Hi all,

This PR has proposed adding:const command to Vim.

Problem

Scope of variables in Vim script function is very dynamic.

leti=0if some_condition" In heavily nested or big statements...leti=1" Unexpectedly using the same name variableendifechoi" => 1

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

leti=0lockvariif some_conditionleti=1" Errorendifechoi

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.

constnumber=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:

consti=0if some_conditionleti=1" E995endifechoi" => 1

Here:const is used instead of:let.:const internally locks the variablei. Soi 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:

letx=0constx=1" => E995

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

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

consts:SEP=has('win32') ?'\' :'/'

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