Re: setfenv/getfenv
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- Subject: Re: setfenv/getfenv
- From: Nevin Flanagan <Alestane@...>
- Date: 2010年1月13日 23:49:07 -0500
On Jan 13, 2010, at 9:53 PM, Ben Kelly wrote:
> When you're done, could you pretty please explain why in-do made it in
> to 5.2, but continue didn't and neither did an error handling mechanism
> (try/catch?) less ugly than pcall? :)
This seems kind of adversarial. Have the authors ever expressed that they think either of these things are critical to the language?
Lexical environments make a real change to the semantics of the language (usefulness of that change aside). "Continue" and exception handling, while both possibly convenient and obviously desired by multiple users, would both be sugar for existing constructs as far as I understand things.
Exception handling in particular is awkward, due to Lua's extremely limited selection of actual types. You could possibly use a construction like
try errName do
code
catch errExpr1 do
handler
catch errExpr2 do
handler
end
where the various errExpr clauses would be evaluated in order with errName in scope as the value supplied to the triggering use of error(). But if the try block follows the usual conventions for blocks, we would lose the ability to pass back values that we have with pcall.
I don't have a major issue with using pcall for explicit error-catching, especially since they're adding proper arguments to xpcall in this same patch. I don't really see how the above clause would be noticeably better than
function catch(valid, ...)
if valid then return ...
elseif expr1((...)) then
handler1
elseif expr2((...)) then
handler2
end
end
other than being defined dynamically at the point of use (not an insignificant advantage, but possible to work around).