GNU Emacs Manual - Fill Prefix

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### The Fill Prefix

To fill a paragraph in which each line starts with a special marker (which might be a few spaces, giving an indented paragraph), you can use the fill prefix feature. The fill prefix is a string that Emacs expects every line to start with, and which is not included in filling. You can specify a fill prefix explicitly; Emacs can also deduce the fill prefix automatically (see section Adaptive Filling).

`C-x .`
  Set the fill prefix (`set-fill-prefix`).
`M-q`
  Fill a paragraph using current fill prefix (`fill-paragraph`).
`M-x fill-individual-paragraphs`
  Fill the region, considering each change of indentation as starting a new paragraph.
`M-x fill-nonuniform-paragraphs`
  Fill the region, considering only paragraph-separator lines as starting a new paragraph.

To specify a fill prefix, move to a line that starts with the desired prefix, put point at the end of the prefix, and give the command `C-x .` (`set-fill-prefix`). That's a period after the `C-x`. To turn off the fill prefix, specify an empty prefix: type `C-x .` with point at the beginning of a line.

When a fill prefix is in effect, the fill commands remove the fill prefix from each line before filling and insert it on each line after filling. Auto Fill mode also inserts the fill prefix automatically when it makes a new line. The `C-o` command inserts the fill prefix on new lines it creates, when you use it at the beginning of a line (see section Blank Lines). Conversely, the command `M-^` deletes the prefix (if it occurs) after the newline that it deletes (see section Indentation).

For example, if `fill-column` is 40 and you set the fill prefix to ``;; '`, then `M-q` in the following text

    ;; This is an
    ;; example of a paragraph
    ;; inside a Lisp-style comment.

produces this:

    ;; This is an example of a paragraph
    ;; inside a Lisp-style comment.

Lines that do not start with the fill prefix are considered to start paragraphs, both in `M-q` and the paragraph commands; this gives good results for paragraphs with hanging indentation (every line indented except the first one). Lines which are blank or indented once the prefix is removed also separate or start paragraphs; this is what you want if you are writing multi-paragraph comments with a comment delimiter on each line.

You can use `M-x fill-individual-paragraphs` to set the fill prefix for each paragraph automatically. This command divides the region into paragraphs, treating every change in the amount of indentation as the start of a new paragraph, and fills each of these paragraphs. Thus, all the lines in one "paragraph" have the same amount of indentation. That indentation serves as the fill prefix for that paragraph.

`M-x fill-nonuniform-paragraphs` is a similar command that divides the region into paragraphs in a different way. It considers only paragraph-separating lines (as defined by `paragraph-separate`) as starting a new paragraph. Since this means that the lines of one paragraph may have different amounts of indentation, the fill prefix used is the smallest amount of indentation of any of the lines of the paragraph. This gives good results with styles that indent a paragraph's first line more or less that the rest of the paragraph.

The fill prefix is stored in the variable `fill-prefix`. Its value is a string, or `nil` when there is no fill prefix. This is a per-buffer variable; altering the variable affects only the current buffer, but there is a default value which you can change as well. See section Local Variables.

The `indentation` text property provides another way to control the amount of indentation paragraphs receive. See section Indentation in Formatted Text.

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