Constraint Logic Programming Using ECLiPSe

9
The suspend library
9.1 Introduction 155
9.2 Solving CSPs and COPs using ECL i PS e 156
9.3 Introducing the suspend library 157
9.4 Core constraints 160
9.5 User defined suspensions 167
9.6 Generating CSPs 170
9.7 Using the suspend library 175
9.8 Summary 180
9.9 Exercises 181
9.1 Introduction
I
N THIS CHAPTER we introduce the first ECL i PS e library, called
suspend. It allows us to implement constraints. This library lacks
the important facility of constraint propagation (briefly mentioned in
Chapter 6) and consequently is of little use for solving computationally
intensive problems. On the other hand, it is used to implement all the
other, more sophisticated, constraint solvers in ECL i PS e and is an excellent
pedagogical tool for introducing constraints in the context of ECL i PS e .
The suspend library still supports only passive constraints and as a result
is only the first step towards realising constraint programming. It improves
upon the computation model of Prolog by automatically preventing the occurrence
of run-time errors for arithmetic constraints, by delaying them until
all their variables are fully instantiated. This modified computation mechanism
is provided for a larger class of constraints that also includes Boolean
constraints. As we shall see, this leads to a more flexible programming style
in the presence of constraints.
In particular, using the suspend library we can easily generate CSPs and
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