Author Topic: Defer on functions.  (Read 169 times)

lerno

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Defer on functions.
« on: March 22, 2019, 11:35:51 AM »
What if functions could add defer to the scope where they are invoked?

Here is some code:

Code: [Select]
func bool do_stuff(i32 resource_id)
{
  Resource *x = get_resource(resource_id);
  defer release_resource(x);
  if (!play_around_with(x)) return false;
  do_some_other_thing(x);
  return foo(x);
}

This is fine, the resource is released no matter what path is taken. However, this always requires the defer.

I envisioned a defer sugar, like this:

Code: [Select]
func bool do_stuff(i32 resource_id)
{
  Resource* x = get_resource(resource_id) @defer(release_resource);
  if (!play_around_with(x)) return false;
  do_some_other_thing(x);
  return foo(x);
}

This would sort of make it more succinct and would also make it possible to use it in an expression.

However, what if there was a "safe" version?

Code: [Select]
func Resource get_resource_with_release(i32 resource_id) @defer(release_resource)
{ ... }

This would be fully equivalent to the code with defer above, but could then be written as:

Code: [Select]
func bool do_stuff(i32 resource_id)
{
  Resource* x = get_resource_with_release(resource_id); // inserts an implicit defer!
  if (!play_around_with(x)) return false;
  do_some_other_thing(x);
  return foo(x);
}

Although resource management is a fine example of this, it's *really* nice for refcounting as if you write the following imaginary code:

Code: [Select]
Foo@ foo = @rcmalloc(sizeof(Foo)); // What is the refcount of foo after this?
foo_something(@rcmalloc(sizeof(Foo))); // Does this leak?

If @rcmalloc returns 1, then rc would be 2 in the first case (except if we have special handling of assignment of RC) and a leak on the second line.
However, if @rcmalloc returns 0, then the second line also leaks.

However, if we let @rcmalloc return rc = 1 AND have the profile of @defer(release), then an implicit defer would ensure that in the scope where called the rc would eventually be decreased (unless assigned to). And this is basically what @autorelease in ObjC does too, but in a less controlled manner.

Even if the above example doesn't make sense, or refcounting shouldn't have language support, it's still a very good way to cheaply enable manual RC built on top of the language.

 

bas

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Re: Defer on functions.
« Reply #1 on: March 24, 2019, 11:20:23 AM »
Interesting idea. I recently read an article about functional languages where one of the basic requirements
of a function call in any language was that the caller could predict its own state after the call returned. This is
not the case anymore when defers are added by the called function..

lerno

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Re: Defer on functions.
« Reply #2 on: March 24, 2019, 04:21:54 PM »
That's why the defer is part of the function signature and not of the function itself, acting as a decorator macro.

It has some interesting consequences: what about the function pointer?

We could also consider this even simpler by introducing a qualifier, eg

Code: [Select]
// func const FILE open(...) { ... } =>
func const managed FILE open(...) { ... }

And here any struct that is "managed" must have a "StructName.release(StructName *struct)" call.

There are various ways to do it actually.