Question

Is it possible to :map to a function that can detect the mode it was
called in? This is what I have:

func! s:ShowModeFunction()
    echomsg mode(1)
endfunc

noremap <expr> <Plug>ShowMode <SID>ShowModeFunction()

map \m <Plug>ShowMode

The map \m eventually calls the top function, which echos a string
indicating normal mode, operator-pending mode, or any of the Visual
modes.

This only works with <expr> on the middle map though; any colon
command would render the final mode() call useless, because it would
then always return “normal mode”. Unfortunately, <expr> strictly
requires an expression on the right-hand side, and since the function
doesn’t return anything, the implicit return value 0 is used, which
moves the cursor to the first column.

Answer 1

Just make the function return nothing, this is then a no-op in the expression mapping:

func! s:ShowModeFunction()
    echomsg mode(1)
    return ''
endfunc

If there’s any other limitation in map-expr, just set a variable in there, and do the real work (including evaluating the variable) in a “normal” mapping or :call that you can append to your <Plug>ShowMode mapping (or another intermediate one, to keep the customizability).