Effect handlers have been gathering momentum as a mechanism for modular programming with user-defined effects. Effect handlers allow for non-local control flow mechanisms such as generators, async/await, lightweight threads and coroutines to be composably expressed. The Multicore OCaml project retrofits effect handlers to the OCaml programming language to serve as a modular basis of concurrent programming. In this talk, I will introduce effect handlers in OCaml, walk through several examples that illustrate their utility, describe the retrofitting challenges and how we overcome them without breaking the existing OCaml code. Our implementation imposes negligible overhead on code that does not use effect handles and is efficient for code that does. Effect handlers are slated to land in OCaml after the addition of parallelism support.

Most practical programming languages include ways of performing various non-functional effects, such as exceptions, generators, dynamic binding and concurrency. Languages typically offer ways for programmers to define new functions, new types, and sometimes even new syntax, but the set of available effects is usually baked into the language and cannot be extended within programs.

In this talk I'll show how a single language construct in OCaml, the effect handler, makes it possible to define all the above effects and more within a program, transferring power from language designers to language users. In particular, I will talk about how effect handlers allow expressing custom concurrency primitives for Multicore OCaml. 

OCaml is a statically typed functional language with a fast native code compiler and incremental garbage collector. In this tutorial, we will first introduce the basics of OCaml using an interactive environment running in a web browser, as well as a local install of OCaml using the OPAM package manager. We will also explore how to compile OCaml to JavaScript using the js_of_ocaml tool.

This tutorial is aimed at people with programming experience. Previous experience with a functional language will be useful but is not required. No prior knowledge of OCaml is assumed.