Auto Fill mode is a minor mode that fills lines automatically as text is inserted. This section describes the hook used by Auto Fill mode. For a description of functions that you can call explicitly to fill and justify existing text, see Filling.
Auto Fill mode also enables the functions that change the margins and justification style to refill portions of the text. See Margins.
The value of this variable should be a function (of no arguments) to be
called after self-inserting a space or a newline. It may be nil,
in which case nothing special is done in that case.
The value of auto-fill-function is do-auto-fill when
Auto-Fill mode is enabled. That is a function whose sole purpose is to
implement the usual strategy for breaking a line.
In older Emacs versions, this variable was named
auto-fill-hook, but since it is not called with the standard convention for hooks, it was renamed toauto-fill-functionin version 19.