Test if variable name is not set [duplicate] - fish

Question

How can I do something like
set variable (some_command_that_may_return_a_string)
if [ variable is set ]
do_something
and inversely how do I check if the variable is empty?

Answer 1

set -q var (note the missing "$" - this uses the variable name) can be used to check if a variable has been set.
set -q var[1] can be used to check whether the first element of a variable has been assigned (i.e. whether it is non-empty as a list).
test -n "$var" [fn0] (or [ -n "$var" ]) can be used to check whether a variable expands to a non-empty string (and test -z is the inverse - true if it is empty).
These will be true/false in slightly different circumstances.
When no set var has been performed at all (and it has not been inherited from the parent process), set -q var, set -q var[1] and test -n "$var" will be false, test -z "$var" will be true.
When something like set var has been done (without any additional arguments), set -q var will be true, set -q var[1] will be false.
When something like set var "" has been done, both set versions will be true.
When something like set var "somestring" (or even set var "" "" [fn1]) has been done, the sets will be true and test -z "$var" will be false.
[fn0]: You never want to use test (or [) without quoting the variable. One particularly egregious example is that test -n $var will return true both if the variable contains something and if it is list-empty/unset (no set at all or set var without arguments). This is because fish's test is one of the few parts that follow POSIX, and that demands that test with any one argument be true. Also it does not handle lists properly - test -n $var will have weird results if var has more than one element.
[fn1]: This is because a list will be expanded as a string by joining the elements with spaces, so the list consisting of two empty strings will expand to " " - one space. Since that isn't empty, test -z returns false.

Answer 2

#faho provided an awesome answer but I thought it would be easier if there's a code example.
function if-test-no-arg
set var
if set -q var
# true
end
if set -q var[1]
# false
end
# Always put quotation marks
if test -n "$var"
# false
end
if test -z "$var"
# true
end
end
function if-test-empty-arg
set var ""
if set -q var
# true
end
if set -q var[1]
# true
end
if test -n "$var"
# false
end
if test -z "$var"
# true
end
end
function if-test-valid-arg
set var "hello"
if set -q var
# true
end
if set -q var[1]
# true
end
if test -n "$var"
# true
end
if test -z "$var"
# false
end
end
function if-test-no-set
if set -q var
# false
end
if set -q var[1]
# false
end
if test -n "$var"
# false
end
if test -z "$var"
# true
end
end

Answer 3

The modern version of fish shell (3.5.1) has string commands.
string length --quiet $var checks that the variable is not empty
not string length --quiet $var checks that the variable is empty