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<pre class='metadata'>
Title: CSS Functions and Mixins Module
Shortname: css-mixins
Level: 1
Status: ED
Work Status: Exploring
Group: CSSWG
ED: https://drafts.csswg.org/css-mixins/
TR: https://www.w3.org/TR/css-mixins-1/
Editor: Miriam E. Suzanne, Invited Expert, http://miriamsuzanne.com/contact, w3cid 117151
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Abstract: This module provides the ability to define custom functional notations.
Default Highlight: css
</pre>
<pre class=link-defaults>
spec:infra; type:dfn; text:list
spec:css-properties-values-api; type:dfn; text:supported syntax component name
spec:css-properties-values-api; type:dfn; text:syntax component
spec:css-syntax-3; type:dfn; text:descriptor;
spec:css-values-4; type:dfn; text:keyword;
spec:css-values-4; type:dfn; text:identifier;
</pre>
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Introduction {#intro}
=====================
<em>This section is not normative.</em>
Issue: TODO
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Defining Custom Functions {#defining-custom-functions}
======================================================
A [=custom function=] can be thought of as an advanced [=custom property=],
which instead of being substituted by a single fixed value
provides its substitution value based on [=function parameters=]
and [=local variables=].
Whenever a declaration's value contains a reference to a [=custom function=]
(a <<dashed-function>>),
the value behaves as if it contained a ''var()'',
with the actual check against the property's grammar delayed until computed-value time.
<div class='example'>
A simple [=custom function=] to negate a value can be defined as follows:
<pre class='lang-css'>
@function --negative (--value) {
result: calc(-1 * var(--value));
}
</pre>
Then, that function can be referenced with <code>--negative()</code>
in some declaration (assuming <code>--gap</code> is defined elsewhere):
<pre class='lang-css'>
html { padding: --negative(var(--gap)); }
</pre>
</div>
A <dfn>custom function</dfn> consists of a name (<<function-name>>),
a list of [=function parameter|parameters=],
a list of [=function dependency|dependencies=],
a function body,
and optionally a <dfn>return type</dfn> described by a [=syntax definition=].
A <dfn>function parameter</dfn> consists of a name (<<custom-property-name>>);
optionally a <dfn>parameter type</dfn>, described by a [=syntax definition=];
and optionally a <dfn>default value</dfn>.
A <dfn>function dependency</dfn>,
is a special [=function parameter=],
that represents
a [=local variable=], [=function parameter=], or [=custom property=]
being implicitly passed as an argument from the calling context.
The <dfn>@function</dfn> Rule {#function-rule}
----------------------------------------------
The ''@function'' rule defines a [=custom function=],
and its syntax is:
<pre class="prod def" nohighlight>
<@function> = @function <<function-name>> [ ( <<function-parameter-list>> ) ]?
[ using ( <<function-dependency-list>> ) ]?
[ returns <<css-type>> ]?
{
<<declaration-rule-list>>
}
<dfn><<function-name>></dfn> = <<dashed-ident>>
<dfn><<function-parameter-list>></dfn> = <<function-parameter>>#
<dfn><<function-dependency-list>></dfn> = <<function-parameter>>#
<dfn><<function-parameter>></dfn> = <<custom-property-name>> <<css-type>>? [ : <<declaration-value>> ]?
<dfn><<css-type>></dfn> = <<syntax-component>> | <<type()>>
<dfn><type()></dfn> = type( <<syntax>> )
</pre>
The name of the resulting [=custom function=] is given by the <<function-name>>,
the [=function parameters=] are optionally given by <<function-parameter-list>>,
the [=function dependencies=] are optionally given by <<function-dependency-list>>,
and the [=return type=] is optionally given by the <<css-type>> following the "returns" keyword.
<div class='example'>
If the <<css-type>> of a [=function parameter=],
[=function dependency=],
or [=custom function=] return value
can be described by a single <<syntax-component>>,
then the <code>type()</code> function may be omitted:
<pre class='lang-css'>
@function --foo(--a <length>) { /* ... */ }
@function --foo(--a <color>) { /* ... */ }
@function --foo(--a <length>+) { /* ... */ }
</pre>
However,
any <<syntax>> that requires a <<syntax-combinator>>
needs to be wrapped in the <code>type()</code> function:
<pre class='lang-css'>
@function --foo(--a type(<number> | <percentage>)) { /* ... */ }
</pre>
</div>
Issue: Should duplicates be disallowed <em>across</em> parameters/dependencies
as well?
If more than one ''@function'' exists for a given name,
then the rule in the stronger cascade layer wins,
and rules defined later win within the same layer.
The <<function-name>> of a ''@function'' rule is a [=tree-scoped name=].
If the <<function-parameter-list>>
contains the same <<custom-property-name>> more than once,
or if the <<function-dependency-list>>
contains the same <<custom-property-name>> more than once,
then the ''@function'' rule is invalid.
The body of a ''@function'' rule accepts [=conditional group rules=],
such as ''@media''.
Additionally, it accepts the following descriptors:
* The '@function/result' descriptor,
which determines the result of [=evaluating a custom function|evaluating the function=].
* [=Custom properties=],
acting like [=local variable=] descriptors.
Unknown descriptors are invalid and ignored,
but do not make the ''@function'' rule itself invalid.
The '@function/result' Descriptor {#the-result-descriptor}
----------------------------------------------------------
<pre class='descdef'>
Name: result
Value: <<declaration-value>>?
For: @function
Initial: n/a (see prose)
</pre>
The '@function/result' descriptor
determines the result of [=evaluate a custom function|evaluating=]
the [=custom function=] that is defined by a ''@function'' rule.
Using [=locally substitute a var()|locally substituted=] ''var()'' functions,
it can reference [=function parameters=], [=function dependencies=], [=local variables=],
as well as other [=custom functions=] via <<dashed-function>>s.
The '@function/result' descriptor itself does not have a type,
but its [=resolved local value|resolved=] value is type checked
during the [=substitute a dashed function|substitution=] of a <<dashed-function>>.
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Using Custom Functions {#using-custom-functions}
================================================
Similar to how the value of a [=custom property=] can be substituted
into the value of another property with ''var()'',
the result of a [=custom function=] evaluation can be substituted
with a <<dashed-function>>.
A <dfn><<dashed-function>></dfn> is a [=functional notation=]
whose function name starts with two dashes (U+002D HYPHEN-MINUS).
Its syntax is:
<pre class="prod informative" nohighlight>
--*( <<declaration-value>># )
</pre>
Issue: Mention semicolon upgrades.
A <<dashed-function>> can only be used where ''var()'' is allowed.
If a property contains one or more <<dashed-function>>s,
the entire property’s grammar must be assumed to be valid at parse time.
At computed-value time,
every <<dashed-function>> must be [=substitute a dashed function|substituted=]
before finally being checked against the property's grammar.
When a value is being computed,
substitution of ''var()'', ''env()'' and ''attr()''
must take place <em>before</em> <<dashed-function>> substitution.
Note: This means arguments passed to a custom function
never contain ''var()'', or similar functions.
A ''var()'' function within a [=local variable=],
or within the ''@function/result'' descriptor,
invokes [=locally substitute a var()|local substitution=],
rather than the computed-value based substitution
described in [[!css-variables]].
<div algorithm>
To <dfn>substitute a dashed function</dfn> in a value,
with |dashed function| being a <<dashed-function>>:
1. Let |function| be the result of dereferencing
the |dashed function|'s name as a [=tree-scoped reference=].
If no such name exists, return failure.
2. Let |dependency values| be an initially empty [=list=].
3. For each |dependency| in |function|'s [=function dependency|dependencies=]:
* Let |dependency value| be the value that would be substituted
if a ''var()'' function had been specified explicitly
at the end of |dashed function|'s argument list,
with |dependency| as its only argument.
* If that substitution would have made a containing declaration
[=invalid at computed-value time=],
set |dependency value| to the [=guaranteed-invalid value=].
* Append the result of [=resolving an argument=] to |dependency values|,
using |dependency value| as value,
and |dependency| as parameter.
4. [=Evaluate a custom function=],
using |function|, |dashed function| and |dependency values|.
5. If failure was returned, return failure.
6. Otherwise,
replace the <<dashed-function>> with the [=equivalent token sequence=]
of the value resulting from the evaluation.
</div>
If [=substitute a dashed function=] fails,
and the substitution is taking place on a property's value,
then the declaration containing the <<dashed-function>> becomes
[=invalid at computed-value time=].
Evaluating Custom Functions {#evaluating-custom-functions}
----------------------------------------------------------
<div algorithm>
To <dfn>evaluate a custom function</dfn>,
with |function| being a [=custom function=],
|dashed function| being the <<dashed-function>> invoking that |function|,
and |dependency values| being a [=list=] of values.
1. If the number of values in |dashed function|'s argument list
is greater than the number of values in |function|'s [=function parameter|parameters=],
return failure.
2. For each value |parameter| in |function|'s [=function parameter|parameters=],
let |argument| be the corresponding value in |dashed function|'s argument list
at the same index:
* If |argument| does not exist,
set |argument| to the [=guaranteed-invalid value=].
* Replace the value in |dashed function|'s argument list
with the result of [=resolving an argument=],
using |argument| as value,
and |parameter| as parameter.
3. Let |result| be the [=resolved local value=]
of the '@function/result' descriptor,
using |function|, |dashed function|, and |dependency values|.
4. If |function| has a [=return type=],
set |result| to the result of [=resolve a typed value|resolving a typed value=],
using |result| as the value,
and the [=syntax definition=] associated with the [=return type=] as the syntax.
5. If |result| is the [=guaranteed-invalid value=],
return failure.
6. Otherwise,
return |result|.
</div>
<div algorithm>
To <dfn>resolve an argument</dfn>,
with value |value|,
and [=function parameter|parameter=] |parameter|:
1. If |value| is not the [=guaranteed-invalid value=],
and |parameter| has a [=parameter type|type=],
set |value| to the result of [=resolve a typed value|resolving a typed value=]
using |value| as the value,
and the [=syntax definition=] associated with |parameter|'s type as the syntax.
<span class=note>This step may cause |value| to become [=guaranteed-invalid value|guaranteed-invalid=].</span>
2. If |value| is the [=guaranteed-invalid value=],
and |parameter| has a [=default value=],
set |value| to one of the following:
<dl class="switch">
: If |parameter| has a [=parameter type|type=]
:: The result of [=resolve a typed value|resolving a typed value=]
using the |parameter|'s [=default value=] as the value,
and the [=syntax definition=] associated with |parameter|'s type as the syntax.
: Otherwise
:: The |parameter|'s [=default value=].
</dl>
3. Return |value|.
</div>
<div algorithm>
To <dfn>resolve a typed value</dfn>,
with value |value|,
and [=syntax definition=] |syntax|:
1. If |value| is the [=guaranteed-invalid value=],
return |value|.
2. <a href="https://drafts.css-houdini.org/css-properties-values-api-1/#calculation-of-computed-values">Compute</a>
|value| as if it were the value associated with a [=registered custom property=]
whose [=syntax definition=] is |syntax|.
3. If this would lead to a declaration being [=invalid at computed-value time=],
return the [=guaranteed-invalid value=].
4. Otherwise, return that value.
</div>
Parameters and Locals {#parameters}
-----------------------------------
The [=function parameters=] and [=function dependencies=] of a [=custom function=]
are available for [=locally substitute a var()|local substitution=]
as if they were declared as [=local variables=]
at the start of the ''@function'' rule body.
Note: A [=local variable=] with the same name
as a [=function parameter=]/[=function dependency=] is allowed,
but will make the parameter/dependency unreachable
for [=locally substitute a var()|substitution=]
A <dfn>local variable</dfn>
is a custom property defined with the body of a [=custom function=].
It is only visible within the function where it is defined.
<div algorithm>
To <dfn>locally substitute a var()</dfn> within a value,
with |function| being a [=custom function=],
|dashed function| being the <<dashed-function>> invoking that |function|,
and |dependency values| being a [=list=] of values:
1. Let |substitution value| be one of the following options,
depending on the [=custom property=] named in the first argument of the ''var()'' function:
<dl class="switch">
: If the [=custom property=] name matches a [=local variable=] within |function|
:: The [=resolved local value=] of that [=local variable=].
: Otherwise, if the [=custom property=] name matches a [=function parameter|parameter=] within |function|
:: The corresponding argument value within the |dashed function|.
: Otherwise, if the [=custom property=] name matches a [=function dependency|dependency=] within |function|
:: The corresponding value of that [=function dependency|dependency=]
within |dependency values|.
: Otherwise
:: The [=guaranteed-invalid value=].
</dl>
2. If |substitution value| is not the [=guaranteed-invalid value=],
replace the ''var()'' function by that value.
3. Otherwise, if the ''var()'' function has a fallback value as its second argument,
replace the ''var()'' function by the [=resolved local value|locally resolved=] fallback value.
4. Otherwise, return failure.
</div>
A <dfn>resolved local value</dfn> is the value of a [=local variable=] or [=descriptor=], except:
* Any ''var()'' functions are replaced by [=locally substitute a var()|local substitution=].
* Any ''env()'' or ''attr()'' functions are substituted normally.
* Any <<dashed-function>>s are replaced by [=substitute a dashed function|dashed function substitution=].
If any substitution algorithm returns failure,
then the [=resolved local value=] of a [=local variable=]
is the [=guaranteed-invalid value=].
Cycles {#cycles}
----------------
Issue: TODO
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Execution Model of Custom Functions {#execution-model}
======================================================
Like the rest of CSS,
[=custom functions=] adhere to a declarative model.
The [=local variable=] descriptors
and '@function/result' descriptor
can appear in any order,
and may be provided multiple times.
If this happens, then declarations appearing later win over earlier ones.
<div class='example'>
<pre class='lang-css'>
@function --mypi() {
result: 3;
result: 3.14;
}
</pre>
The value of the '@function/result' descriptor of <code>--mypi</code>
is <code>3.14</code>.
</pre>
</div>
<div class='example'>
<pre class='lang-css'>
@function --circle-area(--r) {
result: calc(pi * var(--r2));
--r2: var(--r) * var(--r);
}
</pre>
[=Local variable=] descriptors may appear before or after
they are referenced.
</pre>
</div>
Conditional Rules {#conditional-rules}
--------------------------------------
A [=conditional group rule=] that appears within a ''@function''
becomes a [=nested group rule=],
with the additional restriction
that only descriptors allowed within ''@function''
are allowed within the [=nested group rule=].
[=Conditional group rules=] within ''@function''
are <a href="https://drafts.csswg.org/css-conditional-3/#processing">processed</a> as normal,
acting as if the contents of the rule were present
at the [=conditional group rule=]'s location
when the condition is true,
or acting as if nothing exists at that location otherwise.
<div class='example'>
<pre class='lang-css'>
@function --suitable-font-size() {
result: 16px;
@media (width > 1000px) {
result: 20px;
}
}
</pre>
The value of the '@function/result' descriptor
is <code>20px</code> if the media query's condition is true,
and <code>16px</code> otherwise.
</pre>
</div>
<div class='example'>
Note that due to the execution model,
"early return" is not possible within a ''@function'':
<pre class='lang-css'>
@function --suitable-font-size() {
@media (width > 1000px) {
result: 20px;
}
result: 16px;
}
</pre>
The value of the '@function/result' descriptor
is always <code>16px</code> in the above example.
</pre>
</div>
<div class='example'>
[=Local variables=] are also valid within conditional rules:
<pre class='lang-css'>
@function --suitable-font-size() {
--size: 16px;
@media (width > 1000px) {
--size: 20px;
}
result: var(--size);
}
</pre>
</div>
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CSSOM {#cssom}
==============
The {{CSSFunctionRule}} interface represents a ''@function'' rule.
<pre class='idl' export>
[Exposed=Window]
interface CSSFunctionRule : CSSGroupingRule { };
</pre>
While declarations may be specified directly within a ''@function'' rule,
they are not represented as such in the CSSOM.
Instead, consecutive segments of declarations
appear as if wrapped in {{CSSNestedDeclarations}} rules.
Note: This also applies to the "leading" declarations in the ''@function'' rule,
i.e those that do not follow another nested rule.
<div class='example'>
<pre class='lang-css'>
@function --bar() {
--x: 42;
result: var(--y);
@media (width > 1000px) {
/* ... */
}
--y: var(--x);
}
</pre>
The above will appear in the CSSOM as:
<pre class='lang-css'>
@function --bar() {
/* CSSNestedDeclarations { */
--x: 42;
result: var(--y);
/* } */
@media (width > 1000px) {
/* ... */
}
/* CSSNestedDeclarations { */
--y: var(--x);
/* } */
}
</pre>
</div>
Issue: Should we indeed use {{CSSNestedDeclarations}} for this purpose?
The <code>style</code> attribute of the {{CSSNestedDeclarations}} rule
should probably not be a regular {{CSSStyleDeclaration}},
since only custom properties
and the '@function/result' descriptor
are relevant.