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[csswg-drafts] [css-custom-functions] Custom CSS Functions (#5440)

From: Joe Pea via GitHub <sysbot+gh@w3.org>
Date: Tue, 18 Aug 2020 00:26:09 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-680603071-1597710368-sysbot+gh@w3.org>
trusktr has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-custom-functions] Custom CSS Functions ==
I wish there was a way to make re-usable CSS "functions".

Currently, we can't make re-usable logic without unfortunately coupling to HTML markup. See:

- https://stackoverflow.com/questions/63459791/why-do-these-css-variables-not-work-when-passed-into-calc
- https://stackoverflow.com/questions/52015737/css-scoped-custom-property-ignored-when-used-to-calculate-variable-in-outer-scop

As you can tell from those pages, in order to make re-usable expressions ("functions"), we must couple the re-usability to the HTML markup, which is not ideal.

It would be great to be able to create re-usable logic/functions in a way that does not require a CSS author to ever touch any HTML code.

How might an proposal syntax look like, to work around the current limitation?

Perhaps:

```css
:root {
  --sum: function( calc( var(--a) + var(--b) ) )
}

/* Then anywhere in any CSS rules, without dependence on HTML markup: */
.foo.bar.whatever {
  --a: 4px; --b: 6px;
  font-size: call(--sum);
}
```


^ In that example, the user does not have to worry about "evaluating CSS variables at the level where they are needed" (which requires the CSS author to touch the HTML markup as shown by @Afif13's nicely-written answer in the second SO question).

There's probably a bunch of other ways we could do it.

Here's another idea for sake of spinning up some ideas:

```css
:root {
  --sum: function(--a, --b, calc( arg(--a) + arg(--b) ) )
}

/* Then anywhere in any CSS rules, without dependence on HTML markup: */
.foo.bar.whatever {
  font-size: call(--sum, 4px, 6px);
}
```

Or, maybe we can create a new special type of block (it may be cleaner):

```css
@function sum(a, b) {
  return: calc(arg(a) + arg(b))
}

/* Then anywhere in any CSS rules, without dependence on HTML markup: */
.foo.bar.whatever {
  font-size: sum(4px, 6px);
}

/* Maybe `call()` or similar is required to distinguish from builtins? */
.foo.bar.whatever {
  font-size: call(sum, 4px, 6px);
}
```

Maybe functions can also return a rule expression, which could be "mixed" onto a rule (not sure about syntax though):

```css
@function foo(a, b, c) {
  return {
    background: a b c;
  }
}

.whatever {
  call(foo, lightblue, url("foo.gif"), no-repeat); /* How to handle commas? */
  color: red;
}
```

results in equivalent of

```css
.whatever {
  background: lightblue url("foo.gif") no-repeat;
  color: red;
}
```

or something.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5440 using your GitHub account


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