The last required step to complete our new option type extension is to
provide a constructor function that wraps around
make-instance on the corresponding option class. I won’t insult
you by explaining how to write a constructor. Let me just give four good
reasons why providing constructors is important.
Providing a constructor for every new option type is important because:
defsynopsis and defgroup (see Synopsis Items
and Group Definition).
Let’s look at the enumeration example now.
(defun make-enum (&rest keys
&key short-name long-name description
argument-name argument-type
enum env-var fallback-value default-value
hidden)
"Make a new enum option.
- SHORT-NAME is the option's short name (without the dash).
It defaults to nil.
- LONG-NAME is the option's long name (without the double-dash).
It defaults to nil.
- DESCRIPTION is the option's description appearing in help strings.
It defaults to nil.
- ARGUMENT-NAME is the option's argument name appearing in help strings.
- ARGUMENT-TYPE is one of :required, :mandatory or :optional (:required and
:mandatory are synonyms).
It defaults to :optional.
- ENUM is the set of possible values.
- ENV-VAR is the option's associated environment variable.
It defaults to nil.
- FALLBACK-VALUE is the option's fallback value (for missing optional
arguments), if any.
- DEFAULT-VALUE is the option's default value, if any.
- When HIDDEN, the option doesn't appear in help strings."
(declare (ignore short-name long-name description
argument-name argument-type
enum env-var fallback-value default-value
hidden))
(apply #'make-instance 'enum keys))
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Woah, what a mouthful for a single line of code… Yeah, I’m a maniac and I like redundancy. I always restate all the available keys explicitely, and everything again in the docstring so that all the interesting information is directly available (I might change my mind as I grow older though).