Static Type Checking for the Q Functional Language in Prolog

Abstract

We describe an application of Prolog: a type checking tool for the Q functional language. Q is a terse vector processing language, a descendant of APL, which is getting more and more popular, especially in financial applications. Q is a dynamically typed language, much like Prolog. Extending Q with static typing improves both the readability of programs and programmer productivity, as type errors are discovered by the tool at compile time, rather than through
debugging the program execution. We designed a type description syntax for Q and implemented a parser for both the Q language
and its type extension. We then implemented a type checking algorithm using constraints. As most built-in function names of Q are overloaded, i.e. their meaning depends on the argument types, a quite complex system of constraints had to be implemented. Prolog proved to be an ideal implementation language for the task at hand. We used Definite Clause Grammars for parsing and Constraint Handling Rules for the type checking algorithm. In the paper we describe the main problems solved and the experiences gained in the development of the type checking tool.