Cleaning Up Interpolation

Interpolation—the ability to add variables and other snippets using #{...}—is one of the handiest all-purpose features of Sass. You can use it just about everywhere you might need to inject a variable, a function call, or some other expression. In most of those places it just plops the value into the surrounding text. It's straightforward, easy to understand, and useful, which is exactly what we want from a feature.

Unfortunately, that's only true in most places. For complicated historical reasons, there's one place where interpolation goes a little bit bananas: inside an expression but outside quotes. Most of the time, it makes sense; if you write display: -#{$prefix}-box, you'll get what you expect. But if any operators like + are used next to the interpolation, you start to get weird output. For example, $name + #{$counter + 1} might return an unquoted string containing the text name + 3.

This is really weird behavior. Why does + behave differently here than it does everywhere else? Why is it treated as plain text when $name gets evaluated normally? This behavior is confusing, inconsistent, and not particularly useful, which are very much not things we want in a feature. So why do they exist in the first place?

Complicated Historical Reasons

If you don't care for a history lesson, skip on down to A Brave New World.

Way back in the dawn of time, when the indented syntax was the only syntax, Sass had a distinction between "static" and "dynamic" properties. A static property was basically plain CSS; it was declared using property: value, and the value was used as-is without any further processing. If you wanted to use a variable or a function, you had to use a dynamic property, which was declared using property= value. A You'd see a lot of stylesheets like this:

.border

  border-width: 4px

  border-style: solid

  border-color= !background_color

Also, in the dawn of time, variables used ! instead of and couldn't include hyphens. The dawn of time kind of sucked. But it was in this context that we first added interpolation. We wanted to allow properties like border with multiple values to be partially dynamic, so we decided to follow in Ruby's footsteps and allow #{} to be used to drop in values. Soon stylesheets started looking like this:

.border

  border: 4px solid #{!background_color}

That's so much better! And for a while, all was calm.

Then Came SCSS

It eventually became clear that users really strongly wanted their stylesheets to look like CSS, so we sat down and started work on the syntax that would become SCSS in the release that would become Sass 3. As part of this work, we decided to get rid of the distinction between static and dynamic properties altogether. Having all properties work the same way was obviously great for users, but it meant we had to figure out how to merge the two syntaxes with a minimum of pain.

This was mostly straightforward, since the old expression syntax was pretty much universally invalid CSS or something that emitted its CSS value anyway. But interpolation proved tricky. Backwards compatibility is really important to us, so we wanted to be sure that all the places interpolation was used—or could theoretically be used—in Sass 2 would continue to work in Sass 3, even though everything around them was now fully parsed.

Our solution was to make basically anything around #{} that wasn't obviously part of a plain-CSS expression turn into a string. That way, hopefully any weird corner cases that people had would keep working when they upgraded. This led to the weird behavior I described above, but at the time our top priority was making it as easy as possible for users to migrate to Sass 3. We decided the weirdness was worth it, and shipped it.

A Brave New World

Flash forward to today. We're now starting work on the next major release, Sass 4, and (I dearly hope) no one's written any Sass 2 stylesheets in years. A major release is a great opportunity to clean up this bit of historical cruft, and after discussing it extensively on the issue tracker we decided to make the change.

There are three major steps in a backwards-incompatible change like this. The first is to design the new syntax, which was pretty easy here, since it's basically just "do what everyone thought it did already." We just had to take that general notion and suss out the specifics.

We ended up framing it as #{} being, syntactically, part of an identifier. When you write -#{$prefix}-box, Sass parses it as a single identifier containing "-" followed by the value of $prefix followed by "-box". Even if you write #{$font} all on its own, it's parsed as an identifier that only contains the value of $font. This way, interpolation doesn't have weird behavior around operators any more than identifiers ever did.

Once we had a design, the second step was to deprecate the old behavior. The meat of deprecation is figuring out when to print a warning, and that was pretty tough here. We didn't want to warn for situations that would continue to work, even when they involved operators—for example, 12px/#{$line-height} will print the right thing in the old and new worlds (although for slightly different reasons), but 12px+#{$line-height} won't.

I won't go into the gory details of how we got deprecation working here; that's what the GitHub issue is for. Suffice it to say that it involved a lot of special cases, including some where a deprecation warning can be printed based on how a value is used rather than how it's written. I'm pretty happy with where it ended up, though; I suspect it'll catch 99% of cases that will actually break in practice.

Another exciting bonus was the ability to automatically update code. This doesn't always work when introducing backwards-incompatibilities, but in this case we were able to make sass-convert convert deprecated uses of interpolation into Sass 4-compatible code. It has some false negatives—it only converts cases it can prove will be incompatible—but it's enough to get users a long way there.

The final step once the deprecation was in place was to move to the master branch (which will eventually become Sass 4), rip out all the old behavior, and implement the new. And it was wonderful. Deleting gross code and replacing it with something clean feels like taking a shower after spending a day hiking through dust under a hot sun. And after working on this feature for weeks, I was happy to see the other end of it.

Checking it Out

Sass 3.4.20, released today, was the first release to include the deprecation warnings for the old syntax. If you want to check whether you've got any deprecated interpolations lurking in your stylesheets, just gem install sass and recompile your stylesheet. And if you do find some, try running sass-convert --recursive --in-place . to fix a bunch automatically.

If you want to try out the new syntax, 4.0.0.alpha.1 was also released today. You can get it with gem install sass --prerelease. But beware: it is alpha software, so it may change in the future. We generally try to keep even our prereleases pretty stable, but there's also a chance you'll run into a bug.

If you do find a bug, please file it on the issue tracker. Even if it's something as simple as a typo, we want to know. If we've deprecated something that should be valid, we especially want to know. And if you just have a question, feel free to tweet at @SassCSS or post it on the mailing list.