Eff is a functional programming language based on algebraic effects and their handlers.
Algebraic effects are a way of adding computational effects to a pure functional setting. In a technical sense they are subsumed by the monadic approach to computational effects, but they offer new ways of programming that are not easily achieved with monads. In particular, algebraic effects are combined seamlessly, whereas monad transformers are needed in the monadic style.
The main idea of eff is that computational effects are accessed through a set of operations, for example
update for state,
write for I/O,
raise for exceptions, etc. The behavior of operations is determined by handlers. Just like an exception handler determines what happens when an exception is raised, a general handler describes the actions taken when an operation is triggered. Examples of handlers include state, transactions, non-determinism, stream redirection, backtracking, delimited continuations, and many others.
Because eff supports first-class effects and handlers, programmers may define new computational effects, combine existing ones, and handle effects in novel ways. For instance, ML-style references are a defined concept in eff.
Eff code looks and feels like that of Ocaml because eff uses Ocaml syntax extended with constructs for effects and handlers. Furthermore, eff is a statically typed language with parametric polymorphism and type inference. The types are similar to those of OCaml and other variants of ML in the sense that they do not express any information about computational effects.