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GNU Emacs Lisp Reference Manual

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14.4.3 Splicing into Lists

The :inline feature lets you splice a variable number of elements into the middle of a list or vector. You use it in a set, choice or repeat type which appears among the element-types of a list or vector.

Normally, each of the element-types in a list or vector describes one and only one element of the list or vector. Thus, if an element-type is a repeat, that specifies a list of unspecified length which appears as one element.

But when the element-type uses :inline, the value it matches is merged directly into the containing sequence. For example, if it matches a list with three elements, those become three elements of the overall sequence. This is analogous to using `,@' in the backquote construct.

For example, to specify a list whose first element must be t and whose remaining arguments should be zero or more of foo and bar, use this customization type:

(list (const t) (set :inline t foo bar))

This matches values such as (t), (t foo), (t bar) and (t 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.

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