Monthly Downloads: 116
Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Web     Css    
Latest version: v0.2.0.0
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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:

  1. a SelectorGroup is a group of one or more Selectors, these are comma-separated;
  2. 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;
  3. 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 SelectorFilters;
  4. A SelectorFilter is a Hash, a Class, or an Attrib;
  5. Both a TypeSelector and an AttributeName have a namespace. A namespace can be any (*), empty, or a namespace (which should be a valid identifier);
  6. A Hash is a valid identifier prepended with a number sign (#);
  7. A Class is a valid identifier prepended with a dot (.);
  8. 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.


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 Selectors in a Selector group, and sorting the SelectorFilters 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.

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.


You can contribute by making a pull request on the GitHub repository.

You can contact the package maintainer by sending a mail to [email protected].

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.