Exercise 5.8. The following register-machine code is ambiguous, because the label here is defined more than once:
start
(goto (label here))
here
(assign a (const 3))
(goto (label there))
here
(assign a (const 4))
(goto (label there))
there
With the simulator as written, what will the contents of register a be when control reaches there? Modify the extract-labels procedure so that the assembler will signal an error if the same label name is used to indicate two different locations.
根据之前的分析， extract-labels 将会返回 labels 和 instructions 的 list。
lookup-labels 使用 assoc 来获取某个 labels 的对应。 assoc 的实现只找第一个符合条件的值，所以这里的 a 中将保存 3。
新的 extract-labels 如下：
(load "machine-assemble-module.scm")
(define (extract-labels text receive)
(if (null? text)
(receive '() '())
(extract-labels
(cdr text)
(lambda (labels insts)
(let ((next-inst (car text)))
(cond ((and (symbol? next-inst) (assoc next-inst labels))
(error 'extract-labels "Duplicate labels: " next-inst))
((symbol? next-inst)
(receive
(cons (make-label-entry next-inst insts) labels)
insts))
(else
(receive
labels
(cons (make-instruction next-inst) insts)))))))))
(let ((text
'(start
(goto (label here))
here
(assign a (const 3))
(goto (label there))
here
(assign a (const 4))
(goto (label there))
there)))
(assert/exn
(extract-labels text (lambda (labels insts) labels))
"Duplicate"))