How to return control characters from a function?

Question

I have a keybinding that works:

inoremap <silent> <Tab> <C-r>=
  \ pumvisible() ? "\<lt>C-N>" :
  \ CursorAfterOnlyWhitespace() ? "\<lt>Tab>" :
  \ "\<lt>C-\>\<lt>C-O>:ALEComplete\<lt>CR>"<CR>

I tried to put it in a function that takes arguments:

function! TabComplete(complete, indent)
  if pumvisible()
    " Tab through completions.
    return a:complete
  endif
  if CursorAfterOnlyWhitespace()
    " Indent.
    return a:indent
  endif
  " Find completions.
  return '\<C-\>\<C-O>:ALEComplete\<CR>'
endfunction

inoremap <silent> <Tab> <C-R>=TabComplete('\<lt>C-N>', '\<lt>Tab>')<CR>
inoremap <silent> <S-Tab> <C-R>=TabComplete('\<lt>C-P>', '\<lt>S-Tab>')<CR>

But this does not work. It just inserts the literal string returned by the function. How do I get Vim to decode the control characters in the string returned by my function?

Answer 1

Replace the single quotes with double quotes.

return "\<C-\>\<C-O>:ALEComplete\<CR>"

Single quoted Strings are "literal strings". Every character is used as entered. They are useful to express regular expressions etc as you don't have to double the backslashes.

In double quoted strings backslash-escapes like \<C-O> are interpreted as one character.

See :help literal-string and :help String.