stack level parameter

[David Manura <dm.lua@...>]

On Sat, Jan 9, 2010 at 4:34 PM, Cosmin Apreutesei wrote: [1]
> I always felt that the stack level parameter in
> setfenv() and error() is just a hack, and I'm happy to see it go away.

I agree with the sentiment: Whenever I add a level parameter to an
error to setfenv/getfenv, it usually feels fragile or potentially
wrong.  How does code know the appropriate level parameter of its
caller?  For example, the setfenv in package.clean [2] makes an
implementation assumption of stack levels inside the module function.
It gets more complicated if a function can be called from multiple
levels (e.g. directly or from a wrapper function).  There's also a
theoretical dilemma: given a function f, you can't necessary define a
function g that wraps f and behaves identically [3], though you may if
f exposes a level parameter as an optional argument.  Idioms like "f =
assert(io.open(.....))" don't attempt to provide a level parameter,
but you can localize a redefinition of assert to do so.

There are mechanisms, not out-of-the-box, one can devise to deal with
it, and early on I had some experiments with LuaCarp a la Perl Carp
and then [3], but unfortunately I don't have a solution in mind that I
can with certainty say works really well in practice in all use cases,
tail calls, and code by different authors and that I might suggest for
5.2.

[1] http://lua-users.org/lists/lua-l/2010-01/msg00420.html
[2] http://lua-users.org/wiki/ModuleDefinition
[3] http://lua-users.org/lists/lua-l/2009-09/msg00437.html
     http://lua-users.org/lists/lua-l/2009-09/msg00384.html