## 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.