Open main menu
This page needs to be proofread.

SCHEME to reflect lambda-calculus semantics much more closely than dynamically scoped LISP systems. SCHEME also permits the treatment of functions as full- fledged data objects; they may be passed as arguments, returned as values, made part of composite data structures, and notated as independent, unnamed ("anonymous") entities. (Contrast this with most ALGOL-like languages, in which a function can be written only by declaring it and giving it a name; imagine being able to use an integer value only by giving it a name in a declaration!) The property of lexical scoping allows this to be done in a consistent manner without the possibility of identifier conflicts (that is, SCHEME "solves the FUNARG problem" [Moses]). In [SCHEME] we also discussed the technique of "continuation-passing style", a way of writing programs in SCHEME such that no function ever returns a value. In [Imperative] we explored ways of exploiting these properties to implement most traditional programming constructs, such as assignment, looping, and call-by-name, in terms of function application. Such applicative (lambda- calculus) models of programming language constructs are well-known to theoreticians (see [Stoy], for example), but have not been used in a practical programming system. All of these constructs are actually made available in SCHEME by macros which expand into these applicative definitions. This technique has permitted the speedy implementation of a rich user-level language in terms of a very small, easy-to-implement basis set of primitive constructs. In [Imperative] we continued the exploration of continuation-passing style, and noted that the escape operator CATCH is easily modeled by transforming a program into this style. we also pointed out that transforming a program into this style enforces a particular order of argument evaluation, and makes all intermediate computational quantities manifest as variables. In [Declarative] we examined more closely the issue of tail-recursion,