CMP.12:4.1 - Recover the expression structure and the intended observations

Identify the constructors of admitted expressions and how their parts bind and compose. Work with a parsed structure when precedence, grouping or scope matters. A name occurrence must reach the declaration or external input intended by the language. If the supplied text admits different parses that change the answer, resolve that difference before constructing one evaluator.

Specify what evaluation receives and what its user can observe. Inputs can include the expression, name bindings, stored state and external responses. Outputs can include a value, changed state, a sequence of interactions or a failure. Distinguish a language-defined error from the absence of a defined operation; a translator can be required to preserve the former without being required to assign a meaning to the latter.

Choose the required relation between source and target observations. Equal returned values may suffice for a terminating pure expression. An interactive program may require the same ordered events. A source with several permitted behaviors may allow the target to choose among them; retaining every source possibility is a stronger requirement. Timing, memory use or a probability law belongs in this relation only when the receiving question uses it.