每个人都知道Dijkstra的《致编辑的信》:goto语句被认为是有害的(这里。html transcript和这里。pdf)，从那时起，就有一种强大的推动力，尽可能避免使用goto语句。虽然可以使用goto来生成不可维护的、庞大的代码，但它仍然存在于现代编程语言中。即使Scheme中先进的连续控制结构也可以被描述为复杂的后向。
在什么情况下需要使用goto?什么时候最好避免?
作为一个后续问题:C提供了一对函数setjmp()和longjmp()，它们不仅提供了在当前堆栈帧内进行跳转的能力，还提供了在任何调用帧内进行跳转的能力。这些应该被认为和goto一样危险吗?更危险?
Dijkstra自己也对这个头衔感到后悔，因为这不是他的责任。在EWD1308的结尾(也在这里。pdf)，他写道:
Finally a short story for the record.
In 1968, the Communications of the ACM
published a text of mine under the
title "The goto statement considered
harmful", which in later years would
be most frequently referenced,
regrettably, however, often by authors
who had seen no more of it than its
title, which became a cornerstone of
my fame by becoming a template: we
would see all sorts of articles under
the title "X considered harmful" for
almost any X, including one titled
"Dijkstra considered harmful". But
what had happened? I had submitted a
paper under the title "A case against
the goto statement", which, in order
to speed up its publication, the
editor had changed into a "letter to
the Editor", and in the process he had
given it a new title of his own
invention! The editor was Niklaus
Wirth.
Donald E. Knuth写了一篇关于这个主题的经典论文，它与Dijkstra的论文相匹配，那就是结构化编程与语句。阅读既有助于重建上下文，也有助于对主题的非教条性理解。在本文中，Dijkstra对这个案例的观点被报道，并且更加强烈:
Donald E. Knuth:我相信通过呈现这样一个
事实上，我并不是不同意
Dijkstra的观点，因为
他最近写道:
“请不要落入。
相信我是可怕的
教条的关于[去]的教条的
声明)。我觉得很不舒服
感觉别人在做
宗教出来了，好像
编程的概念问题
只用一个小技巧就能解决，用什么
一种简单的编码纪律!”
如果GOTO本身是邪恶的，那么编译器也是邪恶的，因为它们生成jmp。如果跳转到代码块，特别是跟随一个指针，在本质上是邪恶的，那么RETurn指令就是邪恶的。相反，邪恶在于滥用的可能性。
有时我不得不编写应用程序，必须跟踪许多对象，其中每个对象必须遵循复杂的状态序列来响应事件，但整个事情绝对是单线程的。一个典型的状态序列，如果用伪代码表示，将是:
request something
wait for it to be done
while some condition
request something
wait for it
if one response
while another condition
request something
wait for it
do something
endwhile
request one more thing
wait for it
else if some other response
... some other similar sequence ...
... etc, etc.
endwhile
我相信这不是新的，但我在C(++)中处理它的方式是定义一些宏:
#define WAIT(n) do{state=(n); enque(this); return; L##n:;}while(0)
#define DONE state = -1
#define DISPATCH0 if state < 0) return;
#define DISPATCH1 if(state==1) goto L1; DISPATCH0
#define DISPATCH2 if(state==2) goto L2; DISPATCH1
#define DISPATCH3 if(state==3) goto L3; DISPATCH2
#define DISPATCH4 if(state==4) goto L4; DISPATCH3
... as needed ...
然后(假设初始状态为0)上面的结构化状态机转换为结构化代码:
{
DISPATCH4; // or as high a number as needed
request something;
WAIT(1); // each WAIT has a different number
while (some condition){
request something;
WAIT(2);
if (one response){
while (another condition){
request something;
WAIT(3);
do something;
}
request one more thing;
WAIT(4);
}
else if (some other response){
... some other similar sequence ...
}
... etc, etc.
}
DONE;
}
在此基础上，可以有CALL和RETURN，因此一些状态机可以像其他状态机的子例程一样工作。
不寻常吗?是的。维护者是否需要学习一些知识?是的。这种学习有回报吗?我想是的。如果没有跳转到方块中的goto，游戏还能完成吗?不。
因为goto可以用于令人困惑的元编程
Goto既是高级控件表达式，也是低级控件表达式，因此它没有适合大多数问题的合适设计模式。
它是低级的，因为goto是一个基本操作，它实现了一些高级操作，比如while或foreach之类的。
从某种意义上说，它是高级的，当以某种方式使用时，它将以一种清晰的顺序执行的代码，以一种不间断的方式(除了结构化循环)，并将其转换为逻辑片段，这些逻辑片段有足够的gotos，可以动态地重新组装逻辑。
所以，有平淡的一面，也有邪恶的一面。
平淡的一面是，一个向上指向的goto可以实现一个完全合理的循环，而一个向下指向的goto可以执行一个完全合理的中断或返回。当然，实际的while、break或return语句可读性更强，因为可怜的人不需要为了了解全局而模拟goto语句的效果。总的来说，这是个坏主意。
The evil side involves a routine not using goto for while, break, or return, but using it for what's called spaghetti logic. In this case the goto-happy developer is constructing pieces of code out of a maze of goto's, and the only way to understand it is to simulate it mentally as a whole, a terribly tiring task when there are many goto's. I mean, imagine the trouble of evaluating code where the else is not precisely an inverse of the if, where nested ifs might allow in some things that were rejected by the outer if, etc, etc.
Finally, to really cover the subject, we should note that essentially all early languages except Algol initially made only single statements subject to their versions of if-then-else. So, the only way to do a conditional block was to goto around it using an inverse conditional. Insane, I know, but I've read some old specs. Remember that the first computers were programmed in binary machine code so I suppose any kind of an HLL was a lifesaver; I guess they weren't too picky about exactly what HLL features they got.
说了这么多，我曾经在我写的每个程序中都加了一个go，“只是为了惹恼那些纯粹主义者”。
