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2. Overview

Semantic is a tool primarily for the Emacs-Lisp programmer. However, it comes with "applications" that non-programmer might find useful. This chapter is mostly for the benefit of these non-programmers as it gives brief descriptions of basic concepts such as grammars, parsers, compiler-compilers, parse-tree, etc.

The grammar of a natural language defines rules by which valid phrases and sentences can be composed using words, the fundamental units with which all sentences are created. In a similar fashion, a "context-free grammar" defines the rules by which programs can be composed using the fundamental units of the language, i.e., numbers, symbols, punctuations, etc. Context-free grammars are often specified in a well-known form called Backus-Naur Form, BNF for short. This is a systematic way of representing context-free grammars such that programs can read files with grammars written in BNF and generate code for "parser" of that language. YACC (Yet Another Compiler Compiler) is one such program that has been part of UNIX operating systems since the 1970's. YACC is pronounced the same as "yak", the long-haired ox found in Asia. The parser generated by YACC is usually a C program. Bison is also a "compiler compiler" that takes BNF grammars and produces parsers in C language. The difference between YACC and Bison is that Bison is free software and upward-compatible with YACC. It also comes with an excellent manual.

Semantic is similar in spirit to YACC and Bison. Semantic, however, is referred to as a bovinator rather than as a parser, because it is a lesser cousin of YACC and Bison. It is lesser in that it does not perform a full parse like YACC or Bison. Instead, it bovinates. "Bovination" refers to partial parsing which creates parse trees of only the top most expressions rather than parsing every nested expression. This is sufficient for the purposes for which semantic was designed. Semantic is meant to be used within Emacs for providing editor-related features such as code browsers and translators rather than for compiling which requires far more complex and complete parsers. Semantic is not designed to be able to create full parse trees.

One key benefit of semantic is that it creates parse trees (perhaps the term bovine tree may be more accurate) with the same structure regardless of the type of language involved. Higher level applications written to work with bovine trees will then work with any language for which the grammar is available. For example, a code browser written today that supports C, C++, and Java may work without any change on other languages that do not even exist yet. All one has to do is to write the BNF specification for the new language. The rest of the work is done by semantic. For certain languages, it is hard if not impossible to specify the syntax of the language in BNF form, e.g., texinfo and other document oriented languages. Semantic provides a parser for texinfo nevertheless. Instead of BNF grammar, texinfo files are "parsed" using @xref{Regexps,regular-expressions,regular-expressions,emacs}.

Semantic comes with grammars for these languages:

Several tools employing semantic that provide user observable features are listed in 11. Tools section.

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