Fundamental scientific computing with Numpy for OCaml

ocaml-sklearn allows using Python's
scikit-learn machine learning library
from OCaml.

Read the online scikit-learn OCaml API documentation

If you are not familiar with scikit-learn, consult its Python getting
started documentation

and user guide.

As of version 0.22-0.3.0, most classes and functions from scikit-learn
and Numpy should be usable. Many examples have been ported from Python
to OCaml successfully (see below). However, the APIs have not yet
proved stable and will probably evolve in the next releases.

Example : support vector regression with RBF kernel

module Np = Np.Numpy
let n_samples, n_features = 10, 5 in
Np.Random.seed 0;
let y = Np.Random.uniform ~size:[n_samples] () in
let x = Np.Random.uniform ~size:[n_samples; n_features] () in
let open Sklearn.Svm in
let clf = SVR.create ~c:1.0 ~epsilon:0.2 () in
Format.printf "%a\n" SVR.pp @@ clf ~x ~y;
Format.printf "%a\n" Np.pp @@ SVR.support_vectors_ clf;;

This outputs:

SVR(C=1.0, cache_size=200, coef0=0.0, degree=3, epsilon=0.2, gamma='scale',
    kernel='rbf', max_iter=-1, shrinking=True, tol=0.001, verbose=False)
[[0.14509922 0.16277752 0.99033894 0.84013554 0.96508279]
 [0.8865312  0.80655193 0.07459775 0.36058768 0.22130337]
 [0.21844203 0.09612442 0.49908686 0.1154579  0.98202969]
 [0.07306658 0.97225754 0.20558949 0.16423512 0.57400651]
 [0.08153976 0.41462111 0.66190418 0.70208221 0.3600998 ]
 [0.20502873 0.04244781 0.21800856 0.28184598 0.4282653 ]
 [0.89211037 0.51466381 0.23432621 0.29850877 0.13323457]]

There are more examples in
for instance


opam install sklearn

Finding Python's scikit-learn at runtime

You do not need a Python installation when compiling your OCaml
program using ocaml-sklearn. However, when running, your program will
need to load the sklearn, numpy and scipy Python libraries, so these
must be installed where the OCaml program is deployed.

A version of ocaml-sklearn is tied to a version of Python's
scikit-learn, numpy and scipy. For instance, a version of ocaml-sklearn for Python's
scikit-learn 0.22.2 will refuse to initialize (by throwing an
exception) if scikit-learn's version is not 0.22 (it can however be
0.22.1, 0.22.2 or 0.22.2.post1).

One way to make sure you run with the right versions is to create a
virtualenv, install scikit-learn the Python packages inside, and run
your OCaml program in the activated virtualenv.

Do this once to create the virtualenv in directory .venv and install
scikit-learn, numpy and scipy inside:

python3 -mvenv .venv
source .venv/bin/activate
pip install scikit-learn==%%SKLEARN_FULL_VERSION%% numpy==%%NUMPY_FULL_VERSION%% scipy==%%SCIPY_FULL_VERSION%% pytest

Then run your compiled OCaml program inside the virtualenv:

source .venv/bin/activate


We attempt to bind all of scikit-learn's APIs. However, not all of the
APIs are currently tested, and some are probably hard to use or
unusable at the moment.

Each Python module or class gets its own OCaml module. For instance
Python class sklearn.svm.SVC can be found in OCaml module
Sklearn.Svm.SVC. This module has a create function to construct an
SVC and functions corresponding to the Python methods and

Most data is passed in and out of sklearn through module Ndarray (in
module Np.Np.Numpy).

You should generally build a dense array using the constructors in Np.Numpy:

module Np = Np.Numpy
let x = Np.matrixi [|[| 1; 2 |]; [| 3; 4 |]|]

To get data out of an Ndarray, use to_int_array, to_float_array
or to_string_array (all of these return a flattened copy of the
data, and will raise an exception if the data type is wrong).

Attributes are exposed read-only, each with two getters: one that
raises Not_found if the attribute is None, and the other that returns
an option.

Bunches (as returned from the sklearn.datasets APIs) are exposed as

Arguments taking string values are converted (in most cases) to
polymorphic variants.

Each module has a conversion function to Py.Object.t (called
to_pyobject), so that you can always escape and use pyml directly
if the API provided here is incomplete.

No attempt is made to expose features marked as deprecated.

Development notes

ocaml-sklearn's sources are generated using a Python program (see
lib/ that loads up sklearn and uses introspection to
generate bindings based on pyml. To determine types, it parses
scikit-learn's documentation.

Dev tl;dr

python3 -mvenv .venv
source .venv/bin/activate
pip install -r requirements-dev.txt
opam switch create . 4.11.1 --deps-only
dune runtest

Python requirements

The requirements for developing (not using) the bindings are in file
requirements-dev.txt. Install it using:

# sudo apt install python3-venv
python3 -mvenv .venv
source .venv/bin/activate
pip install -r requirements-dev.txt

Running tests

dune runtest

The tests are in examples/auto. They are based on examples extracted
from the Python documentation. A good way to develop is to pick one of
the files and start porting examples.

The following examples have been ported completely:

The following examples still need to be ported:

Generating documentation


Documentation can then be found in html_doc/. Serve
it locally with something like:

python3 -mhttp.server --directory html_doc
xdg-open http://localhost:8000


BSD-3. See file LICENSE.

>= "20200222"
>= "4.07.1"
>= "2.4"
Reverse Dependencies
>= "sp1.5-0.3.1"
>= "sk0.23-0.3.1"