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a1369209993

Djikstra's meaning was that gotos should be removed entirely. By replacing them with the high-level control-flow constructs them were being used to implement. If your language doesn't have a high-level equivalent of `goto fail`... well Djikstra would probably say you shouldn't use that language in the first place, but given that you are, it's no worse than using `jlt .forloop` in assembly.

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You are going to wind up with some kind of 'goto' regardless, whether it's having to navigate through independent files or various other constructs one takes for granted as being immutable. The point being, when you notice them, automate them out of the reasoning space so you don't drown in the complexity of one mundane detail being used to represent the overall state of the program (all the stuff you have to shove in your head to understand what your program is doing).

These things are hard to notice, and even harder to find elegant and effective solutions!

The point is to reduce the problem space so all you are left to look at, when you do come back to your code, is what's important to reason about without removing important details or adding trivialities. These problems haven't gone away just because the literal word goto isn't used anymore.

> If your language doesn't have a high-level equivalent of `goto fail`

Such as?

I am aware of writing this in different ways, of architecting the logic quite differently. But the only control-flow construct I've seen as alternatives are to abuse exceptions or labelled while with named breaks.

What were you thinking of when you wrote this? I'm genuinely interested.

Golang's defer statement gives some of that power, (and some extra guarantees) but doesn't as far as I know allow return statements.

You can however give named return values and reference those names, and I believe it will work.

I've not seen any other constructs in any language that would come close, and I still maintain `goto fail` is a reasonable construct.

Deferred statements (in go) are really useful for delaying the runtime of blocks of code to the end of a context, but leaving them in the logical context where you're setting up a resource.

I'm actually not sure offhand how multiple deferred statements are handled. I could envision that the specification might make no explicit guarantee about the runtime order or it might create and pop off of a stack (at least in behavior).

Thank you. For clarity, here would be the GP example:

   Foo foo = acquireFoo()
   defer releaseFoo(foo)

   if !isValid(foo) {
      return FAIL
   }

   moreWork(foo)
   return SUCCESS

That is impressively clear. I don't use go, myself.

I agree, I am quite comfortable using goto in C/C++ in this context. Though it may be better in C++ to use RAII in this specific example.

Java and C# have the `finally` block which is essentially the `goto fail`. Typically it is used like:

  try {
    doSomething();
  }
  catch (Exception e) {
    handleException();
  }
  finally {
    cleanUpResources();
  }

When the code exits the try block, you are guaranteed that it will run the finally block, possibly entering the catch block first if an exception was thrown. Since the most common use of `goto fail` pattern is freeing memory, the `finally` block isn't actually used a lot in Java/C# code in practice.

Thank you. Since the example was nothing to do with exceptions, and we don't want to abuse exceptions by using them when they are not needed, I will rewrite your example to cover the GP case:

   Foo foo = acquireFoo();
   try {
       if (!isValid(foo)) {
          return FAIL;
       }
       moreWork(foo);
       return SUCCESS;
   } finally {
       releaseFoo(foo);
   }

Python could also use the same approach. Though it also has context managers / the with-statement for this specific application.

Common Lisp has `with-open-file`, which guarantees files get closed regardless of what happens in the body, even if an exception is thrown (the macro closes the file and then propagates the exception in that case):

    (with-open-file (fhandle path)
        (if (has-record fhandle)
            (process-record (get-record fhandle))))

Python basically has this, but Python doesn't give you the macros needed to write your own for other kinds of resource.

http://clhs.lisp.se/Body/m_w_open.htm

> If a new output file is being written, and control leaves abnormally, the file is aborted and the file system is left, so far as possible, as if the file had never been opened.

Shades of PCLSRing!

> Python doesn't give you the macros needed to write your own for other kinds of resource.

Python doesn't use macros, true. But it absolutely gives you the ability to do 'with' for other kinds of resource (and anything else). Look for the with-statement and documentation on defining your own context manager.

with-open-file is just a convenience macro around the underlying functionality which is provided by unwind-protect which is pretty much identical to try/finally in Java.

The ease of writing macros such as with-open-file means that most libraries that need to have similar types of features for their resources that needs releasing also provides similar functionality.

> What were you thinking of when you wrote this? I'm genuinely interested.

That there is no high-level equivalent, and Djikstra, while he sometimes had useful insights, was ultimately a self-righteous twit.

Sorry to disappoint; I would also be interested a (not-even-worse-than-goto-like-exceptions-are) answer to your question.

I could be misunderstanding here, but I'm fairly sure that in the spirit of the paper `goto fail` would be preferable to throwing an exception and unwinding the stack at any arbitrary function call.

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