How to make a keybind in vim, that lets the user insert, the repeats the insertion?

I would like to make a keybind that will let the user type something in insert mode, then goes to a different line, and puts what the user typed there. The purpose of this is for LaTeX \begin and \end. This is what I have so far:

autocmd Filetype tex inoremap `d \begin{}<CR><++><CR>\end{}<esc>2kf{a

At the end, a lets the user type in the \begin field, and I want to make it so that, when I exit insert mode, what I typed is put into the \end field. I honestly have no idea how to go about this. Any help is appreciated.

Solution

This is fairly hard to do for a single mapping.

If you really want to roll your own, I think the simplest approach I can suggest here is to prompt the user for the environment type (with input()) and then use that name twice when inserting the block.

This should work:

function! LatexEnvironment()
    let name = input('Environment name: ')
    return "\\begin{".name."}\r\\end{".name."}\<C-o>O"
endfunction
autocmd Filetype tex inoremap <expr> `d LatexEnvironment()

A much better approach here is to use a snippet engine.

It's typically very easy to support multiple fields with repetitions and you can even set default values for fields in a snippet engine.

For example, in UltiSnips, you can use the following snippet:

snippet "\\?b(egin)?" "begin{} / end{}" br
\begin{${1:something}}
    ${0:${VISUAL}}
\end{$1}
endsnippet

Which is in fact a standard snippet you can find in the honza/vim-snippets library.