Checking model instances are not types

A very common gotcha for users (not necessarily beginners) is calling a method with a model type instead of a model instance. Sometimes the consequence is a confusing error message.

Sometimes we check this (eg, here) but sometimes we don't (eg, base models in a Stack).

In some cases (eg, pipelines) we catch a type and automatically instantiate a default instance, if necessary. I have mixed feelings about this. For example, some model wrappers, eg, TunedModel, EnsembleModel, cannot be called with zero arguments (an atomic model must be specified). So I propose we eventually deprecate this behaviour

For now, I would like a review of MLJBase.jl which introduces the check everywhere a user is expected to call a method with an instance, but might supply a type by accident. An informative error is to be thrown. This could start with some infrastructure along the lines of:

const MSG_MODEL_EXPECTED  = "Type encountered where model instance expected"
function check_is_instance(model, message=MSG_MODEL_EXPECTED)
    model isa Model || throw(AssertionError(message))      # in future, use `ismodel(model)`
    return nothing
end

cc @rikhuijzer

JuliaAI / MLJBase.jl

Checking model instances are not types #782