How @extend Works
Posted 23 November 2013 by Natalie Weizenbaum
This was originally published as a gist.
Aaron Leung is working on
libsass and was wondering how
@extend is implemented in the Ruby implementation of Sass. Rather than just
tell him, I thought I’d write up a public document about it so anyone who’s
porting Sass or is just curious about how it works can see.
Note that this explanation is simplified in numerous ways. It’s intended to
explain the most complex parts of a basic correct
@extend transformation, but
it leaves out numerous details that will be important if full Sass compatibility
is desired. This should be considered an explication of the groundwork for
@extend, upon which full support can be built. For a complete understanding of
@extend, there’s no substitute for consulting the Ruby Sass
code and its tests.
This document assumes familiarity with the selector terminology defined in the Selectors Level 4 spec. Throughout the document, selectors will be treated interchangeably with lists or sets of their components. For example, a complex selector may be treated as a list of compound selectors or a list of lists of simple selectors.
Following are a set of primitive objects, definitions, and operations that are
necessary for implementing
@extend. Implementing these is left as an exercise
for the reader.
A selector object is obviously necessary, since
@extendis all about selectors. Selectors will need to be parsed thoroughly and semantically. It’s necessary for the implementation to know a fair amount of the meaning behind the various different forms of selectors.
A custom data structure I call a "subset map" is also necessary. A subset map has two operations:
Map.get(Set) => [Object]. The former associates a value with a set of keys in the map. The latter looks up all values that are associated with subsets of a set of keys. For example:
map.set([1, 2], 'value1') map.set([2, 3], 'value2') map.set([3, 4], 'value3') map.get([1, 2, 3]) => ['value1', 'value2']
S1is a "superselector" of a selector
S2if every element matched by
S2is also matched by
S1. For example,
.foois a superselector of
ais a superselector of
div a, and
*is a superselector of everything. The inverse of a superselector is a "subselector".
unify(Compound Selector, Compound Selector) => Compound Selectorthat returns a selector that matches exactly those elements matched by both input selectors. For example,
.foo.bar. This only needs to work for compound or simpler selectors. This operation can fail (e.g.
unify(a, h1)), in which case it should return
trim([Selector List]) => Selector Listthat removes complex selectors that are subselectors of other complex selectors in the input. It takes the input as multiple selector lists and only checks for subselectors across these lists since the prior
@extendprocess won’t produce intra-list subselectors. For example, if it’s passed
[[a], [.foo a]]it would return
.foo ais a subselector of
paths([[Object]]) => [[Object]]that returns a list of all possible paths through a list of choices for each step. For example,
paths([[1, 2], , [4, 5, 6]])returns
[[1, 3, 4], [1, 3, 5], [1, 3, 6], [2, 3, 4], [2, 3, 5], [2, 3, 6]].
The AlgorithmThe Algorithm permalink
@extend algorithm requires two passes: one to record the
are declared in the stylesheet, and another to transform selectors using those
@extends. This is necessary, since
@extends can affect selectors earlier in
the stylesheet as well.
Recording PassRecording Pass permalink
In pseudocode, this pass can be described as follows:
let MAP be an empty subset map from simple selectors to (complex selector, compound selector) pairs for each @extend in the document: let EXTENDER be the complex selector of the CSS rule containing the @extend let TARGET be the compound selector being @extended MAP.set(TARGET, (EXTENDER, TARGET))
Transformation PassTransformation Pass permalink
The transformation pass is more complicated than the recording pass. It’s described in pseudocode below:
let MAP be the subset map from the recording pass define extend_complex(COMPLEX, SEEN) to be: let CHOICES be an empty list of lists of complex selectors for each compound selector COMPOUND in COMPLEX: let EXTENDED be extend_compound(COMPOUND, SEEN) if no complex selector in EXTENDED is a superselector of COMPOUND: add a complex selector composed only of COMPOUND to EXTENDED add EXTENDED to CHOICES let WEAVES be an empty list of selector lists for each list of complex selectors PATH in paths(CHOICES): add weave(PATH) to WEAVES return trim(WEAVES) define extend_compound(COMPOUND, SEEN) to be: let RESULTS be an empty list of complex selectors for each (EXTENDER, TARGET) in MAP.get(COMPOUND): if SEEN contains TARGET, move to the next iteration let COMPOUND_WITHOUT_TARGET be COMPOUND without any of the simple selectors in TARGET let EXTENDER_COMPOUND be the last compound selector in EXTENDER let UNIFIED be unify(EXTENDER_COMPOUND, COMPOUND_WITHOUT_TARGET) if UNIFIED is null, move to the next iteration let UNIFIED_COMPLEX be EXTENDER with the last compound selector replaced with UNIFIED with TARGET in SEEN: add each complex selector in extend_complex(UNIFIED_COMPLEX, SEEN) to RESULTS return RESULTS for each selector COMPLEX in the document: let SEEN be an empty set of compound selectors let LIST be a selector list comprised of the complex selectors in extend_complex(COMPLEX, SEEN) replace COMPLEX with LIST
A keen reader will have noticed an undefined function used in this pseudocode:
weave is much more complicated than the other primitive operations,
so I wanted to explain it in detail.
At a high level, the "weave" operation is pretty easy to understand. It’s best
to think of it as expanding a "parenthesized selector". Imagine you could write
.foo (.bar a) and it would match every
a element that has both a
parent element and a
.bar parent element.
weave makes this happen.
In order to match this
a element, you need to expand
.foo (.bar a) into the
following selector list:
.foo .bar a, .foo.bar a, .bar .foo a. This matches
all possible ways that
a could have both a
.foo parent and a
weave does not in fact emit
.foo.bar a; including merged selectors
like it would cause exponential output size and provide very little utility.
This parenthesized selector is passed in to
weave as a list of complex
selectors. For example,
.foo (.bar a) would be passed in as
[.foo, .bar a].
(.foo div) (.bar a) (.baz h1 span) would be passed in as
[.foo div, .bar a, .baz h1 span].
weave works by moving left-to-right through the parenthesized selector,
building up a list of all possible prefixes and adding to this list as each
parenthesized component is encountered. Here’s the pseudocode:
let PAREN_SELECTOR be the argument to weave(), a list of complex selectors let PREFIXES be an empty list of complex selectors for each complex selector COMPLEX in PAREN_SELECTOR: if PREFIXES is empty: add COMPLEX to PREFIXES move to the next iteration let COMPLEX_SUFFIX be the final compound selector in COMPLEX let COMPLEX_PREFIX be COMPLEX without COMPLEX_SUFFIX let NEW_PREFIXES be an empty list of complex selectors for each complex selector PREFIX in PREFIXES: let WOVEN be subweave(PREFIX, COMPLEX_PREFIX) if WOVEN is null, move to the next iteration for each complex selector WOVEN_COMPLEX in WOVEN: append COMPLEX_SUFFIX to WOVEN_COMPLEX add WOVEN_COMPLEX to NEW_PREFIXES let PREFIXES be NEW_PREFIXES return PREFIXES
This includes yet another undefined function,
subweave, which contains most of
the logic of weaving together selectors. It’s one of the most complicated pieces
of logic in the entire
@extend algorithm – it handles selector combinators,
superselectors, subject selectors, and more. However, the semantics are
extremely simple, and writing a baseline version of it is very easy.
weave weaves together many complex selectors,
subweave just weaves
two. The complex selectors it weaves together are considered to have an implicit
identical trailing compound selector; for example, if it’s passed
.x .y .z, it weaves them together as though they were
.foo .bar E and
.x .y .z E. In addition, it doesn’t merge the two selectors in most cases, so
it would just return
.foo .bar .x .y .z, .x .y .z .foo .bar in this case. An
extremely naive implementation could just return the two orderings of the two
arguments and be correct a majority of the time.
Delving into the full complexity of
subweave is out of scope here, since it
falls almost entirely into the category of advanced functionality that this
document is intentionally avoiding. The code for it is located in
and should be consulted when attempting a serious implementation.