11.3  Modelling with CLP and ECLⁱPS^e

When modelling problems with constraints, the basic idea is to set up a network of variables and constraints. Figure 11.1 shows such a constraint network.

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Figure 11.1: A Constraint Network

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It can be seen that the Constraint Logic Programming (CLP) formulation

• is a natural declarative description of the constraint network
• can serve as a program to set up the constraint network

The main ECLⁱPS^e language constructs used in modelling are

Built-in constraints

X #> Y

Abstraction

before(task(Si,Di), task(Sj,Dj)) :- Si+Di #<= Sj.

Conjunction

between(X,Y,Z) :- X #< Y, Y #< Z.

Disjunction (but see below)

neighbour(X,Y) :- ( X #= Y+1 ; Y #= X+1 ).

Iteration

not_among(X, L) :- ( foreach(Y,L),param(X) do X #\= Y ).

Recursion

not_among(X, []).
not_among(X, [Y|Ys]) :- X #\= Y, not_among(X, Ys).