3.6 Type Checking Messages

This section describes Cecil's type checking rules for message sends and method declarations. Section 3.7 describes type checking for other, simpler kinds of expressions. Parameterized types are described in section 4.

In Cecil, all control structures, instance variable accesses, and basic operators are implemented via message passing, so messages are the primary kind of expression to type-check. For a message to be type-correct, there must be a single most-specific applicable method implementation defined for all possible argument objects that might be used as an argument to the message. However, instead of directly checking each message occurring in the program against the methods in the program, in Cecil we check messages against the set of signatures defined for the argument types of the message, and then check that each signature in the program is implemented conformingly, completely, and consistently by some group of methods.

Using signatures as an intermediary for type checking has three important advantages. First, the type-checking problem is simplified by dividing it into two separable pieces. Second, checking signatures enables all interfaces to be checked for conformance, completeness, and consistency independent of whether messages exist in the program to exercise all possible argument types. Finally, signatures enable the type checker to assign blame for a mismatch between implementor and client. If some message is not implemented completely, the error is either "message not understood" or "message not implemented correctly." If the signature is absent, it is the former, otherwise the latter. Signatures inform the type checker (and the programmer) of the intended interfaces of abstractions, so that the system may report more informative error messages. Of course, the "missing signature" error is sometimes the appropriate message to report, but the type checker cannot accurately distinguish this from the "message not understood" alternative.

Subsection 3.6.1 describes checking messages against signatures, and subsection 3.6.2 describes checking signatures against method implementations.