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0 pointsadrian_b2y ago0 comments

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.

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lispm2y ago

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_bOP2y ago

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.

lispm2y ago

UNWIND-PROTECT is a building block.

Resource management in the MIT Lisp OS ca. 1980, approximate example.

One would define a resource of arrays, where arrays can be allocated and deallocated. They will be managed via a pool.

    (defresource 2d-array (rows columns)
      :constructor (make-array (list rows columns)))

Now user code would use the USING-RESOURCE macro, where it spans a dynamic scope. Entering the scope allocates the resource. Inside the scope the resource is allocated. Leaving the scope will automatically deallocate the resource and put it back into the pool. A deinitializer may free memory as needed.

    (using-resource (my-array 2d-array 100 100) ; get me a 100x100 array from a pool
      (setf (aref my-array 42 42) 'the-answer)  ; setting the array
      (print (aref my-array 42 42)))            ; reading the array

To make sure that the resource gets deallocated, the above macro form will expand to something using UNWIND-PROTECT:

    ...
     (unwind-protect                                            

       (progn                                                   ; protected form
         (setf my-array (allocate-resource '2d-array 100 100))  ;   allocate the array
         (setf (aref my-array 42 42) 'the-answer)               ;   setting the array
         (print (aref my-array 42 42)))                         ;  reading the array

       (when my-array                                           ; exit form
         (deallocate-resource '2d-array my-array)))             ;   deallocate the array
    ...

This is an example of manual memory management using a pool of resources, where the DEALLOCATE is done always via UNWIND-PROTECT.

Thus the user will not explicitly use UNWIND-PROTECT, but some macros which use it in their expansion...

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0 pointsadrian_b2y ago0 comments

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.

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"

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.

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lispm2y ago

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.

adrian_bOP2y ago

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.

lispm2y ago

UNWIND-PROTECT is a building block.

Resource management in the MIT Lisp OS ca. 1980, approximate example.

One would define a resource of arrays, where arrays can be allocated and deallocated. They will be managed via a pool.

    (defresource 2d-array (rows columns)
      :constructor (make-array (list rows columns)))

    (using-resource (my-array 2d-array 100 100) ; get me a 100x100 array from a pool
      (setf (aref my-array 42 42) 'the-answer)  ; setting the array
      (print (aref my-array 42 42)))            ; reading the array

To make sure that the resource gets deallocated, the above macro form will expand to something using UNWIND-PROTECT:

    ...
     (unwind-protect                                            

       (progn                                                   ; protected form
         (setf my-array (allocate-resource '2d-array 100 100))  ;   allocate the array
         (setf (aref my-array 42 42) 'the-answer)               ;   setting the array
         (print (aref my-array 42 42)))                         ;  reading the array

       (when my-array                                           ; exit form
         (deallocate-resource '2d-array my-array)))             ;   deallocate the array
    ...

This is an example of manual memory management using a pool of resources, where the DEALLOCATE is done always via UNWIND-PROTECT.

Thus the user will not explicitly use UNWIND-PROTECT, but some macros which use it in their expansion...

2 more replies

j / k navigate · click thread line to collapse