Can function get empty arguments? - fish

Question

My code
function foo
argparse -n foo 'u/user=' 'g/group=' 's/shell=' -- $argv
bar $_flag_u $_flag_g $_flag_s
end
function bar -a user group shell
echo $user
echo $group
echo $shell
end
foo $argv
Execute and Result
> fish test.fish -u user_name -g group_name -s shell_name
user_name
group_name
shell_name
> fish test.fish -g group_name -s shell_name
group_name
shell_name
> fish test.fish -u user_name -s shell_name
user_name
shell_name
>
Expectation I want
> fish test.fish -u user_name -g group_name -s shell_name
user_name
group_name
shell_name
> fish test.fish -g group_name -s shell_name
group_name
shell_name
> fish test.fish -u user_name -s shell_name
user_name
shell_name
>
Can I get the expectation?
If -u option is not used, my expectation is that $user in bar function is empty.
(it means that result of "echo $status" is 1 after "set -q user".)
If I have no choice, I'll do bellow:
(Reference: https://github.com/fish-shell/fish-shell/issues/3926)
function foo
argparse -n foo 'u/user=' 'g/group=' 's/shell=' -- $argv
set -q _flag_u; or set _flag_u ''
set -q _flag_g; or set _flag_g ''
set -q _flag_s; or set _flag_s ''
bar $_flag_u $_flag_g $_flag_s
end
function bar -a user group shell
if test -n $user
# do something
end
if test -n $group
# do something
end
if test -n $shell
# do something
end
end
foo $argv

Answer 1

Your issue is this:
function bar -a user group shell
This means bar calls its first argument "user", the second "group" and the third "shell".
When you then run
bar $u $g $s
fish expands the variables. If they are set, they'll expand to as many arguments as they have elements (in fish, all variables are lists!).
So you'll get $u into $user, $g into $group and $s into $shell.
However, if one isn't set (because the option wasn't given in the call to foo), then the variable will expand to nothing, so if e.g. --group wasn't given, this works like
bar $u $s
which will assign the contents of $u to $user and $s to $group - because it is the second argument that bar sees.
To inhibit this, either use set -q or double-quote the variable:
bar "$_flag_u" "$_flag_g" "$_flag_s"