Exercise 5.9. The treatment of machine operations above permits them to operate on labels as well as on constants and the contents of registers. Modify the expression-processing procedures to enforce the condition that operations can be used only with registers and constants.
make-operation-exp 的问题在于，它无法检查在 assemble 的阶段检查出 (test (op =) (label label1) (const 1)) 的错误来。
我们可以这样来改进
(load "machine-stack-module.scm")
(load "machine-register-module.scm")
(load "machine-machine-module.scm")
(load "machine-assemble-module.scm")
(define (make-operation-exp exp machine labels operations)
(let ((op (lookup-prim (operation-exp-op exp) operations))
(aprocs
(map (lambda (e)
(cond ((label-exp? e)
(error 'make-operation-exp "Bad expression type " e))
(else
(make-primitive-exp e machine labels))))
(operation-exp-operands exp))))
; apply the op to all operands
(lambda ()
(apply op (map (lambda (p) (p)) aprocs)))))
(load "../testframe.scm")
(assert/exn
(make-machine '(n) (list (list '- -))
'((assign n (op -) (label after-expt) (const 1))
after-expt
expt-done))
"Bad expression")