letrec -- r7rs Definition
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syntax;

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(letrec <bindings> <body>)

Syntax: <Bindings> has the form

((<variable_1> <init_1>) ...)

and <body> is a sequence of zero or more definitions followed by one or more expressions as described in section on lambda. 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 unspecified 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 values of the last expression in <body> 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)
                #t
                (odd? (- n 1)))))
         (odd?
          (lambda (n)
            (if (zero? n)
                #f
                (even? (- n 1))))))
  (even? 88))
                ===>  #t

One restriction on letrec is very important: if it is not possible to evaluate each <init> without assigning or referring to the value of any <variable>, it is an error. The restriction is necessary because letrec is defined in terms of a procedure call where a lambda expression binds the <variable>s to the values of the <init>s. In the most common uses of letrec, all the <init>s are lambda expressions and the restriction is satisfied automatically.

The text herein was sourced and adapted as described in the "R7RS attribution of various text snippets" appendix.

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