Re: try..catch for Lua 5.2?

[David Given <dg@...>]

On 13/09/10 14:43, steve donovan wrote:
[...]
> And also that a badly thought-out language feature is a backwards step
> - try..catch is not as easy as it looks (as Duncan observes) and in
> fact I found that it is not possible to do it with a token filter
> precisely because an explicit 'return nil' has very different
> semantics from simply ending the block.

I managed to make this work for Objective Lua's @try...@catch. It was
horrible. What it does is:

- return inside a try/catch block returns the arguments;
- exiting inside a try/catch block returns a special {} value;
- we test what the pcall returns to see whether it's the special {}
value, and if it is, it returns from the enclosing function.

So it's really slow and clunky (but works).

In addition, I've discovered that in an untyped language, exceptions are
rather awkward to use, because you can't do this:

@try
  ...
@catch (TypeOfException e)
  ...
@catch (AnotherTypeOfException e)
  ...
@end

Instead you have to do runtime type checks:

@try
  ...
@catch (e)
  if e:IsTypeOfException() then
    ...
  elseif e:IsAnotherTypeOfException() then
    ...
  else
    @throw e
  end
@end

...except it's even worse, because you need to check to make sure e
really is an exception before you can call methods on it! (Because Lua
doesn't have a formal exception system, code tends to throw strings or
arbitrary tables instead of well-formed exception objects.) Javascript
suffers from exactly the same problem, which is why they're not used
much there. At least Javascript has instanceof, which helps.

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