First-class calc and unary operator

[flachware]

Hi, as slash as division is deprecated by now, I’d like to follow the first-class calc() recommendation:

Recommendation: math.div(-$var, 2) or calc(-$var / 2)

$var: 16px;

div {
  margin: calc(-$var / 2);
}

Error: Expected digit.
  ╷
9 │   margin: calc(-$var / 2);
  │                 ^
  ╵

The negation produces an error though – is this expected?
I’m using sass 1.58.1.

[nex3]

This is an interesting issue. Our recommendation generation logic currently assumes that any SassScript expression is drop-in compatible in a calc() context, but our calc() grammar doesn't currently allow unary minus. This is inconsistent with SassScript, but consistent with plain-CSS calc(), which doesn't support unary minus anywhere (except in numeric and infinity literals, where it's considered part of the literal and not the operation). Talking to @tabatkins, it sounds like this is intentional—it would be syntactically painful to support it for variables or other function calls, since -var(--foo) just parses as a function named -var, so it's forbidden even in syntactically-safe places (like -(1 + 2)) for consistency.

I think we should probably lean towards consistency with plain CSS here, given that we already restrict the use of other SassScript arithmetic features in calc(). That does mean we should change the recommendation here to suggest calc(-1 * $var / 2).

[flachware]

Thanks for clarifying. May I ask which other arithmetic features are restricted in calc()?

[nex3]

Sass's calculation support covers all the standard calculation features defined in Values and Units 3, as well as the ability to use Sass variables and Sass functions. The only expression-level syntax it doesn't include (that would be meaningful in a calculation context) is unary operations and the % operator.

[flachware]

All clear, thank you.