Mappings with @="^E" result in @="" and do nothing

Question

It seems that mappings with @="^E" or @="<c-e>" result in @="" and do nothing.

For example if I use nnoremap ^N @="^E"^M then press ctrl-n I will get a @="" message and nothing happens. However, when I use nnoremap ^N @="^Y"^M then press ctrl-n I receive a @="^Y" message and the screen scrolls down.

I should also mention that if I enter @="^E"^M manually, -- and not as a mapping -- it works just fine.

To clarify I want to know how to pass <c-e> into the expression register because I want to be able to use it in repeatable mappings. Such as nnoremap <c-p> @="<c-e>j" to scroll the window and move cursor down n lines.

Answer 1

From :help c_ctrl-e:

CTRL-E or <End>                 *c_CTRL-E* *c_<End>* *c_End*
                cursor to end of command-line

In command-line mode, <C-e> moves the cursor to the end of the line, and when you are inside the expression register =, you are on the command-line.

So, when you hit <C-N> in normal mode, here's what Vim does:

• type @="
• move the cursor to the end of the command-line (it's already at the end anyway) because of ^E ; it doesn't insert the character ^E because the latter was interpreted as a motion
• type "^M

In the end, it's as if you had typed: @="", which is an empty string. So, Vim doesn't execute anything.

It would work if there was something in the string, for example if you type @="dd" in normal mode, it will delete the current line. But here, there's nothing left in the string, because ^E was interpreted as a motion.

For this reason, you can't use ^E in your mapping.

---

Your mapping works for ^Y because of this (:help c_ctrl-y):

CTRL-Y      When there is a modeless selection, copy the selection into
            the clipboard. |modeless-selection|
            If there is no selection CTRL-Y is inserted as a character.

Like ^E, ^Y is associated with an action on the command-line. But not if there is no selection.
In this case, contrary to ^E, it simply inserts a literal control character.

---

Depending on the context, there are various ways to write a control character like ^E:

• ^E (Ctrl-VCtrl-E) to insert it on the command-line or in a buffer
• <C-E> in a mapping
• "\<C-E>" in a string
• "\<lt>C-E>" in a string read by a mapping command such as :nnoremap

So, if for some reason you really wanted to use the expression register (maybe because you wanted to create a mapping accepting a count), you could use "\<lt>C-E>", like this:

nnoremap ^N @="\<lt>C-E>"^M

In this mapping, 2 translations occur at different times.

First, when your :nnoremap command is executed and add an entry in the mappings table, the keycode <lt> is translated as the character <, and the resulting string is "\<C-E>".
Then, when you hit <C-N> in normal mode, the string "\<C-E>" is translated into the control character ^E.

The keycode <lt> is necessary to protect <C-E> from being automatically translated into a literal control character when :nnoremap is executed.
If you don't protect <C-E> with <lt>, the same problem as before will repeat; ^E will be intepreted as a motion on the command-line instead of a character to insert.

Answer 2

I think you're not using the good way to do what you want:

The expression register ("=) is a way to evaluate an expression but what you are trying to use (Ctrl-E) is simply a normal mode command.

So a in a normal mode mapping you can simply use it directly with :nnoremap <C-n> <C-e> (or :nnoremap <C-n> <C-y> or whatever). You don't need to consider that as an expression to be executed.

Generally speaking I don't think it is a good idea to use the expression register since it adds complexity to your mapping whereas you often have an other solution to do what you're trying to do.