Request for Comments: Strict Unary Operators
Posted 15 June 2022 by Natalie Weizenbaum
Do you know what
margin: $a -$b does in Sass? If you said "the same thing as
margin: $a (-$b), I’m sorry, but you’re wrong. It’s actually the same thing
margin: $a - $b. Don’t worry, you’re not the first person to get tripped up
by this weird corner of Sass’s parser! But our new language proposal aims to fix that.
In the Strict Unary Operators proposal, which is currently open for community
feedback, we propose to first deprecate and then eventually disallow expressions
of the form
$a -$b. We know deprecations are never pleasant, but this should
be fairly painless as they go: you can simply write
$a - $b or
depending which you intend. We’ll also provide a Sass migrator migration to
automatically update your stylesheets.
$a -$bwill no longer be allowed, because it’s unclear what the author intended and the current behavior is likely to be incorrect.
$a - $bwill continue to work, since it’s clearly supposed to indicate subtraction.
$a (-$b)will continue to work, since the parentheses make the unary minus unambiguous.
$a - $b or
$a (-$b) options are supported by all widely-used Sass
versions, so there shouldn’t be any trouble for libraries to avoid this
deprecation warning and continue to support older Sass versions. In addition,
you can always use the
--quiet-deps command-line flag or the
API option to silence warnings from dependencies you don’t control.
Why does it work this way?Why does it work this way? permalink
Why, you might wonder, does
$a -$b parse this way in the first place? The
short answer is, "because other programming languages do it that way". In most
programming languages, operators are parsed the same way regardless of the
whitespace that may or may not surround them. If you parse
$a - $b as
subtraction, you should parse
$a -$b as subtraction as well.
The problem for Sass is that we also inherit CSS’s use of space-separated lists of values, so in some contexts users expect to be able to write two expressions next to one another and have them parse the same way they would if they were each used on their own. These two principles come into conflict and produce the confusion this proposal seeks to address.
Why not just change the way it works?Why not just change the way it works? permalink
In theory, we could change Sass so that
$a -$b parses the same as
a space-separated list of two values, the latter with a unary minus. We chose
not to do that for two reasons:
Pragmatically, it’s more painful to make a breaking change that changes the behavior of existing syntax than it is to make one that just forbids the syntax entirely. It requires more releases and more different versions of Sass with different behaviors. It also opens the door for a stylesheet that upgrades many versions at once to switch to the new behavior without producing an error, which could lead to the worst-case scenario: shipping incorrect styles.
It’s not obvious that
$a -$bshould parse as
$a (-$b)in every case. Users coming from other programming languages may expect it to parse the same way it does in those languages. Even in Sass,
$a -$bwill continue to be a valid binary operation within
calc(). It may not be elegant style, but sometimes formatting isn’t at the top of an author’s mind!
Let us know what you think!Let us know what you think! permalink
If you’ve got thoughts or opinions about this change, please read over the full proposal and then file an issue with your feedback. We’ll be leaving this open to comments for a month, after which we’ll finalize the proposal and start implementing it.