Type Checking

Abstract

Lexing and parsing will reject many texts as not being correct programs. However, many languages have well-formedness requirements that can not be handled exclusively by the techniques seen so far. These requirements can, for example, be static type correctness or a requirement that pattern-matching or case-statements are exhaustive. These properties are most often not context-free, i.e., they can not be checked by membership of a context-free language. Consequently, they are checked by a phase that (conceptually) comes after syntax analysis (though it may be interleaved with it). These checks may happen in a phase that does nothing else, or they may be combined with the actual execution or translation to another language. Often, the translator may exploit or depend on type information, which makes it natural to combine calculation of types with the actual translation. In Chap. 4, we covered type-checking during execution, which is normally called dynamic typing. We will in this chapter assume that type checking and related checks are done in a phase previous to execution or translation (i.e., static typing), and similarly assume that any information gathered by this phase is available in subsequent phases.

“The most touching epitaph I ever encountered was on the tombstone of the printer of Edinburgh. It said simply:

He kept down the cost and set the type right.”

Gregory Nunn (1955–)