Mode Checking in HAL

Recent constraint logic programming (CLP) languages, such as HAL and Mercury, require type, mode and determinism declarations for predicates. This information allows the generation of efficient target code and the detection of many errors at compile-time. However, mode checking in such languages is difficult since the compiler is required to appropriately re-order literals in the predicate’s definition for each predicate mode declaration. The task is further complicated by the need to handle complex instantiations which interact with type declarations, higher order functions and predicates, and automatic initialization of solver variables. Here we give the first formal treatment of mode checking in strongly typed CLP languages which require reordering of clause body literals during mode checking. We also sketch the mode checking algorithms used in the HAL compiler.