Bind visual mode 'I' and 'A' to always use visual block mode before inserting

Question

Is there a way to bind I and A in visual mode and visual line mode to switch the selected region to a visual block selection with the same boundaries?

I'd like to be able to use visual line mode to select a group of lines and then use I or A to add text before or after all of the lines.

For instance, in the following file (with line numbers)

  1 a
  2 b
  3 c
  4 d
  5 e
  6 f
  7 g
  8 h

The sequence <esc> gg V G I # <space> <esc> produces

  1 # a
  2 b
  3 c
  4 d
  5 e
  6 f
  7 g
  8 h

and I'd like it to produce this, just as visual block would.

  1 # a
  2 # b
  3 # c
  4 # d
  5 # e
  6 # f
  7 # g
  8 # h

I've tried using the following keybinding to switch to visual block mode right before inserting in every mode except visual block, but it loses the selection region.

vnoremap <expr> I   mode()=~?'\cv' ? 'I' : '<c-v>I'
vnoremap <expr> A   mode()=~?'\cv' ? 'A' : '<c-v>A'

Answer 1

Your attempt was close, but there are two problems I see:

1. I'm sure you already know this, since you mentioned it in your comment, but mode() will return <C-v> in visual block mode, not \cv.

2. Pressing <C-v> from visual line mode will always leave the cursor where it is. And <C-v>I will not put the text at the beginning of the line, but right before the cursor. Similarly with <C-v>A. So you'll want to use either 0 or $ to make sure your cursor is where you want it.

This should do what you're looking for:

vnoremap <expr> I mode()=~? '<C-v>' ? 'I' : '<c-v>0I'
vnoremap <expr> A mode()=~? '<C-v>' ? 'A' : '<c-v>$A'