@akhleung is working on hcatlin/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
@extend
is 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.set(Set, Object)
andMap.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']
-
A selector
S1
is a "superselector" of a selectorS2
if every element matched byS2
is also matched byS1
. For example,.foo
is a superselector of.foo.bar
,a
is a superselector ofdiv a
, and*
is a superselector of everything. The inverse of a superselector is a "subselector". -
An operation
unify(Compound Selector, Compound Selector) => Compound Selector
that returns a selector that matches exactly those elements matched by both input selectors. For example,unify(.foo, .bar)
returns.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 returnnull
. -
An operation
trim([Selector List]) => Selector List
that 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@extend
process won't produce intra-list subselectors. For example, if it's passed[[a], [.foo a]]
it would return[a]
since.foo a
is a subselector ofa
. -
An operation
paths([[Object]]) => [[Object]]
that returns a list of all possible paths through a list of choices for each step. For example,paths([[1, 2], [3], [4, 5, 6]])
returns[[1, 3, 4], [1, 3, 5], [1, 3, 6], [2, 3, 4], [2, 3, 5], [2, 3, 6]]
.
The @extend
algorithm requires two passes: one to record the @extend
s that
are declared in the stylesheet, and another to transform selectors using those
@extend
s. This is necessary, since @extend
s can affect selectors earlier in
the stylesheet as well.
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))
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
. 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 .foo
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 .bar
parent.
However, 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]
.
Similarly, (.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.
Where 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 .foo .bar
and .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
lib/sass/selector/sequence.rb
and should be consulted when attempting a serious implementation.
It would be good to note that WEAVE is just a simplified, manual implementation of the functionality exposed by CSS's
:matches()
pseudoclass, and that once:matches()
is supported everywhere, WEAVE can be drastically simplified to just do a:matches()
instead, producing simpler and more complete/correct results.In particular, it means that weaving together ".foo" and ".bar" really would allow the full combinatorial set of ".foo .bar", ".bar .foo", and ".foo.bar", because it would no longer be a combinatorial explosion of cases, just a simple ":matches(.foo, .bar)".
Same with more complex selectors like ".foo .bar" and ".x .y .z" - producing ":matches(.foo .bar, .x .y .z)" is complete and correct, with none of the craziness and case-skipping needed to produce reasonably-sized selectors manually.