  Algebraic effects and handlers have emerged in the programming languages
community as a convenient, modular abstraction for controlling computational
effects. They have found several applications including concurrent programming,
meta programming, and more recently, probabilistic programming, as part of
Pyro's Poutines library. We investigate the use of effect handlers as a
lightweight abstraction for implementing probabilistic programming languages
(PPLs). We interpret the existing design of Edward2 as an accidental
implementation of an effect-handling mechanism, and extend that design to
support nested, composable transformations. We demonstrate that this enables
straightforward implementation of sophisticated model transformations and
inference algorithms.
