In conventional programming languages, the block is used to introduce local variables, whose names and types are chosen by the programmer. In AFL, however, the block is used only to introduce a fixed set of local variables, each of which is associated with the current state of one component of the audio formatter.
The main advantage we gain from having a restricted block is simplicity of semantics and implementation. The hardware for each audio component is associated with a particular variable. The contents of the variable can be changed only through an explicit assignment to the variable, so it is not difficult in the implementation to change the hardware when the variable is assigned. AFL is optimized so that only those dimensions that are actually changed have to be conveyed to the hardware. Further, the particular variable that is associated with the hardware can be switched only by the beginning and end of execution of a block. Hence, the implementation of the change of association is simple.
The use of the restricted block leads to the rule that any audio event that is begun in a block must end in that block. In all our extensive experimentation and experience with AsTeR , we have not found this rule to be an inconvenience. In fact, we have been helped by the imposed discipline. On the other hand, taking advantage of a more flexible block (were it available) would only complicate the structure and interaction of various rendering rules, in much the same way that the goto can cause undue complications in conventional programming languages.
Consider, for example, rendering rules of the following form:
When rendering a fraction whose numerator is a superscripted variable, e.g., [tex2html_wrap5608], the above would result in the sound getting turned off by the superscript rendering rule, and this effect would persist for the rest of the rendering of the fraction. Since rendering rules are written for specific object types, many of which can be nested, the above kind of bug would become common if component states were not local to every block.