Re: Functions Without Returns

[John Calsbeek <nexuapex@...>]

How about the semantics of the following?

function multireturn(num)
     if num == 0
        return
     elseif num == 1
        return 1
     elseif num == 2
        return 1, 2
    end
end

function multiparam(...)
    print(select("#", ..))
end

multiparam(multireturn(0))

That would print 1, if an empty return is coerced to a nil return.

-John

On Jan 25, 2008, at 12:16 PM, Ken Smith wrote:

> I ran into a bit of Lua behavior I'd like to discuss in the forum.
> Consider this example.  (Tested under Lua 5.1.3 on Mac OS X.)
>
> cat<<EOF>test.lua
> function nothing()
> end
>
> local result = nothing()
>
> print(tostring(result))
>
> print(tostring(nothing()))
> EOF
>
> The first print yields, "nil", whereas the second one causes the
> following error.
>
> lua: test.lua:8: bad argument #1 to 'tostring' (value expected)
> stack traceback:
> 	[C]: in function 'tostring'
> 	test.lua:8: in main chunk
> 	[C]: ?
>
> The manual is clear that a function "without...a return
> statement...returns with no results" (2.5.9).  But then the discussion
> of nil mentions that it "usually represents the absence of a useful
> value" (2.2).
>
> My expectation was that a function with no return would return no
> results in the form of a nil.  Granted there is a conundrum, if a
> function returns nil, then it returns a result.
>
> The way I see it, a function without a return returns no values which
> introduces another, distinct nil concept into the Lua runtime.  There
> is nil, which is a placeholder value meaning, "different from all
> other values", then there is true nil as is generated by a function
> with no return which is the true absence of any value.
>
> Clearly, the workarounds are unobtrusive and numerous.  However, I
> find this curious, so I'd like to hear a few more opinions on this
> issue.  Should a function with no return statement return 'nil', or
> nothing?  Are they different semantically?