  When implementing a propagator for a constraint, one must decide about
variants: When implementing min, should one also implement max? Should one
implement linear constraints both with unit and non-unit coefficients?
Constraint variants are ubiquitous: implementing them requires considerable (if
not prohibitive) effort and decreases maintainability, but will deliver better
performance than resorting to constraint decomposition.
  This paper shows how to use views to derive perfect propagator variants. A
model for views and derived propagators is introduced. Derived propagators are
proved to be indeed perfect in that they inherit essential properties such as
correctness and domain and bounds consistency. Techniques for systematically
deriving propagators such as transformation, generalization, specialization,
and type conversion are developed. The paper introduces an implementation
architecture for views that is independent of the underlying constraint
programming system. A detailed evaluation of views implemented in Gecode shows
that derived propagators are efficient and that views often incur no overhead.
Without views, Gecode would either require 180 000 rather than 40 000 lines of
propagator code, or would lack many efficient propagator variants. Compared to
8 000 lines of code for views, the reduction in code for propagators yields a
1750% return on investment.
