Monads and algebraic effects are general concepts that give a definition of what a "side-effect" can be: an instance of monad, or an instance of algebraic effect, is a specific realization of a side-effect. While most programming languages provide a fixed family of built-in side-effects, monads or algebraic effects give a structured way to introduce a new notion of effect as a library.

A recent avenue of programming language research is how to locally define several realizations of the same effect interface/signature. There may be several valid notions of "state" or "non-determinism" or "probabilistic choice", and different parts of a program may not require the same realization of those -- a typical use-case would be mocking an interaction with the outside world, for example. Can we let users locally define a new interpretation of an effect, or write code that is generic over the specific interpretation? There are several existing approaches, such as monad transformer stacks, free monads interpreters, monad reification and, lately, effect handlers, as proposed in the programming language Eff.

Frank, presented in the publication below, is a new language with user-defined effects that makes effect handling a natural part of basic functional programming, instead of a separate, advanced feature. It is a significant advance in language design, simplifying effectful programming. Functions, called operators, do not just start computing a result from the value of their arguments, they interact with the computation of those arguments by having the opportunity to handle any side-effects arising during their evaluation. It feels like a new programming style, a new calling convention that blends call-by-value and effect handling -- Sam Lindley suggested the name call-by-handling.

Frank also proposes a new type-and-effect system that corresponds to this new programming style. Operators handle some of the effects raised by their arguments, and silently forward the rest to the computation context; their argument types indicate which effects they handle. In other words, the static information carried by an argument types is the delta/increment between the effects permitted by the ambient computation and the effects of evaluating this argument. Frank calls this an adjustment over the ambient ability. This delta/increment style results in lightweight types for operators that can be used in different contexts (a form of effect polymorphism) without explicit quantification over effect variables. This design takes part in a research conversation on how to make type-and-effect systems usable, which is the major roadblock for their wider adoption -- Koka and Links are also promising in that regard, and had to explore elaborate conventions to elide their polymorphic variables.

Another important part of Frank's type-system design is the explicit separation between values that are and computations that do. Theoretical works have long made this distinction (for example with Call-By-Push-Value), but programmers are offered the dichotomy of either having only effectful expressions or expressing all computations as values (Haskell's indirect style). Frank puts that distinction in the hands of the user -- this is different from distinguishing pure from impure computations, as done in F* or WhyML.

If you wish to play with the language, a prototype implementation is available.





Do Be Do Be Do (arXiv)

Sam Lindley, Conor McBride, Craig McLaughlin

2017