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Let's say I've got the word foo in a file, I copy it in register a ("ayiw) and I define the following mapping:

inoremap <expr> <F3> "<C-r>a"

If I hit F3 while in insert mode it will paste the content of register a (foo).

Now, let's say I define the following function, and the following mapping:

function! SomeFunction()
      return "<C-R>a"
endfunction

inoremap <expr> <F3> SomeFunction()

When I hit F3, instead of inserting the a register (foo), it inserts literally <C-R>a.

I found a solution, which involves changing the function and using the unicode for <C-R>.
I think u12 is the unicode for the control character <C-R>, because when you insert it literally (<C-V><C-R>) and you hit ga on it, the output is <^R> 18, Hex 12, Octal 022.

So the following function and mapping works:

function! SomeFunction()
      return "\u12" . 'a'
endfunction

inoremap <expr> <F3> SomeFunction()

When I hit F3 it inserts the content of the a register as expected.

But why do I have to use unicode when the control character is returned by a function instead of being directly passed as an argument to :inoremap?

Edit: After reading Christian Brabandt's answer, I've realised that my solution was too complex.
As he has well explained, no need of unicode, a control character must simply be escaped in a string, so the proper way to write the function would be:

function! SomeFunction()
      return "\<C-R>a"
endfunction

And yet I'm still confused, if I type:

inoremap <expr> <F3> "<C-r>a"

It works as expected. So why do I need to escape <C-r> inside the function but not when it's directly passed as an argument to :inoremap?

For that, I think I have an explanation. The Ex commands which define a mapping like :inoremap, :nnoremap... don't need the control characters to be escaped. For example, you don't write:

nnoremap \<F3> :call SomeFunction\<cr>

but

nnoremap <F3> :call SomeFunction<cr>

Why? Probably because these commands are special, they already interpret correctly <cr> as a carriage return and not as 4 literal characters (same thing for <F3>).

So, going back to my previous function, why do I need to escape <C-r>?
Because, when I hit <F3>, :inoremap evaluates the right-hand-side of the mapping as a call to the function SomeFunction()... and that's it. It doesn't interpret anything after that, it just types the output literally.
The output of SomeFunction() is not interpreted by :inoremap, so this time you have to escape the control character so that Vim (not :inoremap) interprets \<C-r> as Ctrl + r.

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If you want to have Keys returned by mapping expressions, you have to escape them as mentioned in the help :h expr-quote. So your Function needs to return "\<c-r>a" (note the backslash), else you couldn't let the function return the string "<c-r>a"

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    Thank your for your answer. Just another question, if I type: inoremap <expr> <F3> "<C-r>a", it works as expected. I don't have to escape <C-r>, why?
    – saginaw
    Jan 8, 2016 at 19:06
  • 1
    That is interesting and certainly unexpected. That might be a bug.
    – Christian Brabandt
    Jan 8, 2016 at 19:08
  • 1
    I may be wrong, but I don't think it's a bug. If you look at the very bottom of :h ins-special-special, they give these 2 examples : inoremap <expr> <Home> col('.') == match(getline('.'), '\S') + 1 ? \ repeat('<C-G>U<Left>', col('.') - 1) : \ (col('.') < match(getline('.'), '\S') ? \ repeat('<C-G>U<Right>', match(getline('.'), '\S') + 0) : \ repeat('<C-G>U<Left>', col('.') - 1 - match(getline('.'), '\S'))) inoremap <expr> <End> repeat('<C-G>U<Right>', col('$') - col('.')). They don't escape <C-G>, neither <Left> nor <Right>.
    – saginaw
    Jan 8, 2016 at 19:45
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    Worse, they don't use double quotes, but single quotes (which prevents Vim from evaluating anything in the string). My theory: they don't need to be escaped because the Ex commands which define mappings such as :inoremap don't need it, they already know that <CR> for example means "carriage return", you don't need to write \<cr> in a mapping. This is why I don't understand why I need to escape <C-r> when it comes from a function but not when it's on the command line. But I like your answer, from now on, I will always use double quotes and escape to represent a control character.
    – saginaw
    Jan 8, 2016 at 19:45
  • 1
    i know the example very well. I am still not sure, whether this is a bug. I'll ask vim-dev
    – Christian Brabandt
    Jan 8, 2016 at 21:40

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