How can I represent a 4 spaces indent width as 2 spaces?

Question

For clarity: I mean indents as represented by individual spaces, not tab characters.

I'm working with a friend on a javascript project, and while I'm an 80col zealot, he wants to have 4 space indent. It's not the end of the world, but it bothers the heck out of my eyes.

I know about the .dir-locals.el trick, so I'll just stick the tab width in there, but I was wondering if there was a display hook or extension that would print any block identation as 2 spaces? For example, show this block

function greet(name) {
    return `Hello, ${name}`
}

as this one

function greet(name) {
  return `Hello, ${name}`
}

while actually writing 4 spaces into the file on disk. We already use prettier, so I can chuck in anything without having to worry what comes out on the other end, but I'd prefer to have a solution that just changes the display of the buffer in emacs. I don't really want to hassle with changing the real file contents every time I open a file.

Answer 1

You can (ab)use prettify-symbols-mode for this.

(push '("  " . ?\ ) prettify-symbols-alist)

followed by M-x prettify-symbols-mode

Answer 2

If the indentation is spaces rather than tabs, give redshift-indent a try. I failed to follow up on some upstream text-property enhancements which are needed to make this idea work in all situations, and consequently tabs can cause problems¹; but if your indentation is spaces, then this will probably still work fine.

See the library Commentary for details, and use the M-x redshift-indent command to try it out:

Invoke `redshift-indent-mode' with a buffer-local cosmological constant.

With no prefix ARG, `redshift-indent-mode' is toggled.
If ARG is positive, `redshift-cosmological-constant' is set to ARG.
If ARG is negative, `redshift-cosmological-constant' is set to 1.0 / (abs ARG).
If ARG is C-u, `redshift-cosmological-constant' is prompted for interactively.

e.g.:
With prefix arg 2, indentation will appear 2 times its normal width.
With prefix arg -2, indentation will appear 0.5 times its normal width.

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¹ In a local copy (with the same code), I have a more specific note that "The current implementation unavoidably breaks for tabs which are visually narrower than tab-width."