Babel, an application of extensible compilers

The normal approach in providing an extensible programming language seems to be to design and implement a base language which has facilities enabling the programmer to define and use extensions. This paper discusses a solution using an alternative approach in which extensions are made by changing the compiler. Of course, in theory, any compiler can be altered (it is only a computer program); in practice it is probably not so easy since difficulties will arise if the compiler has been designed for one fixed standard language on one particular computer. If a compiler is to be altered, it must possess various properties; there should be no danger of accidentally invalidating an existing program, and it must be clear what changes are required to make a desired extension. It is also desirable that only a small number of changes should be required to make an extension.