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4.4.1 Explicit Retrieval

Since Clon lets you retrieve options on demand (at any time), it makes sense to be able to request the value of a specific option explicitely. For instance, you might want to try the ‘--help’ option first, without looking at the rest of the command-line because the application will in fact quit immediately after having displayed the help string.

Function: getopt :KEY VALUE… ¶

Retrieve the value of a specific option. The following :KEYs are currently available.

:short-name

:long-name ¶

Use one of these 2 keys to specify the name of the option you wish to retrieve.

:option ¶

Alternatively, you can use a reference to an option object (see Constructors).

This function return two values: the option’s value and the value’s source (see Value Sources in The Clon End-User Manual).

The value’s source may have the following forms:

(:cmdline NAME) ¶

This is for options found on the command-line. NAME is the name used on the command-line. It can be the option’s long or short name, or a completed long name if the option’s name was abbreviated. A completed name displays the omitted parts in parentheses ("he(lp)" for instance).

(:fallback NAME) ¶

The same but when the fallback value is used, that is, when an option is not provided with its (optional) argument.

(:default NAME) ¶

The same, but when the default value is used (because there is no fallback).

(:environment VAR) ¶

This is for options not found on the command-line but for which there is an associated environment variable set in the application’s environment. VAR is the name of the corresponding environment variable.

:default ¶

This is for options not found anywhere, but for which a default value was provided.

Note that because flags don’t take any argument, getopt returns a virtual value of t when they are found or a corresponding environment variable exists in the environment. For the same reason, a flag’s value source may ony be (:cmdline NAME) or (:environment VAR).

When an option is not found anywhere and there is no default value, getopt just returns nil (no second value). Also, note that when your option accepts nil as a value, you need to handle the second retrun value to make the difference between an option not found, and an actual value of nil.