let -- r7rs Definition
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syntax;

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Description
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(let <bindings> <body>)

Syntax: <Bindings> has the form

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

where each <init> is an expression, and <body> is a sequence of zero or more definitions followed by a sequence of 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 <init>s are evaluated in the current environment (in some unspecified order), the <variable>s are bound to fresh locations holding the results, the <body> is evaluated in the extended environment, and the values of the last expression of <body> are returned. Each binding of a <variable> has <body> as its region.

(let ((x 2) (y 3))
  (* x y))                      ===>  6

(let ((x 2) (y 3))
  (let ((x 7)
        (z (+ x y)))
    (* z x)))                   ===>  35

See also "named let", section on iteration.

(let <variable> <bindings> <body>)

Semantics: "Named let" is a variant on the syntax of let which provides a more general looping construct than do and can also be used to express recursion. It has the same syntax and semantics as ordinary let except that <variable> is bound within <body> to a procedure whose formal arguments are the bound variables and whose body is <body>. Thus the execution of <body> can be repeated by invoking the procedure named by <variable>.

(let loop ((numbers '(3 -2 1 6 -5))
           (nonneg '())
           (neg '()))
  (cond ((null? numbers) (list nonneg neg))
        ((>= (car numbers) 0)
         (loop (cdr numbers)
               (cons (car numbers) nonneg)
               neg))
        ((< (car numbers) 0)
         (loop (cdr numbers)
               nonneg
               (cons (car numbers) neg)))))
  ===>  ((6 1 3) (-5 -2))

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

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