Structure and Interpretation of Computer Programs
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- Bu səhifədəki naviqasiya:
- Using let to create local variables
- Exercise 1.34.
The resulting procedure is just as much a procedure as one that is created using define
. The only difference is that it has not been associated with any name in the environment. In fact, (define (plus4 x) (+ x 4)) is equivalent to (define plus4 (lambda (x) (+ x 4))) We can read a lambda expression as follows: (lambda (x) (+ x 4))
the procedure of an argument x that adds x and 4 Like any expression that has a procedure as its value, a lambda expression can be used as the operator in a combination such as ((lambda (x y z) (+ x y (square z))) 1 2 3) 12 or, more generally, in any context where we would normally use a procedure name. 53
Another use of lambda is in creating local variables. We often need local variables in our procedures other than those that have been bound as formal parameters. For example, suppose we wish to compute the function which we could also express as In writing a procedure to compute f, we would like to include as local variables not only x and y but also the names of intermediate quantities like a and b. One way to accomplish this is to use an auxiliary procedure to bind the local variables: (define (f x y) (define (f-helper a b) (+ (* x (square a)) (* y b) (* a b))) (f-helper (+ 1 (* x y)) (- 1 y))) Of course, we could use a lambda expression to specify an anonymous procedure for binding our local variables. The body of f then becomes a single call to that procedure: (define (f x y) ((lambda (a b) (+ (* x (square a)) (* y b) (* a b))) (+ 1 (* x y)) (- 1 y))) This construct is so useful that there is a special form called let to make its use more convenient. Using let
, the f procedure could be written as (define (f x y) (let ((a (+ 1 (* x y))) (b (- 1 y))) (+ (* x (square a)) (* y b) (* a b)))) The general form of a let
expression is (let ((<var 1 > <exp 1 >) (<var 2 > <exp 2 >)
(<var n > <exp n >))
<body>) which can be thought of as saying let
> have the value <exp 1 > and
<var 2 > have the value <exp 2 > and
<var n > have the value <exp n > in <body> let expression is a list of name-expression pairs. When the let is evaluated, each name is associated with the value of the corresponding expression. The body of the let
is evaluated with these names bound as local variables. The way this happens is that the let expression is interpreted as an alternate syntax for ((lambda (<var 1 > ...<var n >)
<exp 1 >
<exp n >) No new mechanism is required in the interpreter in order to provide local variables. A let
expression is simply syntactic sugar for the underlying lambda application. We can see from this equivalence that the scope of a variable specified by a let
expression is the body of the let . This implies that: Let allows one to bind variables as locally as possible to where they are to be used. For example, if the value of x is 5, the value of the expression (+ (let ((x 3)) (+ x (* x 10))) x) is 38. Here, the x in the body of the let is 3, so the value of the let expression is 33. On the other hand, the x that is the second argument to the outermost + is still 5. The variables’ values are computed outside the let
. This matters when the expressions that provide the values for the local variables depend upon variables having the same names as the local variables themselves. For example, if the value of x is 2, the expression (let ((x 3) (y (+ x 2))) (* x y)) will have the value 12 because, inside the body of the let ,
will be 3 and y will be 4 (which is the outer x plus 2). Sometimes we can use internal definitions to get the same effect as with let
. For example, we could have defined the procedure f above as (define (f x y) (define a (+ 1 (* x y))) (define b (- 1 y)) (+ (* x (square a)) (* y b) (* a b))) We prefer, however, to use let
in situations like this and to use internal define
only for internal procedures. 54
(define (f g) (g 2)) Then we have (f square)
(f (lambda (z) (* z (+ z 1)))) 6 Yüklə 2,71 Mb. Dostları ilə paylaş:
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