how to use optional flag in fish

I'm building a CLI tasks utility, (A cheap version of taskwarrior). I want to add some optional flags, such as -n

else if [ $cmd = 'delete' ]
argparse 'n/index'=? -- $argv
sed -i "$_flag_index d" ~/.tasks/data/Tdo.csv

but this gives an error

~/.tasks/run.sh (line 14): No matches for wildcard “'n/index'=?”. See help expand. argparse 'n/index'=? -- $argv

I'm unable to understand the correct usage of optional flags, and I've not been able to find enough resources, the fish shell documentation isn't sufficient for a novice in shell scripting given lack of examples.

How to accept an optional argument n/index, and further execute some code if the argument has been given, and something else otherwise, and is it possible to add integer constraints on optional arguments?

Solution

Did you help expand like fish told you?

The unquoted ? is being handled as a globbing character. Use 'n/index=?'

$ set argv --index=10

$ argparse 'n/index'=? -- $argv
fish: No matches for wildcard ''n/index'=?'. See `help expand`.
argparse --ignore-unknown 'n/index'=? -- $argv
                          ^

# quote the whole thing
#          v         v
$ argparse 'n/index=?' -- $argv

$ set -S _flag_index
$_flag_index: set in local scope, unexported, with 1 elements
$_flag_index[1]: |10|