A graphical user interface (or GUI, sometimes pronounced "gooey") is a method of interacting with a computer through a metaphor of direct manipulation of graphical images and widgets in addition to text.

GUIs display visual elements such as icons, windows and other gadgets. The precursor to GUIs was invented by researchers at the Stanford Research Institute (led by Doug Engelbart) with the development and use of text-based hyperlinks manipulated with a mouse for the On-Line System. The concept of hyperlinks was further refined and extended to graphics by researchers at Xerox PARC, who went beyond text-based hyperlinks and used GUIs as the primary interface for the Xerox Alto computer. Most modern general-purpose GUIs are derived from this system. For this reason some people call this class of interface a PARC User Interface (PUI) (note that PUI is also an acronym for perceptual user interface). The PUI consists of graphical widgets (often provided by widget toolkit libraries) such as windows, menus, radio buttons, check boxes and icons, and employs a pointing device (such as a mouse, trackball or touchscreen) in addition to a keyboard. Those aspects of PUIs can be emphasized by using the alternative acronym WIMP, which stands for Windows, Icons, Menus and Pointing device...

Spotlight on XSL (Intermediate Article)

The eXtensible Stylesheet Language (XSL) is a family of languages which allows one to describe how files encoded in the XML standard are to be formatted or transformed. There are three languages in the family:

- XSL Transformations (XSLT): an XML language for transforming XML documents

- XSL Formatting Objects (XSL-FO): an XML language for specifying the visual formatting of an XML document

- the XML Path Language (XPath): a non-XML language used by XSLT, and also available for use in non-XSLT contexts, for addressing the parts of an XML document.

These three specifications are available in the form of W3C Recommendations.

Within Microsoft, the term XSL is sometimes used to refer to a Microsoft variant of XSLT developed as an implementation of an early (1998) W3C draft of the XSLT language, with Microsoft-specific extensions and omissions. Other commentators generally refer to this dialect as WD-xsl. The dialect was later superseded by a conformant implementation of the W3C specification.

XSL started off as an attempt to bring the functionality of DSSSL, particularly in the area of print and high-end typeseting, to XML. A W3C Working group on XSL was started in December 1997, with Sharon Adler and Steve Zilles as co-chairs, with James Clark acting as editor (and unofficially as chief designer), and Chris Lilley as W3C staff contact. The group released a first public Working Draft on 18 August 1998. XSLT and XPath became W3C Recommendations on 16 November 1999 and XSL reached Recommendation status on 15 October 2001.

X-Query (Intermediate Article)

XQuery is a query language (with some programming language features) that is designed to query collections of XML data. It is semantically similar to SQL.

XQuery 1.0 is being developed by the XML Query working group of the W3C. The work is closely coordinated with the development of XSLT 2.0 by the XSL Working Group; the two groups share responsibility for XPath 2.0, which is a subset of XQuery 1.0. XQuery 1.0 became a W3C Candidate Recommendation on November 3, 2005.

XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents.

XQuery uses XPath expression syntax to address specific parts of an XML document. It supplements this with a SQL-like "FLWOR expression" for performing joins. A FLWOR expression is constructed from the five clauses after which it is named: FOR, LET, WHERE, ORDER BY, RETURN.

The language also provides syntax allowing new XML documents to be constructed. Where the element and attribute names are known in advance, an XML-like syntax can be used; in other cases, expressions referred to as dynamic node constructors are available. All these constructs are defined as expressions within the language, and can be arbitrarily nested.

The language is based on a tree-structured model of the information content of an XML document, containing seven kinds of node: document nodes, elements, attributes, text nodes, comments, processing instructions, and namespaces.

The type system of the language models all values as sequences (a singleton value is considered to be a sequence of length one). The items in a sequence can either be nodes or atomic values. Atomic values may be integers, strings, booleans, and so on: the full list of types is based on the primitive types defined in XML Schema.

XQuery 1.0 does not include features for updating XML documents or databases. It also lacks full text search capability. These features are both under active development for a subsequent version of the language.

YAML (Novice Article)

YAML is a human-readable data serialization format that takes concepts from languages such as XML, C, Python, Perl, as well as the format for electronic mail as specified by RFC 2822. YAML was first proposed by Clark Evans, who designed it together with Ingy döt Net and Oren Ben-Kiki.

YAML is a recursive acronym meaning "YAML Ain't Markup Language". Early in its development, YAML was said to mean "Yet Another Markup Language", retronymed to distinguish its purpose as data-centric, rather than document markup. However since XML (a true document markup language) is frequently used for data serialization, it is reasonable to consider YAML a lightweight markup language.

YAML was created with the belief that all data can be adequately represented by combinations of lists, hashes (mappings), and scalar (single value) data. The syntax is relatively straightforward and was designed with human readability in mind, but also to be easily mapped to data types common to most high-level languages. In addition, YAML uses a notation based on indentation and/or a set of sigil characters distinct from those used in XML, making the two languages easily composable.

XML Schema (Advanced Article)

An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntax constraints imposed by XML itself. An XML schema provides a view of the document type at a relatively high level of abstraction.

There are languages developed specifically to express XML schemas. The Document Type Definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas. Two other very popular, more expressive XML schema languages are XML Schema and RELAX NG.

The mechanism for associating an XML document with a schema varies according to the schema language. The association may be achieved via markup within the XML document itself, or via some external means.

The process of checking to see if an XML document conforms to a schema is called validation, which is separate from XML's core concept of syntactic well-formedness. All XML documents must be well-formed, but it is not required that a document be valid unless the XML parser is "validating", in which case the document is also checked for conformance with its associated schema. DTD-validating parsers are most common, but some support W3C XML Schema or RELAX NG as well.

Documents are only considered valid if they satisfy the requirements of the schema with which they have been associated. These requirements typically include such constraints as:

Elements and attributes that must/may be included, and their permitted structure
How character data is to be interpreted, e.g. as a number, a date, a URL, a Boolean, etc