(emacs.info) Adaptive Fill

 
 Adaptive Filling
 ----------------
 
    The fill commands can deduce the proper fill prefix for a paragraph
 automatically in certain cases: either whitespace or certain punctuation
 characters at the beginning of a line are propagated to all lines of the
 paragraph.
 
    If the paragraph has two or more lines, the fill prefix is taken from
 the paragraph's second line, but only if it appears on the first line as
 well.
 
    If a paragraph has just one line, fill commands _may_ take a prefix
 from that line.  The decision is complicated because there are three
 reasonable things to do in such a case:
 
    * Use the first line's prefix on all the lines of the paragraph.
 
    * Indent subsequent lines with whitespace, so that they line up
      under the text that follows the prefix on the first line, but
      don't actually copy the prefix from the first line.
 
    * Don't do anything special with the second and following lines.
 
    All three of these styles of formatting are commonly used.  So the
 fill commands try to determine what you would like, based on the prefix
 that appears and on the major mode.  Here is how.
 
    If the prefix found on the first line matches
 `adaptive-fill-first-line-regexp', or if it appears to be a
 comment-starting sequence (this depends on the major mode), then the
 prefix found is used for filling the paragraph, provided it would not
 act as a paragraph starter on subsequent lines.
 
    Otherwise, the prefix found is converted to an equivalent number of
 spaces, and those spaces are used as the fill prefix for the rest of the
 lines, provided they would not act as a paragraph starter on subsequent
 lines.
 
    In Text mode, and other modes where only blank lines and page
 delimiters separate paragraphs, the prefix chosen by adaptive filling
 never acts as a paragraph starter, so it can always be used for filling.
 
    The variable `adaptive-fill-regexp' determines what kinds of line
 beginnings can serve as a fill prefix: any characters at the start of
 the line that match this regular expression are used.  If you set the
 variable `adaptive-fill-mode' to `nil', the fill prefix is never chosen
 automatically.
 
    You can specify more complex ways of choosing a fill prefix
 automatically by setting the variable `adaptive-fill-function' to a
 function.  This function is called with point after the left margin of a
 line, and it should return the appropriate fill prefix based on that
 line.  If it returns `nil', that means it sees no fill prefix in that
 line.
Info Catalog
(emacs.info) Fill Prefix
(emacs.info) Filling
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