Stop flushing the last command when hitting enter

Question

Suppose I wrote this command:

command! -nargs=1 E echo ' -> <args>'

and when user feeds an argument,

:E abc

I intend to see the following effect as they hit Enter:

:E abc -> abc

Of course that's not happening, since vim flushes :E abc when printing -> abc, leaving the command-line showing:

 -> abc

Is there an option to retain the command typed by user in the command-line, before finishing execution? Or are there alternative ways to achieve the same effect?

Answer 1

Came up with a solution with functionality of command E being moved entirely to another function F to be called inside command-line expression prompt, thanks to romainl's hint:

command! -nargs=1 E

cnoremap <CR> <C-\>eF()<CR><CR>

" call me only in c_<C-\>e mode
function! F()
    let cmd = ':' .. getcmdline()
    let parse = v:lua.vim.api.nvim_parse_cmd(cmd, {})
    let cmdname = parse.cmd
    if cmdname ==# 'E' && len(parse.args) > 0
        let arg = parse.args[0]
        return 'echo ''' .. cmd .. ' -> ' .. arg .. ''''
    else 
        return cmd
    endif
endfunction

While this solution parses command string utilising nvim API, vim users may need to come up with some other ways to parse command; I cannot think of any builtin functions with the same effect.