Re: nil, err idiom (was Re: Quest: real world "Lua array with holes" usage)

[Michael Nelson <mikestar1313@...>]

I think the returning nil, err from a function  is useful for error 
reporting. Here nil is taken to mean "unable to produce meaningful data" 
and err is 'here is the reason why". Using false,err is not as 
good--false might be meaningful data. If false is not meaningful in the 
particular case, we can use assert(retval,err) but the more accurate 
assert(retval~=nil,err) works even when false is meaningful (at the cost 
of 5 keystrokes). IMHO, returning false,err is useful if and only if the 
function returns true,realdata  in the case of success--itself a useful 
idiom in cases where failure is common, and needed in the rare case 
where nil is valid data.  Although in that case, I would redesign the 
function if I had control of the code. I feel that treating nil as valid 
data is in general A Bad Thing (TM)....