remap ':' to maintain last command, and erase all char if any key other than <CR> was pressed

Question

As the title stated, remap ':' to maintain last command, and erase all char if any key other than <CR> was pressed.

possible use cases:

• I have typed a long command, and I may or may not modified it, then re-run it.
• I save a lot, so I want type :, then visually verify :w is there, then type <CR> to proceed. -- I have remapped :saveas<CR> to <Leader>s.

things that are not what I pursue:

• I know about q: or <C-f> in cmdline mode to open the mini buffer to edit command.
• I know type @: then @@ to repeat command.
• I know type <C-p> or <Up> in cmdline mode.

a little more on motivation:

Repeat with register/macro is great and is really handy for some task.

But here I want a more consistent and safer (visually verification/feedback) way to re-run last command, with fewer key stroke.

Is that possible? or not trivially implementable at all??

======== EDIT ========

Based on @Karl's comment, I think the problem could be implemented via a function definition and a nnoremap. The function consist three parts:

1. echo last command getreg(':'), then get char with getchar

2. if char == '^M', re-execute last command.

3. otherwise, open command line and act accordingly.

My first attempt is as below: -- (based on @grodzik's answer and @Karl's idea in the comment)

func! Show_last_command_and_exec()
    echo ':'.getreg(':')
    try
        let char = getchar()
    catch /^Vim:Interrupt$/
        let char = "\<Esc>"
    endtry
    if char == "^M"
        exec getreg(':')
        " not work, don't know why, tried with previous comand be `:pwd`
    else
        " not work, don't know which function or command should be used.
        " below are failed attempt.
        " normal! :
        " call feedkeys(char)
        " exec ':'.char
    endif
endfunc

nnore : :call Show_last_command_and_exec()<CR>

But unfortunately, except the trivial first echo part, neither work yet. I post my attempt (though failed currently) here, hope someone interested may implement a better one that work nicely.

P.S. as @grodzik said in his answer, the "^M" should be replaced with real <CR> sign.

Answer 1

The following should solve your issue:

nnoremap <expr> : ShowLastCmd()

function! ShowLastCmd()
  echohl ModeMsg
  echo ':' . getreg(':')
  echohl None

  while 1
    try
      let char = getchar()
      if char == '^\d\+$' || type(char) == 0
        let char = nr2char(char)
      endif
    catch /^Vim:Interrupt$/
      return ''
    endtry

    if char == "\<cursorhold>"
      continue
    elseif char == "\<cr>"
      return ':' . getreg(':') . "\<cr>"
    else
      return ':' . char
    endif
  endwhile
endfunction

The while loops is there to catch the <cursorhold> key event.

Answer 2

After a bit of hacking, I actually found a way to achive that:

function! AutoErase()
  setlocal cursorline
  normal Gk
  redraw!
  try
    let char = getchar()
  catch /^Vim:Interrupt$/
    let char = "\<Esc>"
  endtry
  if char == '^\d\+$' || type(char) == 0
    let char = nr2char(char)
  endif " It is the ascii code.
  if char == "^M"
    call feedkeys('^M')
  else
    execute "normal Gi" . char
    startinsert!
  endif
endfunction

autocmd CmdwinEnter * call AutoErase()
nnoremap : q:

This will show Command Window though and command to be executed when pressed Enter is the one highlited. Also, it may be required to change in .vimrc signs with ^M into real <CR> signs with CtrlvEnter.

Answer 3

Here's a very short mapping that will achieve roughly what you're asking, by using the command-line window and select mode:

nnoremap : :<Up><C-F>gH

It recalls the previous command with <Up>, opens the command-line window with <C-F> and then enters linewise select mode with gH.

If you then press Enter then the command is run, but if you type anything else, the current selection will be replaced.