"Parametric polymorphism is more regular and may be illustrated by an example. Suppose f is a function whose argument is of type A and whose results is of B (so that the type of f might be written A => B), and that L is a list whose elements are all of type A (so that the type of L is A list). We can imagine a function, say Map, which applies f in turn to each member of L and makes a list of the results. Thus Map[f,L] will produce a B list. We would like Map to work on all types of list provided f was a suitable function, so that Map would have to be polymorphic. However its polymorphism is of a particularly simple parametric type which could be written (A =>B, A list) => B list, where A and B stand for any types. Polymorphism of both classes presents a considerable challenge to the language designer, but it is not one which we shall take up here." Christopher Strachey, Fundamental Concepts in Programming Languages, 1967.

A frequent dilemma in programming language design is the choice between a language with a rich set of notations and a small, simple core language. We address this dilemma by proposing extensible grammars, a syntax-definition formalism for incremental language extensions and restrictions.