Add a directory-specific function path for fish using direnv

.envrc

# Note that this script runs in Bash!

# This appends the the subdirectory ./scripts/fish/functions to the $fish_function_path in the environment
# set up by direnv.

realpath.dirname() { REPLY=.; ! [[ $1 =~ /+[^/]+/*$|^//$ ]] || REPLY="${1%${BASH_REMATCH[0]}}"; REPLY=${REPLY:-/}; echo $REPLY; }

repo_fish_functions="$(realpath.dirname "$@")/scripts/fish/functions"

# Load fish shell and export its fish_function_path variable, appending the scripts one
# Note: This is exported as a colon separated string rather than an array (array exports are not a thing).
#       In order to load it properly in fish, you need to tell fish that it is a path variable by adding
#       the following to your config.fish:
#
#       We also need to export it so that direnv can restore it properly once done.
#
#       set -ax --path fish_function_path
#
direnv_load fish -lc 'set -ax --path fish_function_path "'$repo_fish_functions'"; export fish_function_path; direnv dump'

# Other commands go here, e.g.
use nix