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6.13.2 Local Variable Bindings

As opposed to definitions at the top level, which creates bindings that are visible to all code in a module, it is also possible to define variables which are only visible in a well-defined part of the program. Normally, this part of a program will be a procedure or a subexpression of a procedure.

With the constructs for local binding (let, let*, letrec, and letrec*), the Scheme language has a block structure like most other programming languages since the days of ALGOL 60. Readers familiar to languages like C or Java should already be used to this concept, but the family of let expressions has a few properties which are well worth knowing.

The most basic local binding construct is let.

syntax: let bindings body

bindings has the form

((variable1 init1) …)

that is zero or more two-element lists of a variable and an arbitrary expression each. All variable names must be distinct.

A let expression is evaluated as follows.

The init expressions are not allowed to refer to any of the variables.

The other binding constructs are variations on the same theme: making new values, binding them to variables, and executing a body in that new, extended lexical context.

syntax: let* bindings body

Similar to let, but the variable bindings are performed sequentially, that means that all init expression are allowed to use the variables defined on their left in the binding list.

A let* expression can always be expressed with nested let expressions.

(let* ((a 1) (b a))
   b)
≡
(let ((a 1))
  (let ((b a))
    b))
syntax: letrec bindings body

Similar to let, but it is possible to refer to the variable from lambda expression created in any of the inits. That is, procedures created in the init expression can recursively refer to the defined variables.

(letrec ((even? (lambda (n)
                  (if (zero? n)
                      #t
                      (odd? (- n 1)))))
         (odd? (lambda (n)
                  (if (zero? n)
                      #f
                      (even? (- n 1))))))
  (even? 88))
⇒
#t

Note that while the init expressions may refer to the new variables, they may not access their values. For example, making the even? function above creates a closure (see About Closure) referencing the odd? variable. But odd? can’t be called until after execution has entered the body.

syntax: letrec* bindings body

Similar to letrec, except the init expressions are bound to their variables in order.

letrec* thus relaxes the letrec restriction, in that later init expressions may refer to the values of previously bound variables.

(letrec ((a 42)
         (b (+ a 10)))  ;; Illegal access
  (* a b))
;; The behavior of the expression above is unspecified

(letrec* ((a 42)
          (b (+ a 10)))
  (* a b))
⇒ 2184

There is also an alternative form of the let form, which is used for expressing iteration. Because of the use as a looping construct, this form (the named let) is documented in the section about iteration (see Iteration)


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