Re: Odd behaviour with multiple return values and varargs

[David Jones <drj@...>]

On 13 Sep 2007, at 23:33, Adrien de Croy wrote:

> I wonder if there's some reason no other language (that I know of  
> anyway) allows more than one return value for a function.
Common Lisp?
> Maybe because it's not such a great idea to have a non- 
> deterministic number of return values?
> Also, surely if Lua is going to validate the number of input  
> parameters to a function, it should validate the number of return  
> values?
Lua doesn't validate the number of input parameters to a function.   
In fact Lua, _unlike_ most other languages, acts very symmetrically  
with regard to calling and returning.  After all, returning is just  
calling the continuation, so the two cases _should_ be treated the same.
In Lua any number of arguments can be passed to any function  
regardless of how many it expects.  Excess arguments are discarded, a  
shortfall is made up with nils.  There is a mechanism for arbitrary  
numbers of arguments.
Any number of results can be returned from a function, regardless of  
how many are required.  Excess results are discarded, a shortfall is  
made up with nils.  There is a mechanism for arbitrary numbers of  
results.
> I'm not dead against more than one return value, but I think it's a  
> terrible idea to have a variable number of return values (esp  
> dependent on input).  How on earth can a caller know how many  
> return values there actually were?
> If a function wants to return lots of values, it should return a  
> table (which at least then can be iterated). At least then the  
> stack doesn't grow (or does lua clean up unused return values?).
> e.g
>
> for i in 1, 1000000 do
>    c = funcreturning2values()
> end
>
> unless lua pops unused return values off the stack, you'll have a  
> stack size of 10000000 items by the end of this.
This is pure speculation.  You haven't looked at the implementation.   
It doesn't do that.  That would be silly.
drj