An implementation of binary session types in OCaml
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Session-ocaml is an implementation of session types in OCaml.

(NEW: the project page is now open at:

How to try it

Prepare OCaml 4.05 and install findlib, ocamlbuild, ppx_tools. We recommend to use opam

Install the compiler and prerequisite libraries. (NOTE: the version number has changed: 4.03 ==> 4.05)

opam switch 4.05.0
eval `opam config env`
opam install ocamlfind ocamlbuild ppx_tools

Then clone the repository and type following at the top directory:

git clone
cd session-ocaml
./configure --prefix=$(dirname `which ocaml`)/..
make install

Then you can play with session-ocaml:

cd examples
make                       # build examples
rlwrap ocaml -short-paths  # play with OCaml toplevel (utop will also do).
                           # rlwrap is a readline wrapper (recommended)

Argument -short-paths is optional (it makes ocaml show the shortest path for each type). Note that .ocamlinit file automatically pre-loads all required packages into OCaml toplevel and sets -rectypes option. It also does open Session.

If things seem broken, try git clean -fdxthen make (WARNING: this command erases all files except the original distribution). Also, you can uninstall manually by ocamlfind remove session-ocaml.


Macro for branching / selection

For branching on arbitrary labels, we provide a macro match%branch.

  open Session
  match%branch s with
  | `apple  -> send 100
  | `banana -> recv ()
  | `orange -> send "Hello!"

Its protocol type will be:

  [`branch of resp *
    [ `apple of [`msg of req * int * 'a]
    | `banana of [`msg of resp * 'v * 'a]
    | `orange of [`msg of req * string * 'a]]

Similarly, we have a macro for selection, like

  [%select s `label]

author: Keigo IMAI (@keigoi on Twitter / keigoi AT