Request For Comments: Strict Unary Operators

>>> Shared from the Original Post at Sass Blog

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
as 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

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 $a (-$b),
depending which you intend. We’ll also provide a Sass migrator migration to
automatically update your stylesheets.


  • $a -$b will no longer be allowed, because it’s unclear what the author
    intended and the current behavior is likely to be incorrect.

Still allowed:

  • $a - $b will continue to work, since it’s clearly supposed to indicate

  • $a (-$b) will continue to work, since the parentheses make the unary minus

The $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 quietDeps JS
API option
to silence warnings from dependencies you don’t control.

Why does it work this way? permalinkWhy does it work this way?

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? permalinkWhy not just change the way it works?

In theory, we could change Sass so that $a -$b parses the same as $a (-$b):
a space-separated list of two values, the latter with a unary minus. We chose
not to do that for two reasons:

  1. 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.

  2. It’s not obvious that $a -$b should 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, $a -$b will 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! permalinkLet us know what you think!

If you’ve got thoughts or opinions about this change, please read over the full
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.