Re: "Loop or previous error loading module" - how to get the error?

[Jerome Vuarand <jerome.vuarand@...>]

2011/11/27 HyperHacker <hyperhacker@gmail.com>:
> On Sun, Nov 27, 2011 at 01:19, steve donovan <steve.j.donovan@gmail.com> wrote:
>> On Sun, Nov 27, 2011 at 10:06 AM, HyperHacker <hyperhacker@gmail.com> wrote:
>>> previous error loading module 'foo'". How can I see what the previous
>>> error was?
>>
>> By looking at the second returned value from pcall(require,'foo').
>>
>> steve d.
>>
>
> Well I meant without doing that; i.e. someone somewhere earlier in the
> program already did that pcall() and it failed and now I'm trying to
> load the module a second time and want to know what went wrong.
> Anyway, I guess the solution should have been obvious: just set
> package.loaded.foo = nil and require() will throw the original error
> again.

I may be wrong, but I believe that "require" is not behaving very well
with regards to module loaders throwing errors: these errors are not
supposed to be recoverable. You have two bits of code going on here.
First the "require" code itself, that for sure was run partially (only
whatever comes before calling the loader, not everything that comes
after). And the module loader code, that thrown the error. You can
probably look at the "require" implementation, and figure out what
needs to be "cleaned" if the loader throws an error. But you may have
to clear some things modified by the partially executed module loader
function too.

It would be nice if "require" did handle errors in the loader, and
cleaned its state itself before re-throwing the error, but that would
require a mechanism to actually re-throw a caught error, which is not
currently possible in Lua. You can catch it, clean your stuff, and
throw a new error (with the same message), but then whatever error
handler was set outside (for example if you xpcalled require with one)
will be called at the wrong spot (ie. where "require" is calling
"error"/lua_error, not at the actual error point in the module
loader).