How to create function that either prints characters or moves cursor

Question

I'm trying to learn vim/vimscript by making changes to my .vimrc as I find things I want to improve. One example is that I now print a paired closing paren whenever I type an open, using:

:inoremap ( ()<left>

This puts me back in the center of the parens, leaving me with a new problem.

How can I write a function that, upon typing the ) character in insert mode, moves a space to the right (without printing anything) if the cursor is currently over that same character, but prints it normally otherwise?

I've tried the following, but it sounds like there's something I don't understand, since this will just print the literal string '<Esc>la' in the scenario where I want to move a space to the right.

:inoremap <expr> ) PrintParenIfNotOnParen()

:function PrintParenIfNotOnParen()
:  if getline(".")[col(".")-1] == ')'
:    return '<Esc>la'
:  else
:    return ')'
:  endif
:endfunction

What am I missing?

Answer 1

The solution is actually in the help. By typing :helpgrep Eatchar you get:

It is possible to move the cursor after an abbreviation:

   :iab if if ()<Left>

This does not work if 'cpoptions' includes the '<' flag. <>

You can even do more complicated things. For example, to consume the space typed after an abbreviation:

   func Eatchar(pat)
      let c = nr2char(getchar(0))
      return (c =~ a:pat) ? '' : c
   endfunc
   iabbr <silent> if if ()<Left><C-R>=Eatchar('\s')<CR>

In your case you need only to change the last line :

   function! Eatchar(pat)
      let l:c = nr2char(getchar(0))
      return (l:c =~ a:pat) ? '' : l:c
   endfunction
   iabbr <silent> ( ()<Left><C-R>=Eatchar('\s')<CR>

Answer 2

Return either ')' or '<right>'. You don't anything more complex than that for now.

Abbreviations are not the way to proceed. Inoremapping are. A <expr> mapping should work fine.