Best way to source a file or directory after null checking

example.sh

#
# Example ternary to source a directory into PATH if it exists
#
[ -d $HOME/custom/dir ] && export PATH=$PATH:${HOME}/custom/dir

#
# Better way to source a path into the global environment through function calls
#
add_to_path() { [ -d "$1" ] && [[ ":$PATH:" != *":$1:"* ]] && export PATH="$1:$PATH"; }

#
# Better way to source a bash config
#
resource() { [ -f "$1" ] && source "$1"; }

Finish with:

unset -f add_to_path resource