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.
A css selector has the following structure:
SelectorGroupis a group of one or more
Selectors, these are comma-separated;
Selectoris a custom linked list implementation where the "cons" (the
Combineddata constructor) contains a besides a reference to a
SelectorSequence(head) and a
Selector(tail), it specifies what
SelectorCombinatoris used. A
Selectorhas at least one
SelectorSequence, this is constructoed with the
TypeSelector(in case the
Universal, this does not need to be part of the css-selector expression); and a set of zero or more
Class, or an
- Both a
AttributeNamehave a namespace. A namespace can be any (
*), empty, or a namespace (which should be a valid identifier);
Hashis a valid identifier prepended with a number sign (
Classis a valid identifier prepended with a dot (
Attributecan be an
Existobject that imposes a constraint that the attribute should exist for the given tag, or an
Attribthat specifies that the attribute exists, and that the value for this attribute satisfies a given constraint. This constraint is determined by the
AttributeCombinatorand the value of the
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
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.
One can turn equivalent css selectors in a "normalized" form. This is done by
Selectors in a
Selector group, and sorting the
of a certain
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.
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
specificity :: ToCssSelector a => a -> Int
or you can obtain a more detailed result with:
specificity' :: ToCssSelector a => a -> SelectorSpecificity
The types that are members of the
ToCssSelector are members of the
use these in blaze HTML and for example in Hamlet.
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.
ToJSON instance will convert the given object in a JSON string that
contains the css selector.
use this in hamlet, you generate a string that contains the css-selector. This
often strings are used to represent these.
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
css-selectors is not safe Haskell
There are not extensions that are used that make the library itself
unsafe, but it makes use of
blaze-markup, etc. and the packages are
not safe. Hence this package is not safe Haskell.
We want to implement an extra quasiquoter with the ability to specify variables, that can then be used in expressions, or in patterns.
You can contribute by making a pull request on the GitHub repository.
You can contact the package maintainer by sending a mail to
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.