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25.2 Local Variable Bindings
============================
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 (`let', `let*' 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 first local binding construct is `let'. The other constructs
`let*' and `letrec' are specialized versions for usage where using
plain `let' is a bit inconvenient.
-- 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.
* 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 `let'
expression.
* The storage for the VARIABLES is freed.
The INIT expressions are not allowed to refer to any of the
VARIABLES.
-- 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
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 ( Iteration while do.)
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