So far we've seen three ways in which a value can be associated with a name in Scheme:
The names of built-in procedures, such as cons and
quotient, are predefined. When DrScheme starts up,
these names are already bound to the procedures they denote.
The programmer can introduce a new binding by means of a definition. A definition may introduce a new equivalent for an old name, or it may give a name to a newly constructed value.
When a programmer-defined procedure is called, the parameters of the procedure are bound to the values of the corresponding arguments in the procedure call. Unlike the other two kinds of bindings, parameter bindings are local -- they apply only within the body of the procedure. Scheme discards these bindings when it leaves the procedure and returns to the point at which the procedure was called.
A let-expression in Scheme is an alternative way to create
local bindings. A let-expression contains a binding
list and a body. The body can be any expression, or sequence
of
expressions, to be evaluated with the help of the local name bindings.
The
binding list is a pair of structural parentheses enclosing zero or more
binding specifications; a binding specification, in turn, is a pair
of structural parentheses enclosing a name and an expression. Here's an
example of a binding list, taken from a let-expression in a
real Scheme program:
((next (car source)) (char-list '()))
This binding list contains two binding specifications -- one in which the
value of the expression (car source) is bound to the name
next, and the other in which the empty list is bound to the
name char-list. Notice that binding lists and binding
specifications are not procedure calls; their role in a
let-expression simply to give names to certain values while
the body of the expression is being evaluated. The outer parentheses in a
binding list are ``structural,'' like the outer parentheses in a
cond-clause -- they are there to group the pieces of the
binding list together.
When Scheme encounters a let-expression, it begins by
evaluating all of the expressions inside its binding specifications. Then
the names in the binding specifications are bound to those values. Next,
the expressions making up the body of the let-expression are
evaluated, in order. The value of the last expression in the body becomes
the value of the entire let-expression. Finally, the local
bindings of the names are cancelled. (Names that were unbound before the
let-expression become unbound again; names that had different
bindings before the let-expression resume those earlier
bindings.)
Using a let-expression often simplifies an expression that
contains two or more occurrences of the same subexpression. The
programmer
can compute the value of the subexpression just once, bind a name to it,
and then use that name whenever the value is needed again. Sometimes this
speeds things up by avoiding such redundancies as the recomputation of the
discriminant in the second example in exercise 1; in other cases, there is
little difference in speed, but the code may be a little clearer. For
instance, here is a procedure remove-all which removes all
occurences of a given item in a given list.
(define remove-all
(lambda (item ls)
(if (null? ls)
'()
(let ((first-element (car ls))
(rest-of-result (remove-all item (cdr ls))))
(cond ((equal? first-element item) rest-of-result)
((pair? first-element)
(cons (remove-all item first-element) rest-of-result))
(else (cons first-element rest-of-result)))))))
One of the least attractive features of the text's version of this program
was the repetition of the recursive call (remove-all item (cdr
ls)) in three different places. Consolidating the repeated code
and
giving a name to the value it returns makes it a little easier to
understand what the three cond-clauses are doing.
As shown in the third example in exercise 1 of the lab, it is
possible to nest
one let-expression inside another. One might be tempted to
try to combine the binding lists for the nested
let-expressions, thus:
;; Combining the binding lists doesn't work!
;;
(let ((total (+ 8 3 4 2 7))
(mean (/ total 5)))
(* mean mean))
This wouldn't work (try it and see!), and it's important to understand why
not. The problem is that, within one binding list, all of the
expressions are evaluated before any of the names are bound.
Specifically, Scheme will try to evaluate both (+ 8 3 4 2 7)
and (/ total 5) before binding either of the names
total and mean; since (/ total 5)
can't
be computed until total has a value, an error occurs. You
have
to think of the local bindings coming into existence simultaneously rather
than one at a time.
Because one often needs sequential rather than simultaneous binding,
Scheme
provides a variant of the let-expression that rearranges the
order of events: If one writes let* rather than
let, each binding specification in the binding list is
completely processed before the next one is taken up:
;; Using LET* instead of LET works!
;;
(let* ((total (+ 8 3 4 2 7))
(mean (/ total 5)))
(* mean mean))
The star in the keyword let* has nothing to do with
multiplication. Just think of it as an oddly shaped letter.
One can use a let- or let*-expression to create
a
local name for a procedure:
(define hypotenuse-of-right-triangle
(let ((square (lambda (n)
(* n n))))
(lambda (first-leg second-leg)
(sqrt (+ (square first-leg) (square second-leg))))))
Regardless of whether square is defined outside this
definition, the local binding gives it the appropriate meaning within the
lambda-expression that describes what
hypotenuse-of-right-triangle does.
This document is available on the World Wide Web as
http://www.cs.grinnell.edu/~gum/courses/151/readings/local-bindings.xhtml
created February 26, 1997
last revised August 11, 2001
John David Stone (stone@cs.grinnell.edu) and Ben Gum (gum@cs.grinnell.edu)