  A long-lived autonomous agent should be able to respond online to novel
instances of tasks from a familiar domain. Acting online requires 'fast'
responses, in terms of rapid convergence, especially when the task instance has
a short duration, such as in applications involving interactions with humans.
These requirements can be problematic for many established methods for learning
to act. In domains where the agent knows that the task instance is drawn from a
family of related tasks, albeit without access to the label of any given
instance, it can choose to act through a process of policy reuse from a
library, rather than policy learning from scratch. In policy reuse, the agent
has prior knowledge of the class of tasks in the form of a library of policies
that were learnt from sample task instances during an offline training phase.
We formalise the problem of policy reuse, and present an algorithm for
efficiently responding to a novel task instance by reusing a policy from the
library of existing policies, where the choice is based on observed 'signals'
which correlate to policy performance. We achieve this by posing the problem as
a Bayesian choice problem with a corresponding notion of an optimal response,
but the computation of that response is in many cases intractable. Therefore,
to reduce the computation cost of the posterior, we follow a Bayesian
optimisation approach and define a set of policy selection functions, which
balance exploration in the policy library against exploitation of previously
tried policies, together with a model of expected performance of the policy
library on their corresponding task instances. We validate our method in
several simulated domains of interactive, short-duration episodic tasks,
showing rapid convergence in unknown task variations.
