Add a blog post for strict unary operators (sass#645)

See sass/sass#1912

source/blog/033-request-for-comments-strict-unary-operators.md

@@ -0,0 +1,87 @@
+---
+title: "Request for Comments: Strict Unary Operators"
+author: Natalie Weizenbaum
+date: 2022-06-15 15:30 PST
+---
+
+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 - $b`. Don't worry, you're not the first person to get tripped up
+by this weird corner of Sass's parser! But our new language proposal aims to fix
+that.
+
+In the [Strict Unary Operators] proposal, which is currently open for community
+feedback, we propose to first deprecate and then eventually disallow expressions
+of the form `$a -$b`. We know deprecations are never pleasant, but this should
+be fairly painless as they go: you can simply write `$a - $b` or `$a (-$b)`,
+depending which you intend. We'll also provide a [Sass migrator] migration to
+automatically update your stylesheets.
+
+[strict unary operators]: https://github.com/sass/sass/blob/main/proposal/strict-unary.md
+[Sass migrator]: https://sass-lang.com/documentation/cli/migrator
+
+**Deprecated:**
+
+* `$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 - $b` will continue to work, since it's clearly supposed to indicate
+  subtraction.
+
+* `$a (-$b)` will continue to work, since the parentheses make the unary minus
+  unambiguous.
+
+The `$a - $b` or `$a (-$b)` options are supported by all widely-used Sass
+versions, so there shouldn't be any trouble for libraries to avoid this
+deprecation warning and continue to support older Sass versions. In addition,
+you can always use the [`--quiet-deps` command-line flag] or the [`quietDeps` JS
+API option] to silence warnings from dependencies you don't control.
+
+[`--quiet-deps` command-line flag]: https://sass-lang.com/documentation/cli/dart-sass#quiet-deps
+[`quietDeps` JS API option]: https://sass-lang.com/documentation/js-api/interfaces/Options#quietDeps
+
+## Why does it work this way?
+
+Why, you might wonder, does `$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 -$b` as subtraction as well.
+
+The problem for Sass is that we also inherit CSS's use of space-separated lists
+of values, so in some contexts users expect to be able to write two expressions
+next to one another and have them parse the same way they would if they were
+each used on their own. These two principles come into conflict and produce the
+confusion this proposal seeks to address.
+
+## Why not just change the way it works?
+
+In theory, we could change Sass so that `$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!
+
+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.
+
+[the full proposal]: https://github.com/sass/sass/blob/main/proposal/strict-unary.md
+[file an issue]: https://github.com/sass/sass/issues/new