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- Subject: no return value vs. nil return value(s)
- From: Lorenzo Donati <lorenzodonatibz@...>
- Date: Sun, 11 Sep 2011 16:47:34 +0200
I know that it is possible to differentiate between a function returning
no value and a function returning nil(s):
function f() return end
function g() return nil end
select('#',f()) --> 0
select('#',g()) --> 1
I'd like to know two things:
(1) is it safe to rely on such behaviour? I.e. is it intentional or is
it only an implementation detail that may change at any time, even with
minor releases of Lua? Lua 5.2 still complies, but haven't found a clear
reference to this behaviour in the manual.
I only found:
"Results are returned using the return statement (see §2.4.4). If
control reaches the end of a function without encountering a return
statement, then the function returns with no results. "
"The return statement is used to return values from a function or a
chunk (which is just a function). Functions and chunks can return more
than one value, and so the syntax for the return statement is
stat ::= return [explist]
But it is not clear that "return" with an empty explist is the same as
returning with no return statement (currently it behaves that way, but
can we rely on that?)
(2) assuming that (1) is intentional and stable, I'd like to pass a
function (call it "func") to an higher order function that will call it
many times and do different things according to whether func returned no
values instead of only nil values.
How can I detect the number of returned values of "func" without calling
I only came up with this solution (here simplified), but it seems rather
inefficient, since it forces an additional call to an helper function:
function higher( func )
local function helper( ... )
nrets = select( '#', ... )
local ret1, ret2 = helper( func() )
if nrets == 0 then
print "func returned no values"
print "func returned something:"
print( ret1, ret2 )
Is there a simpler idiom to avoid the call to "helper"?
Any hints or pointer appreciated!