with non-trivial arguments, and conditionals with non-trivial predicates.
An algorithm for converting SCHEME programs to continuation-passing style was given in Appendix A of [Declarative]. {Note Old CPS Algorithm} The one used in RABBIT is almost identical, except that for the convenience of the code-generation phase a distinction is made between ordinary LAMBDA-expressions and continuations, and between combinations used to invoke "functions" and those used to invoke continuations. These sets can in fact be consistently distinguished, and it affords a certain amount of error-checking; for example, a LAMBDA-expression should never appear in the "function" position of a continuation-invoking combination. Another fine point is that ASET' can never be applied to a variable bound by a continuation. Except for such differences arising from their uses, the two sets of constructs are treated more or less identically in later phases. An additional difference between the algorithm in [Declarative] and the one in RABBIT is that trivial subforms are treated as single nodes in the CPS version; these nodes have pointers to the non-CPS versions of the relevant code, which are largely ignored by later processing until the final code is to be generated.
It must be emphasized that there is not necessarily a unique CPS version for a given SCHEME program; there is as much freedom as there is in the original program to re-order the evaluation of subexpressions. In the course of the conversion process decisions must be made as to what order to use in evaluating arguments to combinations. The current decision procedure is fairly simple-minded, consisting mostly of not making copies of constants and the values of variables. The point here, as earlier, is not so much that RABBIT has a much better algorithm than other compilers as that it has a far cleaner way of expressing the result. (For a complex decision procedure for argument ordering, see [Wulf].) {Note Non-deterministic CPS Conversion}
