Hi all,

This PR has proposed adding :const command to Vim.

Problem

Scope of variables in Vim script function is very dynamic.

let i = 0
if some_condition
    " In heavily nested or big statements...
    let i = 1 " Unexpectedly using the same name variable
endif
echo i " => 1

This sometimes causes a bug by modifying variables unexpectedly. To prevent this, (if i is intended not to be modified,) :lockvar is effective:

let i = 0
lockvar i
if some_condition
    let i = 1  " Error
endif
echo i

However, using :let and :lockvar has following downsides:

• it is hard to give :lockvar at everywhere it is required
• using :let and :lockvar may not be efficient because it introduces overhead of one more command.

Solution

In JavaScript, const is provided for defining a variable.

const number = 42;
number = 99;  // TypeError: invalid assignment to const `number'

:const does the similar thing in Vim script. With it, above example can be written as follows:

const i = 0
if some_condition
    let i = 1  " E995
endif
echo i " => 1

Here :const is used instead of :let. :const internally locks the variable i. So i is no longer modifiable. And it is more efficient than using both :let and :lockvar because both are done in one command.

In addition, :const raises an error if the variable is already existing:

let x = 0
const x = 1  " => E995

So :const does not lock and overwrite existing variable unexpectedly and x is guaranteed to be a new variable.

Another use case of :const would be defining constants in the script. For example:

const s:SEP = has('win32') ? '\' : '/'

Here the constant is not intended to be modified any more. So :const is better than :let.