What is an extensible programming language?
An extensible programming language supports mechanisms to extend the programming language, compiler/interpreter and runtime environment. The programmer is allowed to define new language constructs such as statements, declaration constructs and operators syntactically and semantically. Most programming languages allow user defined variables, functions and types, but they also use constructs which are hardcoded in the compiler/interpreter. An extensible programming language trys to avoid such hardcoded constructs in normal programs. Extensible programming was an area of active research in the 1960s, but in the 1970s the extensibility movement was displaced by the abstraction movement. Todays software history gives almost no hint that the extensible languages movement had ever occurred. In the historical movement an extensible programming language consisted of a base language providing elementary computing facilities, and a meta-language capable of modifying the base language. A program t