Type-Checking Revisited: Modular Error-Handling
نویسنده
چکیده
Static-semantics determines the validity of a program, while a type-checker provides more speciic type error information. Type-checkers are speciied based on the static semantics speciication, for the purpose of identifying and presenting type errors in invalid programs. We discuss a style of algebraically specifying the static semantics of a language which facilitates automatic generation of a type-checker and a language speciic error reporter. Such a speciication can also be extended in a modular manner to yield human-readable error messages. Static-semantics of a language determines the validity of a program written in that language. Type-checking of a program, to be useful in practice, should not only indicate whether a given program is valid or not, but also summarize the type errors and show the location of the erroneous constructs which caused the errors. Thus, specifying a type-checker that is useful in practice results in (textually) modifying the speciication of static semantics to \knit" in the reporting of the nature and location of type errors. Knitting such reporting information about the details of type-checking into the speciication of static semantics is a lot of work and is error prone. We discuss diierent styles of specifying type-checkers and present a spec-iication style which facilitates the generation of an error reporting tool that automatically locates the erroneous constructs. This speciication style advocates the concise and abstract speciication of static semantics. A signiicant advantage of this style is that error handling (providing a summary of type errors) need not be knit in the type-checker speciication, but can be speciied as a modular extension to the static semantics. To illustrate our speciication style, we use the toy language Pico 1]. Pico programs consists of declarations followed by statements. The syntax and a brief description of Pico can be found in the appendix. The small size of Pico allows us to discuss diierent styles used in the speciications in detail. In Section 2, we brieey discuss algebraic speciications and term rewriting systems, and introduce the speciication formalism used in this paper. In Section 3 we examine a \classical" style of specifying static semantics of Pico 1,
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