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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 lookup and update for state, read and 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.

For further information visit the Eff page or contact the authors Andrej Bauer and Matija Pretnar

Installation & Usage


We have tested Eff on macOS and Linux, and it should work on other Unix-like systems. In principle, nothing prevents Eff from running on Windows, we just have not tested it yet.

To install Eff, you need a standard Unix-style build environment as well as

  1. OCaml, version 4.14.1 or newer,
  2. Js_of_ocaml compiler from OCaml bytecode to Javascript
  3. its syntax extension js_of_ocaml-ppx,
  4. Menhir LR(1) parser generator,
  5. OCamlFormat auto-formatter for OCaml code.

We do not require, but recommend a command-line editing wrapper such as rlwrap or ledit. Eff uses these automatically.

Installing with OPAM

This is the easiest way to install Eff. Follow these steps:

  1. Install the OPAM package manager if you do not have it yet.

  2. Make sure you have the correct OCaml compiler activated. Since Eff compiles with all recent version of OCaml you probably need not worry about this step.

  3. Run

     opam pin add -k git eff

    OPAM will download and build the necessary dependencies first, then download and build Eff itself.

Manual installation

To compile Eff manually, first clone the GitHub repository

git clone
cd eff

and run


If all goes well, you should be able to run Eff in-place by typing ./eff.exe.

You can also run a battery of tests with

make test

Finally, to install the command eff, run

make install

See the file etc/README.txt for editor support.

Using Eff

There are examples of Eff in examples subdirectory that should get you started. The Eff syntax is very close to that of OCaml. You can find further material about Eff on the Eff page.


Eff is distributed under the abbreviated BSD License, see LICENSE.txt for licensing information.