substituted for one or more occurrences of a parameter in the body of the LAMBDA- expression. (This rule is related to the view of LAMBDA as a renaming operator discussed in [Declarative], and together with the two preceding rules make up the rule of beta-conversion.) Such a substitution is permissible only if (a) either the parameter is referred to only once or the argument has no side effects, and (b) the substitution will not alter the order in which expressions are evaluated in such as way as to allow possible side-effects to produce different results. Before performing the substitution it is necessary to show that side-effects will not interfere in this manner. This issue is discussed in [Allen and Cocke], [Geschke], and [Wulf], and characterized more accurately in [Standish]. There is also some difficulty if the parameter appears in an ASET‘. Presently RABBIT does not attempt any form of substitution for such a parameter. (ASET' is so seldom used in SCHEME programs that this restriction makes very little difference.)
This third rule creates an exception to the second. If an argument with a side effect is referred to once, and is substituted for the reference, then the second rule must be invoked to eliminate the original occurrence of the argument, so that the side effect will not occur twice. This requires a little collusion between the two rules.
Even if such a substitution is permissible, it is not always desirable; time/space tradeoffs are involved. The current heuristic is that a substitution is desirable if (1) the parameter is referred to only once; or (2) the argument to be substituted in is a constant or variable; or (3) the argument is a LAMBDA- expression whose body is (3a) a constant, or (3b) a variable reference, or (3c) a combination which has no more arguments than the LAMBDA-expression requires and for which the arguments are all constants or variables. This heuristic was designed to be as conservative as possible while handling most cases which arise from typical macro-expansions.
