4.51 Exercise 4.51

Implementing permanent-set! is actually easier than implementing set! was, because it no longer needs to wrap the innermost failure continuation with code that sets the value back to what it was before.

First, we have the trivial detection and accessor functions. permanent-set? is added as a case to analyze before application?, just like usual:

(define (permanent-assignment? exp)

(tagged-list? exp 'permanent-set!))

(define (permanent-assignment-variable exp) (cadr exp))

(define (permanent-assignment-value exp) (caddr exp))

And here is how we analyze permanent set expressions.

(define (analyze-permanent-assignment exp)

(let ((var (permanent-assignment-variable exp))

(vproc (analyze (permanent-assignment-value exp))))

(lambda (env succeed fail)

(vproc env

(lambda (val fail2)

(set-variable-value! var val env)

(succeed 'ok fail2))

fail))))

Compare this to analyze-assignment – you’ll see that it is a strictly simpler variant:

(define (analyze-assignment exp)

(let ((var (assignment-variable exp))

(vproc (analyze (assignment-value exp))))

(lambda (env succeed fail)

(vproc env

(lambda (val fail2)

(let ((old-value (lookup-variable-value var env)))

(set-variable-value! var val env)

(succeed 'ok

(lambda ()

(set-variable-value! var old-value env)

(fail2)))))

fail))))