- (letrec* <bindings> <body>) syntax
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 4.1.4. 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> (interleaving evaluations and assignments), 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.
;; Returns the arithmetic, geometric, and ;; harmonic means of a nested list of numbers (define (means ton) (letrec* ((mean (lambda (f g) (f (/ (sum g ton) n)))) (sum (lambda (g ton) (if (null? ton) (+) (if (number? ton) (g ton) (+ (sum g (car ton)) (sum g (cdr ton))))))) (n (sum (lambda (x) 1) ton))) (values (mean values values) (mean exp log) (mean / /))))Evaluating (means '(3 (1 4))) returns three values: 8/3, 2.28942848510666 (approximately), and 36/19.