How can I convert this remap to a <expr>?

Question

N.B. my question title might be poor, so feel free to improve it.

Background

I want to write a command to move my cursor to the body of my email, add blank lines, and change to insert mode. Previously, I used the following for new emails.

nnoremap gb <C-Home>/^$<CR>:nohlsearch<CR>o

mutt would create a new email in vim, then I'd press gb. The cursor would move to the top of the email (<C-Home>), find the first blank line and immediately stop highlighting (/^$<CR>:nohlsearch<CR>), create a new line and start editing a new line (o).

However, I often reply to an email, and I wanted to automatically create another "buffer" empty line below my cursor position. Hence, I changed the command to this following.

nnoremap gb <C-Home>/^$<CR>:nohlsearch<CR>o<Esc>ko

This would do as above, then get into normal mode <Esc>, move up (k), and create another new line to edit o.

Conditional

I wanted to create a conditional. After searching for the first blank line (<C-Home>/^$<CR>:nohlsearch<CR>), if this were the last line of the email, I'd be editing a new email, so just o. Otherwise, I'd be replying, so o<Esc>ko. The following doesn't work, because I've mangled together different syntaxes, but it should give an idea of what I want.

nnoremap gb <C-Home>/^$<CR>:nohlsearch<CR>|if line('.') != line('$')|o<Esc>k|endif|o

I suspect nnoremap <expr> gb ... is useful, but I'm not sure how to use it.

Answer 1

There you go, I'm not sure to have understood perfectly your use case, bu you can add your workflow to this example :

function! Mail()
  normal! gg
  call search('^$')
  nohlsearch
  if line('.') != line('$')
    normal! o
    normal! k
  endif
  normal! o
  startinsert
endfunction

nnoremap gb :call Mail()<CR>

Note that you can execute normal! to prevent mapping to be used.