Escape return value key in mapping function

Question

I want to create a function for a mapping. The key for the mapping is configured through a variable. How can I escape the key in the return statement so it works? This works:

let g:some_key = "<Up>"
function! <SID>TestFun()
  " doing something
  return "\<Up>"
endfunction
noremap <expr> g:some_key <SID>TestFun()

But I want something likes this

let g:some_key = "<Up>"
function! <SID>TestFun()
  " doing something
  return "\".g:some_key
endfunction
noremap <expr> g:some_key <SID>TestFun()

Right now my solution was creating a new variable holding the escaped key like this

exec 'let s:escaped_key = "\'.g:some_key.'"'

but I'm there's a better way to do it.

EDIT: The method I'm looking for should also work for a function called using <C-R>=<SID>SomeFunction()<CR> in a imap

Answer 1

All problems in computer science can be solved by another level of indirection except for the problem of too many layers of indirection.

-Kevlin Henney

Use a <Plug> (or <SID>) mapping to g:some_key. Use this new mapping in your function instead of g:some_key.

let g:some_key = '<Up>'
execute 'nnoremap <Plug>(some-key) ' . g:some_key

function! s:TestFun()
  " doing something
  return "\<Plug>(some-key)"
endfunction
execute 'nmap <expr> ' . g:some_key . ' <SID>TestFun()'

There are a few things worth noting:

• Using nmap instead of nnoremap so that we can map <Plug>
• Using double quoted string and escaping the key notation. e.g. "\<Plug>.."
• You can use <SID> mapping instead and use <script> if you want to emulate nnoremap. See :h <SID> and :h :map-script for more information.

For more help see:

:h :execute
:h <Plug>
:h <SID>
:h :map-script
:h key-notation
:h expr-string
:h literal-string