Re: Odd behaviour with multiple return values and varargs

[Richard Warburton <richard@...>]

Tom Miles wrote:
> > > I'm not sure I understand your point.  In my mind, if I have two 
> > > functions returning 2 values, then I have 4 values plain 
> > >       
> > and simple.  
> >     
> > > It would be treated exactly the same as print(1,2,3,4).  Is there a 
> > > reason not to do it that way?
> > >       
> > Hang on, unlike other languages a function can return 
> > different number of arguments depending on input.  Now print 
> > isn't picky about arguments as it is just outputting them to 
> > std out.  However, if you had a function that required x and 
> > y for some parabolic eqn, things could get messy if x relied 
> > on a fn that returned two values when x was negative.
> >
> >
> > local calcX
> >
> > calcX = function(b)
> >     if b<0 then return calcX(-b),"Warning Negative Fixed" end
> >     else return math.sqrt(b) end
> > end
> >
> > for x=-10,10 do
> >     plot(calcX(x),10)
> > end
> >
> > You don't want returns mucking up input arguments
> >
> >
> >     
>
> I guess people are using multiple return values in entirely different
> ways.  Personally, if I have a I know how many values it returns and use
> all the values. In the above example, what is the point of returning the
> error string, as you'll never see it.
I was thinking that calcX may be part of a library that I didn't write.  
I could check for an error message, but if I'm aware of the issue and 
don't care.... 
>   You're correct in saying you
> don't want returns mucking up input arguments, but surely you should
> know what is being returned from a function in all cases and deal with
> it appropriately.
>   
Good point, but Lua was designed AFAIK to be easy for anyone to pick up 
and trying to track down a bug due to a function you didn't write 
spitting out more vars than expected due to rare input might be problematic.

Many lua functions return one value if successful and two on failure.
> I'm really beginning to think that I must be confined to a single way of
> thinking, and am missing something in the bigger picture.
>   
No it's just different.  It could also be due to:
x,y =1,2 (x=1,y=2)
x = 1,2  (2 discarded, x=1)
x,y,z = 1,2,3,4 (4 discarded)

Therefore:
print(fn(),fn()) wouldn't follow the above convention of one value per 
parameter.