The command-line is not the only place where Clon looks for option
values. Other sources for option values are: fallback
values, environment variables and default values. The
existence of a fallback value, environment variable or default value
should be advertised in the application’s help string (see Output).
When Clon attempts to retrieve a value for a particular option, it
always does so in a specific order: first, the command-line is searched.
If an argument is present, it is used. Otherwise, a fallback or default
value is used in that order (note that when an option’s argument is
optional, the option is required to provide at least a fallback or a
default value). Next, an environment variable is tried (when
appropriate). Finally, when everything else fails, the option’s default
value is used (if any).
Clon always scans the command-line from left to right, and stops at the
first match. Please note that the match in question may be a regular
option call or a pack, depending on what appears first on the
command-line. There is no concept of priority amongst option forms.
Also, note that it is possible to provide several calls to the same option on a single command-line. Some applications may take advantage of this: every consecutive request for an option will use the next match on the command-line until there is none left.
Finally, note that fallback or default values don’t make any sense for flags, but flags can still be associated with environment variables. In such a case, the very existence of the variable in the environment, regardless of its value, stands for the presence of the corresponding option on the command-line.