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 (
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
expressions has a few properties which are well worth knowing.
The most basic local binding construct is
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.
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.
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.
let* expression can always be expressed with nested
(let* ((a 1) (b a)) b) ≡ (let ((a 1)) (let ((b a)) b))
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)
odd? variable. But
odd? can’t be called
until after execution has entered the body.
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
(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)