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89 lines
4.2 KiB
Markdown
89 lines
4.2 KiB
Markdown
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---
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title: 'Request for Comments: Strict Unary Operators'
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author: Natalie Weizenbaum
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tags: blog
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# date: 2022-06-15 15:30 PST
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---
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Do you know what `margin: $a -$b` does in Sass? If you said "the same thing as
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`margin: $a (-$b)`, I'm sorry, but you're wrong. It's _actually_ the same thing
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as `margin: $a - $b`. Don't worry, you're not the first person to get tripped up
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by this weird corner of Sass's parser! But our new language proposal aims to fix
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that.
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In the [Strict Unary Operators] proposal, which is currently open for community
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feedback, we propose to first deprecate and then eventually disallow expressions
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of the form `$a -$b`. We know deprecations are never pleasant, but this should
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be fairly painless as they go: you can simply write `$a - $b` or `$a (-$b)`,
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depending which you intend. We'll also provide a [Sass migrator] migration to
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automatically update your stylesheets.
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[strict unary operators]: https://github.com/sass/sass/blob/main/proposal/strict-unary.md
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[Sass migrator]: https://sass-lang.com/documentation/cli/migrator
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**Deprecated:**
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- `$a -$b` will no longer be allowed, because it's unclear what the author
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intended and the current behavior is likely to be incorrect.
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**Still allowed:**
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- `$a - $b` will continue to work, since it's clearly supposed to indicate
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subtraction.
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- `$a (-$b)` will continue to work, since the parentheses make the unary minus
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unambiguous.
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The `$a - $b` or `$a (-$b)` options are supported by all widely-used Sass
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versions, so there shouldn't be any trouble for libraries to avoid this
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deprecation warning and continue to support older Sass versions. In addition,
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you can always use the [`--quiet-deps` command-line flag] or the [`quietDeps` JS
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API option] to silence warnings from dependencies you don't control.
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[`--quiet-deps` command-line flag]: https://sass-lang.com/documentation/cli/dart-sass#quiet-deps
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[`quietDeps` JS API option]: https://sass-lang.com/documentation/js-api/interfaces/Options#quietDeps
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## Why does it work this way?
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Why, you might wonder, does `$a -$b` parse this way in the first place? The
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short answer is, "because other programming languages do it that way". In most
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programming languages, operators are parsed the same way regardless of the
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whitespace that may or may not surround them. If you parse `$a - $b` as
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subtraction, you should parse `$a -$b` as subtraction as well.
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The problem for Sass is that we also inherit CSS's use of space-separated lists
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of values, so in some contexts users expect to be able to write two expressions
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next to one another and have them parse the same way they would if they were
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each used on their own. These two principles come into conflict and produce the
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confusion this proposal seeks to address.
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## Why not just change the way it works?
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In theory, we could change Sass so that `$a -$b` parses the same as `$a (-$b)`:
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a space-separated list of two values, the latter with a unary minus. We chose
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not to do that for two reasons:
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1. Pragmatically, it's more painful to make a breaking change that changes the
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behavior of existing syntax than it is to make one that just forbids the
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syntax entirely. It requires more releases and more different versions of
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Sass with different behaviors. It also opens the door for a stylesheet that
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upgrades many versions at once to switch to the new behavior _without
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producing an error_, which could lead to the worst-case scenario: shipping
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incorrect styles.
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2. It's not obvious that `$a -$b` _should_ parse as `$a (-$b)` in every case.
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Users coming from other programming languages may expect it to parse the same
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way it does in those languages. Even in Sass, `$a -$b` will continue to be a
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valid binary operation within `calc()`. It may not be elegant style, but
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sometimes formatting isn't at the top of an author's mind!
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## Let us know what you think!
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If you've got thoughts or opinions about this change, please read over [the full
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proposal] and then [file an issue] with your feedback. We'll be leaving this
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open to comments for a month, after which we'll finalize the proposal and start
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implementing it.
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[the full proposal]: https://github.com/sass/sass/blob/main/proposal/strict-unary.md
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[file an issue]: https://github.com/sass/sass/issues/new
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