12 Procedures

Procedures are created by evaluating lambda expressions (see Lambda Expressions); the lambda may either be explicit or may be implicit as in a “procedure define” (see Definitions). Also there are special built-in procedures, called primitive procedures, such as car; these procedures are not written in Scheme but in the language used to implement the Scheme system. MIT/GNU Scheme also provides application hooks, which support the construction of data structures that act like procedures.

In MIT/GNU Scheme, the written representation of a procedure tells you the type of the procedure (compiled, interpreted, or primitive):

pp
     ⇒  #[compiled-procedure 56 ("pp" #x2) #x10 #x307578]
(lambda (x) x)
     ⇒  #[compound-procedure 57]
(define (foo x) x)
foo
     ⇒  #[compound-procedure 58 foo]
car
     ⇒  #[primitive-procedure car]
(call-with-current-continuation (lambda (x) x))
     ⇒  #[continuation 59]

Note that interpreted procedures are called “compound” procedures (strictly speaking, compiled procedures are also compound procedures). The written representation makes this distinction for historical reasons, and may eventually change.