Quick-Start Guide to Adding Models

The definitive specification of the MLJ model interface is given in Adding Models for General Use. In the more informal and condensed instructions below, we assume: (i) you have a Julia registered package YourPackage.jl implementing some machine learning models; (ii) that you would like to interface and register these models with MLJ; and (iii) that you have a rough understanding of how things work with MLJ. In particular you are familiar with:

  • what scientific types are

  • what Probabilistic, Deterministic and Unsupervised models are

  • the fact that MLJ generally works with tables rather than bare bone matrices. Here a table is a container satisfying the Tables.jl API (e.g., DataFrame, JuliaDB table, CSV file, named tuple of equi-length vectors)

  • CategoricalArrays.jl (if working with finite discrete data, e.g., doing classification)

If you're not familiar with any one of these points, please refer to relevant sections of this manual, and in particular Getting Started and Adding Models for General Use.

But tables don't make sense for my model! If a case can be made that tabular input does not make sense for your particular model, then MLJ can still handle this; you just need to define a non-tabular input_scitype trait. However, you should probably open an issue to clarify the appropriate declaration. The discussion below assumes input data is tabular.

For simplicity, this document assumes no data front-end is to be defined for your model. Adding a data front-end, which offers the MLJ user some performances benefits, is easy to add post-facto, and is described in Implementing a data front-end.


To write an interface create a file or a module in your package which includes:

  • a using MLJModelInterface or import MLJModelInterface statement

  • MLJ-compatible model types and constructors,

  • implementation of fit, predict/transform and optionally fitted_params for your models,

  • metadata for your package and for each of your models


MLJModelInterface is a very light-weight interface allowing you to define your interface, but does not provide the functionality required to use or test your interface; this requires MLJBase. So, while you only need to add MLJModelInterface to your project's [deps], for testing purposes you need to add MLJBase to your project's [extras] and [targets]. In testing, simply use MLJBase in place of MLJModelInterface.

We give some details for each step below with, each time, a few examples that you can mimic. The instructions are intentionally brief.

Model type and constructor

MLJ-compatible constructors for your models need to meet the following requirements:

  • be mutable struct,
  • be subtypes of MLJModelInterface.Probabilistic or MLJModelInterface.Deterministic or MLJModelInterface.Unsupervised,
  • have fields corresponding exclusively to hyperparameters,
  • have a keyword constructor assigning default values to all hyperparameters.

You may use the @mlj_model macro from MLJModelInterface to declare a (non parametric) model type:

MLJModelInterface.@mlj_model mutable struct YourModel <: MLJModelInterface.Deterministic
    a::Float64 = 0.5::(_ > 0)
    b::String  = "svd"::(_ in ("svd","qr"))

That macro specifies:

  • A keyword constructor (here YourModel(; a=..., b=...)),
  • Default values for the hyperparameters,
  • Constraints on the hyperparameters where _ refers to a value passed.

Further to the last point, a::Float64 = 0.5::(_ > 0) indicates that the field a is a Float64, takes 0.5 as default value, and expects its value to be positive.

Please see this issue for a known issue and workaround relating to the use of @mlj_model with negative defaults.

If you decide not to use the @mlj_model macro (e.g. in the case of a parametric type), you will need to write a keyword constructor and a clean! method:

mutable struct YourModel <: MLJModelInterface.Deterministic
function YourModel(; a=0.5)
    model   = YourModel(a)
    message = MLJModelInterface.clean!(model)
    isempty(message) || @warn message
    return model
function MLJModelInterface.clean!(m::YourModel)
    warning = ""
    if m.a <= 0
        warning *= "Parameter `a` expected to be positive, resetting to 0.5"
        m.a = 0.5
    return warning

Additional notes:

  • Please annotate all fields with concrete types, if possible, using type parameters if necessary.

  • Please prefer Symbol over String if you can (e.g. to pass the name of a solver).

  • Please add constraints to your fields even if they seem obvious to you.

  • Your model may have 0 fields, that's fine.

  • Although not essential, try to avoid Union types for model fields. For example, a field declaration features::Vector{Symbol} with a default of Symbol[] (detected with the isempty method) is preferred to features::Union{Vector{Symbol}, Nothing} with a default of nothing.



The implementation of fit will look like

function MLJModelInterface.fit(m::YourModel, verbosity, X, y, w=nothing)
    # body ...
    return (fitresult, cache, report)

where y should only be there for a supervised model and w for a supervised model that supports sample weights. You must type verbosity to Int and you must not type X, y and w (MLJ handles that).


In the body of the fit function, you should assume that X is a table and that y is an AbstractVector (for multitask regression it may be a table).

Typical steps in the body of the fit function will be:

  • forming a matrix-view of the data, possibly transposed if your model expects a p x n formalism (MLJ assumes columns are features by default i.e. n x p), use MLJModelInterface.matrix for this,

  • passing the data to your model,

  • returning the results as a tuple (fitresult, cache, report).

The fitresult part should contain everything that is needed at the predict or transform step, it should not be expected to be accessed by users. The cache should be left to nothing for now. The report should be a NamedTuple with any auxiliary useful information that a user would want to know about the fit (e.g., feature rankings). See more on this below.

Example: GLM's LinearRegressor


For a classifier, the steps are fairly similar to a regressor with these differences:

  1. y will be a categorical vector and you will typically want to use the integer encoding of y instead of CategoricalValues; use MLJModelInterface.int for this.
  2. You will need to pass the full pool of target labels (not just those observed in the training data) and additionally, in the Deterministic case, the encoding, to make these available to predict. A simple way to do this is to pass y[1] in the fitresult, for then MLJModelInterface.classes(y[1]) is a complete list of possible categorical elements, and d = MLJModelInterface.decoder(y[1]) is a method for recovering categorical elements from their integer representations (e.g., d(2) is the categorical element with 2 as encoding).
  3. In the case of a probabilistic classifier you should pass all probabilities simultaneously to the UnivariateFinite constructor to get an abstract UnivariateFinite vector (type UnivariateFiniteArray) rather than use comprehension or broadcasting to get a vanilla vector. This is for performance reasons.

If implementing a classifier, you should probably consult the more detailed instructions at The predict method.



Nothing special for a transformer.

Example: FillImputer

Fitted parameters

There is a function you can optionally implement which will return the learned parameters of your model for purposes of user-inspection. For instance, in the case of a linear regression, the user may want to get direct access to the coefficients and intercept. This should be as human and machine readable as practical (not a graphical representation) and the information should be combined in the form of a named tuple.

The function will always look like:

function MLJModelInterface.fitted_params(model::YourModel, fitresult)
    # extract what's relevant from `fitresult`
    # ...
    # then return as a NamedTuple
    return (learned_param1 = ..., learned_param2 = ...)

Example: for GLM models

Summary of user interface points (or, What to put where?)

Recall that the fitresult returned as part of fit represents everything needed by predict (or transform) to make new predictions. It is not intended to be directly inspected by the user. Here is a summary of the interface points for users that your implementation creates:

  • Use fitted_params to expose learned parameters, such as linear coefficients, to the user in a machine and human readable form (for re-use in another model, for example).
  • Use the fields of your model struct for hyperparameters, i.e., those parameters declared by the user ahead of time that generally affect the outcome of training. It is okay to add "control" parameters (such a specifying an acceleration parameter specifying computational resources, as here).
  • Use report to return everything else, including model-specific methods (or other callable objects). This includes: feature rankings, decision boundaries, SVM support vectors, clustering centres, methods for visualizing training outcomes, methods for saving learned parameters in a custom format, degrees of freedom, deviance, etc. If there is a performance cost to extra functionality you want to expose, the functionality can be toggled on/off through a hyperparameter, but this should otherwise be avoided. For, example, in a decision tree model report.print_tree(depth) might generate a pretty tree representation of the learned tree, up to the specified depth.


The implementation of predict (for a supervised model) or transform (for an unsupervised one) will look like:

function MLJModelInterface.predict(m::YourModel, fitresult, Xnew)
    # ...

Here Xnew is expected to be a table and part of the logic in predict or transform may be similar to that in fit.

The values returned should be:

model subtypereturn value of predict/transform
Deterministicvector of values (or table if multi-target)
Probabilisticvector of Distribution objects, for classifiers in particular, a vector of UnivariateFinite

In the case of a Probabilistic model, you may further want to implement a predict_mean or a predict_mode. However, MLJModelInterface provides fallbacks, defined in terms of predict, whose performance may suffice.



Adding metadata for your model(s) is crucial for the discoverability of your package and its models and to make sure your model is used with data it can handle. You can individually overload a number of trait functions that encode this metadata by following the instuctions in Adding Models for General Use), which also explains these traits in more detail. However, your most convenient option is to use metadata_model and metadata_pkg functionalities from MLJModelInterface to do this:

const ALL_MODELS = Union{YourModel1, YourModel2, ...}

    name = "YourPackage",
    uuid = "6ee0df7b-...", # see your Project.toml
    url  = "https://...",  # URL to your package repo
    julia = true,          # is it written entirely in Julia?
    license = "MIT",       # your package license
    is_wrapper = false,    # does it wrap around some other package?

# Then for each model,
    input_scitype   = MLJModelInterface.Table(MLJModelInterface.Continuous),  # what input data is supported?
    target_scitype  = AbstractVector{MLJModelInterface.Continuous},           # for a supervised model, what target?
    output_scitype  = MLJModelInterface.Table(MLJModelInterface.Continuous),  # for an unsupervised, what output?
    supports_weights = false,                                                  # does the model support sample weights?
    descr   = "A short description of your model"
	load_path    = "YourPackage.SubModuleContainingModelStructDefinition.YourModel1"

Important. Do not omit the load_path specification. Without a correct load_path MLJ will be unable to import your model.


Adding a model to the model registry

See here.