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<close> is equivalent to golang's defer (either can be implemented in terms of the other) except at the block level. imo calling it RAII mostly leads to confusion. One mailing list user recently asked a bunch of questions around using <close> for RAII. They expected RAII to work even for resources that are never bound to a particular type of local variable, for example if one writes do_thing(io.open("foo.txt")), where do_thing is not responsible for closing files because sometimes it is used with files that will continue to be used. They eventually concluded that the closest thing to RAII available was __gc.

Some users presented separate complaints about resources kept in to-be-closed locals in coroutines not necessarily being closed. You can do something to try to solve this, like setting the __gc and __close metamethods of coroutines to call coroutine.close. A "real" solution would look more like dynamic-wind. Notably golang doesn't attempt to solve this one, so maybe not solving it is fine.






> <close> is equivalent to golang's defer [...] except at the block level.

Does that mean it’s the same as C#'s `using` and Python's `with`?


I wasn't deeply familiar with python's `with`, so I looked it up[0]

<close> differs from `with` in at least the sense that it doesn't have any equivalent to __enter__ and doesn't create its own block. It creates a local variable whose __close metamethod will be called when it goes out of scope. Since Lua has lexical scope at the block level rather than the function level, this works similarly to the way Python calls __exit__.

These snippets of Python and Lua are roughly equivalent. They both open a file, read one line, and close it, or do nothing if there was some error opening the file.

  try:
    with open("foo.txt") as f:
      print(f.readline())
    # f is now closed.
  except:
    pass

  local f,err = io.open("foo.txt")
  if not err then
    local f <close> = f
    print(f:read("*l"))
  end
  -- f is now closed.
C#'s `using`[1] seems much closer, except that it handles nulls by not trying to close them and lua's <close> does not include any such handling.

[0]: https://docs.python.org/3/reference/compound_stmts.html#the-...

[1]: https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...


The reference manual https://www.lua.org/manual/5.4/manual.html#3.3.8 seems to specify that "nil" and "false" values are ignored, so it behaves similarly to C#'s using.

Oh, that's great!

Do file descriptors come with a default __close metamethod or do you have to create your own?

They come with one.



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