This manual describes the SCSS Scheme module.
Copyright © 2011 Julian Graham.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
Appendices
Indices
SCSS is an API for parsing and manipulating W3C Cascading Style Sheets in the Scheme programming language. Although the majority of the W3C recommendation is devoted to the proper rendering of documents styled with CSS, SCSS concerns itself only with the formal structure of the style sheet itself – in other words, SCSS doesn't render anything! You can however, combine SCSS with the XML parser of your choice (SCSS integrates particularly well with SDOM: http://www.nongnu.org/sdom/) to obtain style information about your XML documents so that they may be rendered correctly.
This document contains a desription of the SCSS API; for general information about Cascading Style Sheets, see the W3C's CSS site (http://www.w3.org/Style/CSS/).
The following data structures are defined by the API and are implemented as R6RS record types.
The scss:cascade type holds the three style sheets that make up the CSS cascade: the user style sheet, the user agent style sheet, and the author style sheet. In SCSS, each of these style sheets is an S-expression (potentially obtained from scss:css->scss) in the format described in See SCSS style sheet representation.
A new scss:cascade instance must be initialized with the three style sheets above; #f may be substituted for style sheets to be omitted from the cascade. Each of user-sheet, agent-sheet, and author-sheet, if specified, should be a valid SCSS style sheet, as determined by scss:stylesheet?.
The contents of the cascade may be manipulated using the following functions.
Set stylesheet as the “user” component of cascade. stylesheet and cascade must be a valid style sheet and a valid cascade, as determined by scss:stylesheet? and scss:cascade?, respectively.
Set stylesheet as the “author” component of cascade. stylesheet and cascade must be a valid style sheet and a valid cascade, as determined by scss:stylesheet? and scss:cascade?, respectively.
Set stylesheet as the “user agent” component of cascade. stylesheet and cascade must be a valid style sheet and a valid cascade, as determined by scss:stylesheet? and scss:cascade?, respectively.
The document interface provides a set of functions used for traversing a document tree and extracting information about the nodes it contains. This structure must be initialized with the following arguments.
A name-function, which must be a function of one argument, and which will be passed a node from the document tree. It should return the name of the element as a string.
A parent-function, which must be a function of one argument, and which will be passed a node from the document tree. It should return the node parent if the specified node is not its document's root element, #f otherwise.
A prev-sibling-function, which must be a function of one argument, and which will be passed a node from the document tree. It should return the node's previous sibling if the node is not its document's root element and not its parent's first child, #f otherwise.
An attribute-function, which must be a function of two arguments, and which will be passed a node from the document tree and the name of an attribute as a string. It should return the value of the specified attribute for the specified node if it is present, #f otherwise.
A class-function, which must be a function of one argument, and which will be passed a node from the document tree. It should return as a string the value of the “class” attribute for the specified node in the XML dialect to which the node belongs, or #f if the attribute is not set or cannot be determined.
An id-function, which must be a function of one argument, and which will be passed a node from the document tree. It should return as a string the value of the ID attribute for the node in the XML dialect to which the node belongs, or #f if the attribute is not set or cannot be determined.
Note that a sample instance of scss:document-interface that provides implementations of the functions described above that work well with SDOM is exported by the
(scss interface sdom)R6RS library that comes with this version of SCSS.
The rendering interface wraps functions that provide information to SCSS that cannot be obtained from the document or cascade alone – information that is available to the rendering layer of an application, for example. This structure must be initialized with the following arguments.
A pseudo-class-predicate, which must be a function of two arguments and will be passed a node obtained from the document interface and a string giving the name of a pseudo-class; the function should return #t if the node has the specified pseudo-class, #f otherwise.
The pseudo-class name will be one of the following strings:
- “link”
- “visited”
- “hover”
- “active”
- “focus”
- “lang”
A pseudo-element-predicate, which must be a function of two arguments and will be passed a node obtained from the document interface and a string giving the name of a pseudo-element; the function should return #t if the node has an immediate pseudo-element child of the specified type, #f otherwise.
The pseudo-element type will be one of the following strings:
- “first-line”
- “first-letter”
- “before”
- “after”
A selection context encapsulates the static information that SCSS needs to perform style queries. Instances of scss:selection-context must be initialized with instances of scss:document-interface, scss:rendering-interface, and scss:cascade; their construction is described above.
Represents the value obtained from the cascade for a CSS property at a particular document node. This record type provides the following field accessors.
- scss:selected-value-selector returns an S-expression representing the selector that was used to match the style rule represented by this record against the node passed to scss:select-values.
- scss:selected-value-property returns a symbol giving the name of the property the record represents.
- scss:selected-value-value returns a string giving the value of the property the record represents.
- scss:selected-value-important? returns #t if the rule that produced the selected value in the cascade was marked !important, #f otherwise.
Reads a complete style sheet from port, verifying that it is well- formed and represents valid style information, and returns a Scheme- language representation of the style sheet. See SCSS style sheet representation. Certain language elements not directly related to style information, such as comments, will be discarded during parsing, and @-rules will be evaluated and the results added to the resulting style sheet.
In particular, @import @-rules will cause SCSS to attempt to import the requested remote style sheet into the one currently being parsed. SCSS understands relative and absolute filesystem paths as well as the “file://” URI scheme. (Use scss:set-uri-import-handler! to handle other URI schemes.) As per the W3C recommendation, relatively- specified paths resolve relative to the location of the importing style sheet. If the base-url parameter is specified, it will be used as a base URI against which to resolve relative paths and URIs during the location of imported style documents.
As part of parsing the style sheet, this function will expand comma- delimited groups of selectors so that each selector refers internally to its own copy of the ruleset. This optimization does not affect style queries, and can be reversed while emitting CSS-formatted style information from an SCSS style sheet.
Throws an error if the root style sheet is either lexically or semantically invalid. Errors in imported style sheets will be ignored.
Install the procedure proc as the handler to be called to retrieve data from an @import directive that specifies a URI for the remote style sheet whose protocol is not “file://”. proc should have the following signature:
(lambda (uri) ...)...where uri will be a string giving the original URI from the @import directive. proc should return a valid input port from which the requested style information can be read. If proc does not return a valid input port, the @import directive will be ignored and parsing will proceed normally.
Sends a pretty-printed text representation of stylesheet to port. Throws an error if stylesheet is not a valid style sheet as determined by scss:stylesheet?.
(Note that it is not necessary to understand the information in this section to use the SCSS API. It is provided for users who curious about how SCSS works or want to construct their own cascades or style sheets from scratch.)
As of version 0.3, SCSS's style sheet representation is based on the format fully described in the documentation of Jim Bender's WebIt! package (http://celtic.benderweb.net/webit/), and is outlined in brief below. The S-expr style sheet representations produced by SCSS are wholly compatible with the ones used by WebIt!, provided that only selector types and syntax from the CSS1 recommendation is used. Some (externally incompatible) extensions to this format have been added, in order to support CSS2.1; they are described at the end of this section.
An SCSS style sheet is a list, the head of which is the symbol css, and the tail of which is a list of rules, each of which in turn consists of a head element (a selector) and a tail of property- value lists, each of which can be optionally wrapped in a list whose CAR is !, the importance signifier. The selector may be a symbol, a list of symbols, or a composition of symbols and operators according the rules set forth by the WebIt! specification. Each of the simple selector types defined by the CSS recommendation has a short and an expanded representation in the WebIt! format. The SCSS parser will produce the short form when possible, but will fall back to the long form if an unambiguous short form cannot be produced from the given input. The following table gives the short and expanded forms of the selector types SCSS has in common with WebIt!:
| Selector type | Short form | Expanded form
|
|---|---|---|
| Class | SELECTOR.CLASS | '(class SELECTOR CLASS)
|
| Pseudo-class | SELECTOR:CLASS | '(pclass SELECTOR CLASS)
|
| ID | SELECTOR|#ID# | '(id SELECTOR ID)
|
Descendent path selectors are represented as a list of simple selectors with the symbol // as its head.
Two or more selectors can be grouped by making them into the elements of a list.
The following are extensions to the WebIt! format that have been added to support the selector types and syntax introduced by CSS2.1:
foo:first-line
'(pelement foo before)
foo[bar~=baz]
foo[attrName]
The long expanded syntax is a list of three elements, and has the form
(attrib SUB-SELECTOR ATTRIB-EXPRESSION)
...where SUB-SELECTOR is the rest of this selector, in short or expanded form; and ATTRIB-EXPRESSION is either a symbol giving the attribute's name, meaning that no value is specified for the attribute selector, or a list with three elements, of the form
(EQUALITY-OPERATOR ATTRIB-NAME ATTRIB-VALUE)
...where EQUALITY-OPERATOR is a symbol eq to =, ~=, or |=; ATTRIB-NAME is a symbol; and ATTRIB-VALUE is a string. Examples of the extended syntax:
'(attrib p q)
'(attrib (pclass p r) (|= id "foo"))
'(+ SELECTOR-A SELECTOR-B)
'(> SELECTOR-A SELECTOR-B)
...where SELECTOR-A and SELECTOR-B are selectors in short or expanded form.
(@media MEDIA-TYPE-LIST RULESET)
...where MEDIA-TYPE-LIST is a list containing one or more symbols whose names are equal to the media type names from the CSS recommendation's list, and RULESET is a ruleset in SCSS's regular format. As per the CSS recommendation, media types do not nest.
See the WebIt! documentation and the W3 CSS recommendation for more information.
Evaluates to #t if and only if sheet is a valid result of parsing a style sheet using scss:css->scss or has been constructed according to the SCSS format specification. See SCSS style sheet representation.
Note that this operation performs a deep inspection of the contents of sheet; consequently, its performance is dependent on the size of its argument.
Evaluates to #t if and only if prop-name is a symbol giving the name of a property that can be inherited from a parent node in a document tree, #f otherwise.
Returns a list of scss:selected-value records for properties assigned to the specified document node node by the cascade included in the selection context selection-context. node must be a document node whose representation is compatible with the document interface included in selection-context, which must be an scss:selection-context record constructed as described above.
The returned records may be inspected using the record descriptors described above. See Data structures.
The selector matching process proceeds according to the CSS recommendation – briefly put, matching properties from all matching selectors are retrieved, sorted by importance, then sorted by selector specificity, and then returned.
Evaluates to the default value of the CSS property whose name is prop-name, a symbol. If prop-name is not a valid property name, #f will be returned.
Attempts to derive a value for a more specific property property from the value for a more general convenience property, value. property must be a symbol giving the name of a valid CSS2.1 property. The rules for increasing specificity are individual to the property in question. For example, the expression
(scss:specify 'background-color "background: url(\"chess.png\") gray 50% repeat fixed")evaluates to the color value “gray”.
The following functions are not directly related to the style cascade but may be useful to developers working with style information.
Evaluates to a string representation of a hexadecimal number corresponding to the string given by color, if color is one of “maroon,” “red,” “orange,” “yellow,” “olive,” “purple,” “fuschia,” “white,” “lime,” “green,” “navy,” “blue,” “aqua,” “teal,” “black,” “silver,” or “gray;” otherwise, the empty list.
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document-interface: Data structuresname-function: Data structuresobj: Data structurespseudo-class-predicate: Data structuresscss:color->hex: Utility functionsscss:css->scss: Parsing and emitting style informationscss:get-default-value: Querying the cascadescss:inherited?: Style predicatesscss:scss->css: Parsing and emitting style informationscss:select-values: Querying the cascadescss:set-agent-stylesheet!: Data structuresscss:set-author-stylesheet!: Data structuresscss:set-uri-import-handler!: Parsing and emitting style informationscss:set-user-stylesheet!: Data structuresscss:specify: Querying the cascadescss:stylesheet?: Style predicatesselected-value: Data structuresuser-sheet: Data structures