Freer Monads, More Extensible Effects Oleg Kiselyov

We present a rational reconstruction of extensible effects, the re-cently proposed alternative to monad transformers, as the conflu-ence of efforts to make effectful computations compose. Free mon-ads and then extensible effects emerge from the straightforward term representation of an effectful computation, as more and more boilerplate is abstracted away. The generalization process further leads to freer monads, constructed without the Functor constraint. The continuation exposed in freer monads can then be represented as an efficient type-aligned data structure. The end result is the al-gorithmically efficient extensible effects library, which is not only more comprehensible but also faster than earlier implementations. As an illustration of the new library, we show three surpris-ingly simple applications: non-determinism with committed choice (LogicT), catching IO exceptions in the presence of other effects, and the semi-automatic management of file handles and other re-sources through monadic regions. We extensively use and promote the new sort of ‘laziness’, which underlies the left Kan extension: instead of performing an operation, keep its operands and pretend it is done