[Gretl-devel] Re: Optional arguments (in user-defined hansl functions) and nested function calls
On Tue, Mar 21, 2023 at 6:43 PM Sven Schreiber
<sven.schreiber(a)fu-berlin.de> wrote:
Am 21.03.2023 um 19:51 schrieb Cottrell, Allin:
One more go on this...
On Tue, Mar 21, 2023 at 1:53 PM Sven Schreiber
<sven.schreiber(a)fu-berlin.de> wrote:
<hansl>
function void level2 (string s2)
eval exists(s) ? s : "nothing"
end function
[ BTW note confusion between s2 and s here ]
Yes, that was a typo, sorry. In this example it is inconsequential, however, because the
point is what happens when "null" is passed in.
Not totally inconsequential, in that passing "null" as an argument
keyword would automatically fail for level2() as written, since a
missing argument is not permitted by the function's signature.
The second null-setting is never necessary. Revised version:
...
function void level1 (string s1[null])
level2(exists(s1) ? s1 : null)
end function
This was in fact also my first version of the example, but then I decided to make it
visually more explicit. I mean what you do there in the ternary statement is also a second
and explicit null-setting, that's why you have to write the keyword "null"
again. So I don't think this serves as a valid counter-example. Or am I missing your
point?
Well, I didn't take my example as a case of "null-setting" (as in with
your string, where you created a named, empty string variable). My
case just uses "null" as a keyword, meaning "don't pass any argument
corresponding to this parameter". If one wanted something less terse
than the ternary expression, the equivalent would be
if exists(s1)
level2(s1)
else
level2()
endif
Let me give you another example which might be more realistic and
thus perhaps more useful; it's based on an options bundle that may or may not be
provided by the caller:
<hansl>
function void level1 (bundle opts[null])
bundle defaults = _(x=5)
bcheck(&defaults, opts) # causes an error in this example
end function
level1()
</hansl>
Sorry, I don't really understand this example. If opts is not supplied
there's nothing to be checked, so one should simply do
if exists(opts)
bcheck(&defaults, opts)
endif
Moreover, the second argument to bcheck() is not optional, so passing
null as a substitute is not going to work. If in context the opts
bundle is supposed to contain at least one required key + setting,
then the function whose role is to pass opts to bcheck() should not
permit null in place of opts.
However, thinking about the 3-argument usage of bcheck(), perhaps I'm
beginning to see what you mean. I need to think about it some more.
Allin