  We present a formalism for the numerical implementation of general theories
of dark energy, combining the computational simplicity of the equation of state
for perturbations approach with the generality of the effective field theory
approach. An effective fluid description is employed, based on a general action
describing single-scalar field models. The formalism is developed from first
principles, and constructed keeping the goal of a simple implementation into
CAMB in mind. Benefits of this approach include its straightforward
implementation, the generality of the underlying theory, the fact that the
evolved variables are physical quantities, and that model-independent
phenomenological descriptions may be straightforwardly investigated. We hope
this formulation will provide a powerful tool for the comparison of theoretical
models of dark energy with observational data.
