As opposed to definitions at the top level, which are visible in the whole program (or current module, when Guile modules are used), 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
let expressions has a few properties
which are well worth knowing.
The first local binding construct is
let. The other constructs
letrec are specialized versions for usage where
let is a bit inconvenient.
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.
letexpression is evaluated as follows.
- All init expressions are evaluated.
- New storage is allocated for the variables.
- The values of the init expressions are stored into the variables.
- The expressions in body are evaluated in order, and the value of the last expression is returned as the value of the
- The storage for the variables is freed.
The init expressions are not allowed to refer to any of the variables.
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
letexpressions.(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
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)