Experience using constraint programming to solve real-life problems has shown that finding an efficient solution to the problem often requires experimentation with different constraint solvers or even building a problem-specific constraint solver. HAL is a new constraint logic programming language expressly designed to facilitate this process. It provides a well-defined solver interface, mutable global variables for implementing a constraint store, and dynamic scheduling which support combining, extending and writing new constraint solvers. Equally importantly, HAL supports semi-optional type, mode and determinacy declarations. These allow natural constraint specification by means of type overloading, better compile-time error checking and generation of more efficient run-time code.