letrec* -- r7rs Definition
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syntax;

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Description
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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, each <variable> is assigned in left-to-right order to the result of evaluating the corresponding <init>, the <body> is evaluated in the resulting environment, and the values of the last expression in <body> are returned. Despite the left-to-right evaluation and assignment order, each binding of a <variable> has the entire letrec* expression as its region, making it possible to define mutually recursive procedures.

If it is not possible to evaluate each <init> without assigning or referring to the value of the corresponding <variable> or the <variable> of any of the bindings that follow it in <bindings>, it is an error. Another restriction is that it is an error to invoke the continuation of an <init> more than once.

(letrec* ((p
           (lambda (x)
             (+ 1 (q (- x 1)))))
          (q
           (lambda (y)
             (if (zero? y)
                 0
                 (+ 1 (p (- y 1))))))
          (x (p 5))
          (y x))
  y)
                ===>  5

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

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