Info Catalog elisp: Fields elisp: Text Properties

elisp: Not Intervals

 
 31.19.10 Why Text Properties are not Intervals
 ----------------------------------------------
 
 Some editors that support adding attributes to text in the buffer do so
 by letting the user specify intervals within the text, and adding the
 properties to the intervals.  Those editors permit the user or the
 programmer to determine where individual intervals start and end.  We
 deliberately provided a different sort of interface in Emacs Lisp to
 avoid certain paradoxical behavior associated with text modification.
 
    If the actual subdivision into intervals is meaningful, that means
 you can distinguish between a buffer that is just one interval with a
 certain property, and a buffer containing the same text subdivided into
 two intervals, both of which have that property.
 
    Suppose you take the buffer with just one interval and kill part of
 the text.  The text remaining in the buffer is one interval, and the
 copy in the kill ring (and the undo list) becomes a separate interval.
 Then if you yank back the killed text, you get two intervals with the
 same properties.  Thus, editing does not preserve the distinction
 between one interval and two.
 
    Suppose we attempt to fix this problem by coalescing the two
 intervals when the text is inserted.  That works fine if the buffer
 originally was a single interval.  But suppose instead that we have two
 adjacent intervals with the same properties, and we kill the text of one
 interval and yank it back.  The same interval-coalescence feature that
 rescues the other case causes trouble in this one: after yanking, we
 have just one interval.  Once again, editing does not preserve the
 distinction between one interval and two.
 
    Insertion of text at the border between intervals also raises
 questions that have no satisfactory answer.
 
    However, it is easy to arrange for editing to behave consistently for
 questions of the form, “What are the properties of text at this buffer
 or string position?” So we have decided these are the only questions
 that make sense; we have not implemented asking questions about where
 intervals start or end.
 
    In practice, you can usually use the text property search functions
 in place of explicit interval boundaries.  You can think of them as
 finding the boundaries of intervals, assuming that intervals are always
 coalesced whenever possible.  Property Search.
 
    Emacs also provides explicit intervals as a presentation feature; see
 Overlays.
 

Info Catalog elisp: Fields elisp: Text Properties