Question

I use mappings to normal mode commands that I’d also like to work in insert mode. This can be done by adding <C-o> to insert mode mapping:

nmap <C-Up> 10<Up>
imap <C-Up> <C-o>10<Up>

But this means repeating each mapping twice.
To avoid repetition, I’ve tried to “overload” some other key, then use it for mode-specific part:

" F12 selects prefix suitable for current mode
nmap <F12> <Nop>
imap <F12> <C-o>
" single mapping relying on "overloaded" F12 
map <C-Up> <F12>10<Up>

For some reason, it doesn’t work. F2 in insert mode just inserts <F2> as text.
Any idea what’s wrong and how to fix it?
Bonus points if you can extend the solution to visual mode.

Answer 1

There are no commands to define mappings for all modes: :map maps for normal, operator-pending and visual modes (really visual and select at once) which is clearly stated in documentation. It does not make any sense to have same mapping for all modes, though unlike movement ones saving may be done in all modes with exactly the same rhs:

function s:Save()
    update
    return ''
endfunction
noremap  <expr> <F2> <SID>Save()
noremap! <expr> <F2> <SID>Save()

. noremap! is another multi-mode mapping command, it covers insert and command mode now. You can’t move the cursor from <SID>Save() function (textlock) thus this method is not applicable for cursor movement commands, but you can use variables in order not to repeat the same thing twice:

let s:tendownlhs='10j'
execute ' noremap <C-Down>      '.s:tendownlhs
execute 'inoremap <C-Down> <C-o>'.s:tendownlhs

. Now without command mode as this is tricky and likely useless.