The Mjølner System: Metaprogramming System

A metaprogram is a program that manipulate other programs. Yggdrasil is a metaprogramming system, that supports creation of metaprograms. Yggdrasil is grammar based: a metaprogramming environment may be generated from the grammar of any language

The metaprograms manipulate programs through a common represen tation called abstract syntax trees (ASTs) An AST is modelled as an instance of a pattern. There is a pattern corresponding to each syntactic category (non-terminal) of the grammar. The grammar hierarchy is modelled by a corresponding pattern hierarchy, derived automatically from the grammar.

Grammar Based Interface

The grammar for a specific language result in a grammar based interface-a metaprogramming environment with a set of patterns for the language. A metaprogram using the grammar based interface will thus be language specific. Examples of tools that benefit from using the grammar based interface are: semantic checkers, program analyzers, interpreters, browsers, and graphical presentation tools.

Three Levels of Support

The AST may be manipulated at different levels of specialization:

Tree level: Here the AST is viewed as a tree. Examples of tools accessing the AST using the tree level are table-driven parsers and syntax-directed editors
Context-free level: This is the grammars based interface generated automatically from the grammar. Graphical presentation tools and browsers and examples of tools using the context-free level.
Semantic level: At this level semantic attributes may be added to the AST. The compiler is an example of a tool using the semantic level to access the AST.

Users may develop their own metaprogramming tools.

Yggdrasil supports generation of metaprogramming environments from a grammar. Users can then use this environment to develop their own metaprogramming tools-tools that manipulate other programs.

The Fragment System

The fragment system is a part of Yggdrasil, and enables the management and manipulation of coherent ASTs located in different files. The functionality of the fragment system allows splitting of an AST into a number of sub-ASTs (sub-trees) by allowing some inferior nodes in the original AST to be replaced by special nodes. The AST, originally positioned at the position of that node, is called a fragment and may be located in a totally different file.

More Information

For further information, see

Mjølner System: Yggdrasil: The Meta Programming System.

Yggdrasil is the tree of the world. The ash tree of which the crown covers the whole world. The tree gets power from the gods, from the evil giants and from the kingdom of the death. Everything in the world happens under the mighty crown of Yggdrasil.