Gen.train not updating parameters

[JGameCreation]

Hi! I try to use Gen.train and wrote this very simple test code:

@gen function model()
    @param x_mu::Float64
    x = @trace(normal(x_mu, 1.), :x)
end
function data_generator()
    x = normal(3., 1.)
    constraints = Gen.choicemap()
    constraints[:x] = x
    return ((), constraints)
end
update = Gen.ParamUpdate(Gen.FixedStepGradientDescent(0.1), model);
init_param!(model, :x_mu, 1.0)
scores = Gen.train!(model, data_generator, update,
    num_epoch=200, epoch_size=100, num_minibatch=1, minibatch_size=100, evaluation_size=20, verbose=true);
for param in get_params(model)
    println("$param $(get_param(model, param))")
end

I would expect the parameter x_mu to be trained but it always remains unchanged.

[alex-lew]

@JGameCreation Sorry for the (very) late reply!

The issue here is that the ParamUpdate object needs to either:

• manually specify a list of parameters to train, using the explicit param_list argument (

  Gen.jl/src/optimization.jl

  Line 19 in 44f5f14

  update = ParamUpdate(conf, param_lists...)

  ); or
• be created after the init_param! call, in which case it will train all initialized parameters.

In your example, switching the order of the init_param line and the ParamUpdate line fixes the issue. This is not documented, but definitely should be. I've opened another issue to track that (#455). Thanks for the report and sorry again for the months-late reply!