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.
Installation & Usage
We have tested Eff on Mac OS X 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
- OCaml, version 4.02.3 or newer,
- Menhir parser generator, and
Installing with OPAM
This is the easiest way to install Eff. Follow these steps:
Install the OPAM package manager if you do not have it yet.
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.
opam pin add -k git eff email@example.com:matijapretnar/eff.git
OPAM will download and build the necessary dependencies first, then download and build Eff itself.
To compile Eff manually, first clone the GitHub repository
git clone firstname.lastname@example.org:matijapretnar/eff.git cd eff
If it complains you will have to install missing prerequisites. In case of
make clean distclean might help. The configuration script takes
standard GNU Autoconf arguments, such as
--prefix which determines where to
install Eff. Type
./configure --help for more information. Next, run
If all goes well, you should be able to run Eff in-place by typing
You can also run a battery of tests with
Finally, to install the command
sudo make install
See the file
etc/README.txt for editor support.
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.
Copyright and license
Copyright (c) 2015, Andrej Bauer and Matija Pretnar Copyright (c) 2012, Timotej Lazar
Eff is distributed under the abbreviated BSD License, see