57 views

Skip to first unread message

Mar 2, 1989, 11:27:00 AM3/2/89

to

The following function, which I mailed out to this last last February, was

intended to demonstrate how CALL-WITH-CURRENT-CONTINUATION could be used to

expose the SET! hidden in the definition of LETREC:

intended to demonstrate how CALL-WITH-CURRENT-CONTINUATION could be used to

expose the SET! hidden in the definition of LETREC:

(define (make-cell)

(call-with-current-continuation

(lambda (return-from-make-cell)

(letrec ((state

(call-with-current-continuation

(lambda (return-new-state)

(return-from-make-cell

(lambda (op)

(case op

((set)

(lambda (value)

(call-with-current-continuation

(lambda (return-from-access)

(return-new-state

(list value return-from-access))))))

((get) (car state)))))))))

((cadr state) 'done)))))

Here is another way to do it that has an additional interesting property:

(define (make-cell initial-value)

(letrec ((state (call-with-current-continuation

(lambda (return-new-state)

(list initial-value return-new-state #F)))))

(if (caddr state)

((caddr state) 'done)

(lambda (op)

(case op

((get) (car state))

((set)

(lambda (new-value)

(call-with-current-continuation

(lambda (return-from-set)

((cadr state)

(list new-value (cadr state) return-from-set)))))))))))

Notice that the variable STATE does not occur anywhere in the right hand

side of its own definition in the LETREC. Thus you might think that you

could optimize this into an instance of LET. But that optimization would

be incorrect (assuming that the expansion of LETREC given in R^3RS really

-is- its definition), because then this code will stop producing

side-effectable cells.

A simpler case of this peculiarity of LETREC:

(define (test)

(letrec ((x (call-with-current-continuation

(lambda (c)

(list #T c)))))

(if (car x)

((cadr x) (list #F (lambda () x)))

(eq? x ((cadr x))))))

(TEST) should return #T. If you replace the LETREC with LET (you might be

tempted because X does not appear anywhere in the right hand side of the

definition!), then (TEST) will return #F.

I wonder if any real compilers make this mistaken optimization?

Reply all

Reply to author

Forward

0 new messages

Search

Clear search

Close search

Google apps

Main menu