How do I map return in insert to go to normal mode and then do the o action

Question

I am currently using vim and I'd like to map the Return key (I'm on a mac. I believe that this is generally represented by <ENTER> in maps) to leave insert mode, and then perform the o action. I was trying to put something like this

imap <ENTER> <ESC>o

However this is not performing the desired action. Any help would be fantastic.

Edit: The desired action is that if I am typing in insert mode, each new line is a new action. So if I press undo in normal mode it just undoes the last line instead of all the lines typed while in insert mode.

Answer 1

I'm not sure why this isn't working. I do not know why escaping and using o to open a newline does not add to the change-list. However, lucky for us, there is a command for explicitly adding the current state of the text to the change list. That command is (in insert mode) <C-g>u. From :h i_ctrl-g_u

CTRL-G u    break undo sequence, start new change        *i_CTRL-G_u*

Conveniently, this command doesn't even leave insert mode! Putting it all together, the mapping you're looking for is:

:inoremap <cr> <C-g>u<cr>

Or, you could also do

:inoremap <cr> <cr><C-g>u

which will leave you with a blank line after undoing.