chickadee » scheme » letrec

(letrec <bindings> <body>)syntax

Syntax: <Bindings> should have the form

((<variable[1]> <init[1]>) ...),

and <body> should be a sequence of one or more expressions. It is an error for a <variable> to appear more than once in the list of variables being bound.

Semantics: The <variable>s are bound to fresh locations holding undefined values, the <init>s are evaluated in the resulting environment (in some unspecified order), each <variable> is assigned to the result of the corresponding <init>, the <body> is evaluated in the resulting environment, and the value(s) of the last expression in <body> is(are) returned. Each binding of a <variable> has the entire letrec expression as its region, making it possible to define mutually recursive procedures.

(letrec ((even?
          (lambda (n)
            (if (zero? n)
                (odd? (- n 1)))))
          (lambda (n)
            (if (zero? n)
                (even? (- n 1))))))
  (even? 88))
                        ===>  #t

One restriction on letrec is very important: it must be possible to evaluate each <init> without assigning or referring to the value of any <variable>. If this restriction is violated, then it is an error. The restriction is necessary because Scheme passes arguments by value rather than by name. In the most common uses of letrec, all the <init>s are lambda expressions and the restriction is satisfied automatically.