How to 'enter' or add a carriage return in an <expr> mapping

Question

In the following nnoremap:

nnoremap <expr> <leader>3 ToggleColors()
func ToggleColors()
    if g:colors_name == "OceanicNext"
        return ":colorscheme TextMate"
    else
        return ":colorscheme OceanicNext"
endfunc

I am able to get the function to return the proper command to 'enter', however, I have to actually press return for it to run. If I try entering either:

nnoremap <expr> <leader>3 ToggleColors()<CR>

I get:

E15: Invalid expression: ToggleColors() ^M

Or if I try doing:

return ":colorscheme TextMate <CR>"

It enters a literal and doesn't actually hit enter:

What would be the proper way to 'hit enter' at the end of the mapping so I don't have to do it manually?

Answer 1

First, the key code can be input and written into a file directly.

So let foo = 'FooCtrl-VEnterBar' looks like let foo = 'Foo^MBar'

But usually people want their sources do not contain control codes. So some sort of run-time substitution is preferred.

To put key code into a string in runtime one normally needs a backslash:

let foo = "Foo\<CR>Bar"

Note double quotes to allow string preprocessing.

When it's a part of a :map command, the substitution is made while reading a command before expression evaluation, so backslash is not needed:

nnoremap <A-X> :xit<CR>

or

nnoremap <expr><A-X> ':xit<CR>'

does not make a big difference.

If you want to stop such early expansion use <lt>:

nnoremap <expr><A-X> ":xit\<lt>CR>"

This one produces the same result as the previous. The expression is first expanded into ":xit\<CR>" (while reading the command) and then into ':xit^M' (while executing mapping).