Re: How to make sure some operations in ANY situation? (aka: with statement in Python)

["Drake Wilson" <drake@...>]

On Mon, 20 Jun 2011 11:25 -0300, "Roberto Ierusalimschy" <roberto@inf.puc-rio.br> wrote:
> So, to make sure that some operations MUST happen just put them out of
> the 'if':
> 
>   if pcall(foo) then
>     <error handling code>
>   end
>   <MUST happen code>

This is neat, simple, and subtly wrong (which is perhaps why other
languages have unwind-protect et al. in the first place).

  function risky_operation() ... end
  function handle_error(err)
    local processed = gyrate_and_maybe_oom(err)
    unreliable_disk_io(processed)
  end

  local ok, x = pcall(risky_operation)
  if not ok then handle_error(x) end
  -- Last call doesn't always happen!
  wait_a_minute_what_about_double_faults()

So instead the translation looks more like:

  local ok, x = pcall(risky_operation)
  -- Eats some double faults, but often okay.  Can also do
  -- plain cleanup_everything() to have cleanup errors squash
  -- operation errors and be propagated further, or other
  -- fancier tricks...
  pcall(cleanup_everything)
  if not ok then handle_error(x); return end
  all_righty_then(x)

   ---> Drake Wilson