5.9 Exercise 5.9

Currently, the machine does not reject operations that are performed on labels. For example, this example program is assembled correctly:

(define operating-on-label

(make-machine

'(a)

(list (list '+ +))

'(start

(assign a (op +) (label start) (const 1)))))

Naturally, this program doesn’t actually work – instead, it throws a runtime error when trying to apply the + operation.

Since performing operations on labels is (probably) not a good idea, we can revise the machine to reject these programs. All that we need to do is insert a check in make-operation-exp to verify that primitive expressions are not labels:

(define (make-operation-exp exp machine labels operations)

(let ((op (lookup-prim (operation-exp-op exp) operations))

(aprocs

(map (lambda (e)

(if (label-exp? e)

(error "Cannot perform operations on label -- ASSEMBLE" e)

(make-primitive-exp e machine labels)))

(operation-exp-operands exp))))

(lambda ()

(apply op (map (lambda (p) (p)) aprocs)))))

It’s a little ugly, but I think this is a reasonable place to check the expression type. Obviously we want to check the expression itself rather than the expression procedure, so it’s convenient to do so in a location where each operand expression is already in scope. And we don’t want to push down any new logic into make-primitive-exp, because labels are primitives. As an alternative, you might also consider reporting on all of the operands that are labels.

For now, we can verify that what we’ve written rejects the program above: