[brabel]

One of the only languages where I saw this non-local returns is Kotlin.

You can return from a function from within a lambda (and I believe this works on any number of levels), for example:

    fun go(list: List<String>): Boolean {
        list.forEach { if (it.isEmpty()) return@go true }
        return false
    }

[gpderetta]

Longjmp in standard C, swapcontext in POSIX C. I think GCC allows gotos to the containing function from a local function (but I never used GCC local functions).

Smalltalk, and I believe ruby, allow non local return from blocks.

[gumby]

longjmp simply restores the machine registers (including PC and SP) and thus doesn’t respect any sort of unwind-protect, a concept unknown in C.

unwind-protect is a more general form of the c++ raii (in fact c++ got raii from Common Lisp).

[Findecanor]

`longjmp` does not restore the machine registers on all platforms. The C standard guarantees only the contents of variables marked `volatile`: anything above that is implementation-dependent.

I think that MSVC++ longjmp actually does proper unwinding, calling destructors in C++ functions on the stack, but don't quote me on that. I think it is also dependent on the compiler flags.

[gpderetta]

Implementing exception handling (with cleanup on unwind) on top of setjmp was not uncommon. GCC still does on some targets. But I'm sure you know this more than I do.

I think that RAII is different from unwind-protect and other scope based cleanup (finally, defer, with) as it is tied to object lifetimes. The fact that automatic objects lifetimes are tied to scope is a nice feature, but RAII goes beyond that.

[gumby]

Well lisps don’t really have destructors in the sense C++ does; objects go out of scope and become unreachable. So unwind-protect makes cleanup (closing a file, for example) explicit, which it has to be.

I wouldn’t say RAII is “tied to object lifetime” except in its name; at least I think of it as every {} pair defining an unwind-protect with object creation/destruction being how it is effected by the programmer. Perhaps that is the same thing as you said, simply viewed from opposite sides.

I do like the automatic nature of RAII, though implementing a whole class for it always feels clumsy to me even after doing it for decades.

[gpderetta]

What I mean is that if I add an instance of a class with a non trivial destructor from a container and later remove it, the destructor is automatically invoked. This is different from an unwind protect.

[kazinator]

The object is not exactly removed from the container. It is removed from existence. The object in that situation resides in the container physically. It is constructed there and destroyed. It does not exist outside of the container afterwards, or at any other time.

This is a low-level memory management strategy being conflated with object-orientation, that is alien to higher level languages.

[lispm]

Lisp uses UNWIND-PROTECT to implement that.

For example WITH-OPEN-FILE closes the file automatically when leaving its scope.

     (with-open-file (stream "hello.world")
       (read stream))

WITH-OPEN-FILE is a macro and expands into OPEN and CLOSE operations, protected by an UNWIND-PROTECT.

Thus anything where a destructor would automatically clean things up is done in Lisp behind the scenes automatically using an UNWIND-PROTECT form.

[oumua_don17]

>> in fact c++ got raii from Common Lisp

I know and understand this is true.

Can you point to any sources (projects, papers) that further substantiate this?

[adrian_b]

The concept of RAII has emerged gradually and it would be hard to pinpoint a moment for its appearance. I am not aware of any specific feature of RAII that has appeared in Common Lisp before other languages.

The concepts of constructors and destructors have been introduced by C.A.R. Hoare in November 1965, in "Record Handling", a proposal for the extension of Algol.

At that time, Hoare was using the Cobol terms, i.e. "record" for what later was named "object" and "record class" for what later, in Simula 67, was abbreviated to "class". All the "records" discussed by Hoare were allocated dynamically, in the heap.

Constructor (Hoare, 1965-11): "In order to bring records into existence in the first place, the record class identifier should be used as if it were a function designator"

Destructor (Hoare, 1965-11): "a standard procedure "destroy" is proposed, which takes as parameter a reference to a record, and which reverses the effect of having created that record"

The next step towards RAII has been done by Bjarne Stroustrup in "C with Classes" in April 1980, when he made the invocation of the destructors implicit at the exit from a block, by introducing the special member function "delete" for this purpose. Despite the name, the 1980 "delete" member functions corresponded to what later, in C++, were renamed as destructors.

So in 1980, RAII was complete, but it was not yet promoted as a universal strategy for managing resources.

In 1980, Common Lisp did not exist.

Most older Lisps did not have any concept similar with the Algol block, so it would have been impossible for them to invoke implicitly some cleanup functions at block exits. They relied only on the garbage collector, where there is no RAII in the Stroustrup sense, even if GC and RAII are alternative methods for avoiding the explicit invocations of "free", "close" and the like.

[lispm]

MDL had UNWIND in 1977 and Maclisp added UNWIND-PROTECT in 1978. I'll guess that Lisp Machine Lisp then got it, too. Common Lisp emerged 1981 onwards, largely based on the latter.

In the Lisp Machine OS there is a concept called RESOURCE for manual memory management. For example the CHAOS network stack has to deal with network packets. There is a macro USING-RESOURCE, which allocates/gets an object and on exit frees it. The macro expands into a form using also UNWIND-PROTECT to ensure freeing a resource on non-local exit. I would think that this is from around 1980, for the MIT CADR machine.

[adrian_b]

Thanks for pointing to MDL:

http://www.ifarchive.org/if-archive/infocom/info/MDL_Primer_...

Nevertheless, the MDL UNWIND and the later UNWIND-PROTECT are more limited in applications and they require much more work from the programmer than the mechanism introduced by Stroustrup in 1980.

With implicitly-invoked destructors, the destructor body is written once for each type of data, and normally there is no need to ever invoke it explicitly.

After writing correctly the constructors and destructors, the programmer's work becomes identical with using a garbage collector, because the objects are allocated explicitly, but they are never deallocated explicitly.

On the other hand, UNWIND is intended for handling exceptions. It can also be used as a normal cleanup strategy, but it still must be written every time for handling the exit from a block or from a hierarchy of nested blocks. In the latter variant, there is some economy in code writing, but the lazy deallocation is less efficient.

The UNWIND of MDL has little resemblance to RAII, but it resembles the UNWIND of Mesa (programming language used at Xerox, starting with 1976, which has introduced many innovations that have been included only much later in most programming languages).

It would be difficult to determine whether UNWIND has appeared first in MDL or in Mesa, or if both have taken it from another language, because experiments with exception handling were fashionable during those years and there were many places where various variants were tried.

[gumby]

No, but Tiemann might remember as I remember introducing him to unwind-protect (not that he invented RAII afaik, nor would claim to have)

[EdwardCoffin]

Smalltalk only allows the return from the block's lexically enclosing method, not an arbitrarily specifiable context like Common Lisp does. It's discussed a bit on c2.com [1]

[1] https://wiki.c2.com/?SmalltalkBlockReturn

[xkriva11]

In Pharo or Squeak, you can just send the "return:" message to any context (except contexts that have nowhere to return).

[kazinator]

swapcontext was in POSIX.1-2001; it was removed in 2008.

In relation to all this, in POSIX, there are often good reasons to use sigsetjmp and siglongjmp rather than setjmp and longjmp, because these also save and restore the signal mask. If you jump out of a context that locally disabled certain signals, you likely want them restored, like you would if that code returned normally. (It doesn't necessarily have to be a jump out of a signal handler!)