The :inline feature lets you splice a variable number of
elements into the middle of a list or vector
customization type. You use it by adding :inline t to a type
specification which is contained in a list or vector
specification.
Normally, each entry in a list or vector type
specification describes a single element type. But when an entry
contains :inline t, the value it matches is merged directly
into the containing sequence. For example, if the entry matches a
list with three elements, those become three elements of the overall
sequence. This is analogous to ‘,@’ in a backquote construct
(see Backquote).
For example, to specify a list whose first element must be baz
and whose remaining arguments should be zero or more of foo and
bar, use this customization type:
(list (const baz) (set :inline t (const foo) (const bar)))
This matches values such as (baz), (baz foo), (baz bar)
and (baz foo bar).
When the element-type is a choice, you use :inline not
in the choice itself, but in (some of) the alternatives of the
choice. For example, to match a list which must start with a
file name, followed either by the symbol t or two strings, use
this customization type:
(list file
(choice (const t)
(list :inline t string string)))
If the user chooses the first alternative in the choice, then the
overall list has two elements and the second element is t. If
the user chooses the second alternative, then the overall list has three
elements and the second and third must be strings.
The widgets can specify predicates to say whether an inline value
matches the widget with the :match-inline element.