On Thu, Jul 13, 2017 at 11:32 Viacheslav Usov <via.usov@gmail.com
<mailto:via.usov@gmail.com>> wrote:
On Thu, Jul 13, 2017 at 6:12 PM, Roberto Ierusalimschy
<roberto@inf.puc-rio.br <mailto:roberto@inf.puc-rio.br>> wrote:
> I hope you do not expect that the previous sentence ("Parameters
act as local variables that are initialized with the argument
values") applies also to C functions.
In fact, at some point back in the past, that is exactly what I
expected. I expected that an omitted argument would be seen as nil
in a C function. I learned the actual behaviour only when I had to
chase some subtle bugs introduced by my expectation. I did not
like the experience. That is why I called that arcane and confusing.
Another problem is that your language above implies that we
somehow must expect that setmetatable should behave like a "C
function" and not a like a Lua function. Why? I do not think
"because it is implemented in C is a sensible answer for someone
who has no idea about the implementation of Lua. And why, indeed,
should we expect that C functions behave differently? The
difference is explained very compactly at the end of section 4.3,
but it only makes sense for a C programmer, not for a Lua
programmer (a pure Lua programmer would not even look there), who
still sees there "virtual type LUA_TNONE, which behaves like a nil
value". Does that imply that every user of Lua should understand
the arcane details of Lua's C language API? I think that would be
too much to expect.
> Similarly for the next sentence ("A vararg function [...]
collects all extra arguments and supplies them to the function
through a vararg expression"). So, I see no reason to expect that
this particular sentence would apply to C functions.
In fact, the behaviour of C functions can be most easily
understood by assuming they are always vararg. Even then the
TNONE/TNIL difference remains, but at least the "adjustment" is
taken out.
Cheers,
V.
Your argument is completely valid. To be fair, it's pretty simple to
create a Lua function that errors when an argument is omitted:
(function(...) if select('#',...) <1 then error("no argument") end
return "did stuff" end)() --errors
It's much easier to do this on the C side, or at least it doesn't
require a function call to select.
I think the reason that the choice was made in this way is more of a
matter of design. The thinking might be that by making sure that you
have to use nil explicitly, you are forced to be more verbose about
your intent.
If it was done that way simply because it's a C function, then I think
that is a good argument to be made in favor of a change to the Lua
library, imho. If it's a design question, then patch away and use it
in good health!
