vi / Vim: oddities of composing delete/change (d/c) with motion

Question

Let's start with investigating some motions in vi(m). I use the sample line

AA BBB

as subject of these investigations, and for each experiment we are starting off in normal mode, cursor at beginning of line. The experiments will end up also in normal mode, and I'm adding an enclosing rX action to indicate the final cursor position.

So see these four movements: tB, fB, w, 3l. The first one moves the cursor to the space preceding BBB, the others to the first B. That is,

tBrX        # AAXBBB
fBrX        # AA XBB
wrX         # AA XBB
3lrX        # AA XBB

Now let's see what the doc says about performing these moves in deletion context:

• Vim:

  d{motion}       Delete text that {motion} moves over
  

• FreeBSD vi(1) man page:

  d motion
      Delete the region of text described by ... motion
  

So these express a simple and clear concept, the composition of deletion and movement. At this point almost everything seems to be clear, except maybe whether the deletion range is inclusive or exclusive, ie. whether the position we end up by the movement is deleted or kept.

If it were exclusive, prefixing the above actions with d should just delete everything up to the X:

dtBrX       # XBBB
dfBrX       # XBB
dwrX        # XBB
d3lrX       # XBB

If it were inclusive, the position that is indicated with the X above gets also deleted, so the final position is the followup one, ie. one more character would be devoured, thus we were to get:

dtBrX       # XBB
dfBrX       # XB
dwrX        # XB
d3lrX       # XB

Now let's see how it is in real life:

dtBrX       # XBB
dfBrX       # XB
dwrX        # XBB
d3lrX       # XBB

That is, the composition is inclusive for the first two movements, but exclusive for the latter two! I wonder is there any principle that can explain this?

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Also, there is c, which is described in Vim doc as Delete {motion} text and start insert. I expect it, from the spec, that if we go back to normal mode after c: c{motion}<ESC>l, that has the same effect as d{motion} (as discussed in Why does `ESC` move the cursor back in vim?, <ESC> has the effect of moving the cursor left, so we need an l to compensate if we want to keep position through normal / insert mode roundtrips). Well, almost:

ctB<ESC>lrX # XBB
cfB<ESC>lrX # XB
cw<ESC>lrX  # XBBB
c3l<ESC>lrX # XBB

... just c combined with w has a different effect: this operation keeps even the space preceding BBB, that is, the character preceding the destination of the movement! What's the logic behind this behavior?

Answer 1

d by itself is neither inclusive nor exclusive. The motions are. Each motion's help states whether it is inclusive or exclusive:

                                                       f
f{char}                 To [count]'th occurrence of {char} to the right.  The
                        cursor is placed on {char} |inclusive|.

                                                        t
t{char}                 Till before [count]'th occurrence of {char} to the
                        right.  The cursor is placed on the character left of
                        {char} |inclusive|.
<S-Right>       or                                      <S-Right> w
w                       [count] words forward.  |exclusive| motion.

l               or                                      l
<Right>         or                                      <Right> <Space>
<Space>                 [count] characters to the right.  |exclusive| motion.

Further details are in :h inclusive.

That is, the composition is inclusive for the first two movements, but exclusive for the latter two! I wonder is there any principle that can explain this?

As can be seen, f and t are inclusive, w and l are exclusive.

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just c combined with w has a different effect: this operation keeps even the space preceding BBB.

That's a special case also mentioned in the docs. See a bit further down in :h word:

Special case: "cw" and "cW" are treated like "ce" and "cE" if the cursor is
on a non-blank.  This is because "cw" is interpreted as change-word, and a
word does not include the following white space.