Filling

Filling means adjusting the lengths of lines (by moving the line breaks) so that they are nearly (but no greater than) a specified maximum width. Additionally, lines can be justified, which means inserting spaces to make the left and/or right margins line up precisely. The width is controlled by the variable fill-column. For ease of reading, lines should be no longer than 70 or so columns.

You can use Auto Fill mode (see Auto Filling) to fill text automatically as you insert it, but changes to existing text may leave it improperly filled. Then you must fill the text explicitly.

Most of the commands in this section return values that are not meaningful. All the functions that do filling take note of the current left margin, current right margin, and current justification style (see Margins). If the current justification style is none, the filling functions don't actually do anything.

Several of the filling functions have an argument justify. If it is non-nil, that requests some kind of justification. It can be left, right, full, or center, to request a specific style of justification. If it is t, that means to use the current justification style for this part of the text (see current-justification, below). Any other value is treated as full.

When you call the filling functions interactively, using a prefix argument implies the value full for justify.

— Command: fill-paragraph &optional justify region

This command fills the paragraph at or after point. If justify is non-nil, each line is justified as well. It uses the ordinary paragraph motion commands to find paragraph boundaries. See Paragraphs.
When region is non-nil, then if Transient Mark mode is enabled and the mark is active, this command calls fill-region to fill all the paragraphs in the region, instead of filling only the current paragraph. When this command is called interactively, region is t.

— Command: fill-region start end &optional justify nosqueeze to-eop

This command fills each of the paragraphs in the region from start to end. It justifies as well if justify is non-nil.
If nosqueeze is non-nil, that means to leave whitespace other than line breaks untouched. If to-eop is non-nil, that means to keep filling to the end of the paragraph—or the next hard newline, if use-hard-newlines is enabled (see below).
The variable paragraph-separate controls how to distinguish paragraphs. See Standard Regexps.

— Command: fill-individual-paragraphs start end &optional justify citation-regexp

This command fills each paragraph in the region according to its individual fill prefix. Thus, if the lines of a paragraph were indented with spaces, the filled paragraph will remain indented in the same fashion.
The first two arguments, start and end, are the beginning and end of the region to be filled. The third and fourth arguments, justify and citation-regexp, are optional. If justify is non-nil, the paragraphs are justified as well as filled. If citation-regexp is non-nil, it means the function is operating on a mail message and therefore should not fill the header lines. If citation-regexp is a string, it is used as a regular expression; if it matches the beginning of a line, that line is treated as a citation marker.
Ordinarily, fill-individual-paragraphs regards each change in indentation as starting a new paragraph. If fill-individual-varying-indent is non-nil, then only separator lines separate paragraphs. That mode can handle indented paragraphs with additional indentation on the first line.

— User Option: fill-individual-varying-indent

This variable alters the action of fill-individual-paragraphs as described above.

— Command: fill-region-as-paragraph start end &optional justify nosqueeze squeeze-after

This command considers a region of text as a single paragraph and fills it. If the region was made up of many paragraphs, the blank lines between paragraphs are removed. This function justifies as well as filling when justify is non-nil.
If nosqueeze is non-nil, that means to leave whitespace other than line breaks untouched. If squeeze-after is non-nil, it specifies a position in the region, and means don't canonicalize spaces before that position.
In Adaptive Fill mode, this command calls fill-context-prefix to choose a fill prefix by default. See Adaptive Fill.

— Command: justify-current-line &optional how eop nosqueeze

This command inserts spaces between the words of the current line so that the line ends exactly at fill-column. It returns nil.
The argument how, if non-nil specifies explicitly the style of justification. It can be left, right, full, center, or none. If it is t, that means to follow specified justification style (see current-justification, below). nil means to do full justification.
If eop is non-nil, that means do only left-justification if current-justification specifies full justification. This is used for the last line of a paragraph; even if the paragraph as a whole is fully justified, the last line should not be.
If nosqueeze is non-nil, that means do not change interior whitespace.

— User Option: default-justification

This variable's value specifies the style of justification to use for text that doesn't specify a style with a text property. The possible values are left, right, full, center, or none. The default value is left.

— Function: current-justification

This function returns the proper justification style to use for filling the text around point.
This returns the value of the justification text property at point, or the variable default-justification if there is no such text property. However, it returns nil rather than none to mean “don't justify”.

— User Option: sentence-end-double-space

If this variable is non-nil, a period followed by just one space does not count as the end of a sentence, and the filling functions avoid breaking the line at such a place.

— User Option: sentence-end-without-period

If this variable is non-nil, a sentence can end without a period. This is used for languages like Thai, where sentences end with a double space but without a period.

— User Option: sentence-end-without-space

If this variable is non-nil, it should be a string of characters that can end a sentence without following spaces.

— User Option: fill-separate-heterogeneous-words-with-space

If this variable is non-nil, two words of different kind (e.g., English and CJK) will be separated with a space when concatenating one that is in the end of a line and the other that is in the beginning of the next line for filling.

— Variable: fill-paragraph-function

This variable provides a way to override the filling of paragraphs. If its value is non-nil, fill-paragraph calls this function to do the work. If the function returns a non-nil value, fill-paragraph assumes the job is done, and immediately returns that value.
The usual use of this feature is to fill comments in programming language modes. If the function needs to fill a paragraph in the usual way, it can do so as follows:
          (let ((fill-paragraph-function nil))
            (fill-paragraph arg))

— Variable: fill-forward-paragraph-function

This variable provides a way to override how the filling functions, such as fill-region and fill-paragraph, move forward to the next paragraph. Its value should be a function, which is called with a single argument n, the number of paragraphs to move, and should return the difference between n and the number of paragraphs actually moved. The default value of this variable is forward-paragraph. See Paragraphs.

32.11 Filling