[Declarative].
We realize that other systems of environment and control constructs also are reasonably concise, clear, and elegant, and can be axiomatized in useful ways, for example the guarded commands of Dijkstra. [Dijkstra] However, that of lambda-calculus is extremely well-understood, lends itself well to certain kinds of optimizations in a natural manner, and has behind it a body of literature which can be used directly by RABBIT to express non-primitive constructs.
The desire for uniform lexical scoping arises from other motives as well, some pragmatic, some philosophical. Many of these are described in [SCHEME], [Imperative], [Declarative], and [Revised Report]. It is often difficult to explain some of these to those who are used to dynamically scoped LISP systems. Any one advantage of lexical scoping may often be countered with "Yes, but you can do that in this other way in a dynamically scoped LISP." However, we are convinced that lexical scoping in its totality provides all of the advantages to be described in a natural, elegant, and integrated manner, largely as a consequence of its great simplicity.
There are those to whom lexical scoping is nothing new, for example the ALGOL community. For this audience, however, we should draw attention to another important feature of SCHEME, which is that functions are first-class data objects. They may be assigned or bound to variables, returned as values of other functions, placed in arrays, and in general treated as any other data object. Just as numbers have certain operations defined on them, such as addition, so functions have an important operation defined on them, namely invocation.
The ability to treat functions as objects is not at all the same as the ability to treat representations of functions as objects. It is the latter ability that is traditionally associated with LISP; functions can be represented as S-expressions. In a version of SCHEME which had no S-expression primitives,
