escape operator" which gives the user a handle on the control structure of the interpreter. The expression:

(CATCH <identifier> <expression>)