css-selectors
A library for parsing, manipulating, and rendering css selectors (not css files,
just the selectors).
It has a quasiquoter to enable Haskell to validate the css selector at compile
time.
Currently the css grammar is implemented without the pseudo-classes,
pseudo-elements and negations. One can furthermore calculate the specificity of
a css-selector, and thus perform an analysis over what css-selector will take
precedence.
The package documentation can be found on the GitHub pages.
Selector structure
A css selector has the following structure:
- a
SelectorGroup
is a group of one or more Selector
s, these are
comma-separated;
- A
Selector
is a custom linked list implementation where the "cons" (the
Combined
data constructor) contains a besides a reference to a
SelectorSequence
(head) and a Selector
(tail), it specifies what
SelectorCombinator
is used. A Selector
has at least one
SelectorSequence
, this is constructoed with the Selector
data
constructor;
- A
SelectorSequence
contains a TypeSelector
(in case the TypeSelector
is Universal
, this does not need to be part of the css-selector
expression); and a set of zero or more SelectorFilter
s;
- A
SelectorFilter
is a Hash
, a Class
, or an Attrib
;
- Both a
TypeSelector
and an AttributeName
have a namespace. A namespace
can be any (*
), empty, or a namespace (which should be a valid
identifier);
- A
Hash
is a valid identifier prepended with a number sign (#
);
- A
Class
is a valid identifier prepended with a dot (.
);
- An
Attribute
can be an Exist
object that imposes a constraint that the
attribute should exist for the given tag, or an Attrib
that specifies
that the attribute exists, and that the value for this attribute satisfies
a given constraint. This constraint is determined by the
AttributeCombinator
and the value of the Attrib
object.
Quasiquoter
The main use of this package is a quasiquoter, that can be used both for
expressions and patterns. We thus can construct a SelectorGroup
in an
expression with:
myCssSelector :: SelectorGroup
myCssSelector = [csssel|* html .pun .inbox, * html .pun #bdrdmain, * html .pun .infldset|]
A less common use case is using the quasiquoter in a pattern to check if a given
SelectorGroup
matches exactly with a given css selector. For example:
isMyCssSelector :: SelectorGroup -> Bool
isMyCssSelector [csssel|* html .pun .unbox|] = True
isMyCssSelector _ = False
The quasiquoter can be used in a type signature as well, but will always,
regardless of the content, return the type for SelectorGroup
. If you use the
quasiquoter as a declaration, it will simply not generate any declarations. It
will raise a warning (not an error) about this.
Perhaps in the (far) future, we will make more sensical implementations for the
type and declaration part of the quasiquoter.
Note that you need to enable the -XQuasiQuotes
pragma when you compile.
Selector normalization
One can turn equivalent css selectors in a "normalized" form. This is done by
sorting the Selector
s in a Selector
group, and sorting the SelectorFilter
s
of a certain SelectorSequence
.
The order is determined by the default instances of Ord
of the sequences. This
is thus not an "inherent" ordering of the css selector, but just an order that
the program constructed to convert multiple css selectors that are equivalent
same to a normal form in which these are equal.
We here do not optimize the css selector, for example by removing duplicate
filters, since that can have impact on the specificity of the selector.
Selector specificity
The specificity of a selector is defined by three numbers a, b and c.
Later, one calculates the specificity level with 100 a + 10 b + c. The higher
the specificity level, the more it takes precedence. If there are thus two
selectors and the former selector has 14 as specificity level, and the latter
has 42 as specificity level, then rules defined in the latter, will "overrule"
the rules defined in the former, given these rules "clash".
One can calculate the specificity of a item with as type a member of the ToCssSelector
class with:
specificity :: ToCssSelector a => a -> Int
or you can obtain a more detailed result with:
specificity' :: ToCssSelector a => a -> SelectorSpecificity
ToMarkup
, ToJSON
, and ToJavascript
instances
The types that are members of the ToCssSelector
are members of the ToMarkup
,
ToJSON
, and ToJavascript
type classes as well, such that we can conveniently
use these in blaze HTML and for example in Hamlet.
The ToMarkup
instance will render the css selector as raw content. So if you
add this as an attribute, the css selector will appear, unescaped, in the
rendered page. Note that it will be escaped, so foo > bar
will be generated as
foo > bar
.
The ToJSON
instance will convert the given object in a JSON string that
contains the css selector.
The ToJavascript
will render the content to a javascript string. So if you
use this in hamlet, you generate a string that contains the css-selector. This
is often useful, since javascript itself has no syntax for css selectors, and
often strings are used to represent these.
Binary
instances
The css-elements are all a member of the Binary
typeclass, that converts the
css selector to a compact binary format. This is not standard format. This is
more to write a css-selector to a binary format and back.
Arbitrary
css selectors
One can generate arbitrary CSS selectors (and their subcomponents). It is
however not advisable to use this for anything other than for validation
purposes (like with QuickCheck
).
css-selectors
is not safe Haskell
There are not extensions that are used that make the library itself
unsafe, but it makes use of aeson
, blaze-markup
, etc. and the packages are
not safe. Hence this package is not safe Haskell.
Future plans
We want to implement an extra quasiquoter with the ability to specify variables,
that can then be used in expressions, or in patterns.
Contribute
You can contribute by making a pull request on the GitHub
repository.
You can contact the package maintainer by sending a mail to
hapytexeu+gh@gmail.com
.
This package is dedicated in loving memory to my mother, Veerle Dumon
(1958-2019), in the hope that eventually it will be as stylish as she was.