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Lux Interface¤


If you just want to define compatibility with Lux without actually using any of the other functionality provided by Lux (like layers), it is recommended to depend on LuxCore.jl instead of Lux.jl. LuxCore.jl is a significantly lighter dependency.

First let's set the expectations straight.

  • Do you have to follow the interface? No.
  • Should you follow it? Probably yes.
  • Why? It provides the ability for frameworks built on top of Lux to be cross compatible. Additionally, any new functionality built into Lux, will just work for your framework.


The interface is optional for frameworks being developed independent of Lux. All functionality in the core library (and officially supported ones) must adhere to the interface

Layer Interface¤

Singular Layer¤

If the layer doesn't contain any other Lux layer, then it is a Singular Layer. This means it should optionally subtype Lux.AbstractExplicitLayer but mandatorily define all the necessary functions mentioned in the docstrings. Consider a simplified version of Dense called Linear.

First, setup the architectural details for this layer. Note, that the architecture doesn't contain any mutable structure like arrays. When in doubt, remember, once constructed a model architecture cannot change.


For people coming from Flux.jl background this might be weird. We recommend checking out the Flux to Lux migration guide first before proceeding.

using Lux, Random

struct Linear{F1, F2} <: Lux.AbstractExplicitLayer

function Linear(in_dims::Int, out_dims::Int; init_weight=Lux.glorot_uniform,
    return Linear{typeof(init_weight), typeof(init_bias)}(in_dims, out_dims, init_weight,

l = Linear(2, 4)

Next, we need to implement functions which return the parameters and states for the layer. In case of Linear, the parameters are weight and bias while the states are empty. States become important when defining layers like BatchNorm, WeightNorm, etc. The recommended data structure for returning parameters is a NamedTuple, though anything satisfying the Parameter Interface is valid.

function Lux.initialparameters(rng::AbstractRNG, l::Linear)
    return (weight=l.init_weight(rng, l.out_dims, l.in_dims),
            bias=l.init_bias(rng, l.out_dims, 1))

Lux.initialstates(::AbstractRNG, ::Linear) = NamedTuple()

You could also implement Lux.parameterlength and Lux.statelength to prevent wasteful reconstruction of the parameters and states.

# This works
println("Parameter Length: ", Lux.parameterlength(l), "; State Length: ",

# But still recommened to define these
Lux.parameterlength(l::Linear) = l.out_dims * l.in_dims + l.out_dims

Lux.statelength(::Linear) = 0
Parameter Length: 12; State Length: 0


You might notice that we don't pass in a PRNG for these functions. If your parameter length and/or state length depend on a random number generator, you should think really hard about what you are trying to do and why.

Now, we need to define how the layer works. For this you make your layer a function with exactly 3 arguments – x the input, ps the parameters, and st the states. This function must return two things – y the output, and st_new the updated state.

function (l::Linear)(x::AbstractMatrix, ps, st::NamedTuple)
    y = ps.weight * x .+ ps.bias
    return y, st

Finally, let's run this layer. If you have made this far into the documentation, we don't feel you need a refresher on that.

rng = Random.default_rng()
Random.seed!(rng, 0)

ps, st = Lux.setup(rng, l)

println("Parameter Length: ", Lux.parameterlength(l), "; State Length: ",

x = randn(rng, Float32, 2, 1)

Lux.apply(l, x, ps, st) # or `l(x, ps, st)`
(Float32[-0.15276335; 0.45325348; 1.0207279; 0.78226817;;], NamedTuple())

Container Layer¤

If your layer comprises of other Lux layers, then it is a Container Layer. Note that you could treat it as a Singular Layer, and it is still fine. FWIW, if you cannot subtype your layer with Lux.AbstractExplicitContainerLayer then you should go down the Singular Layer route. But subtyping allows us to bypass some of these common definitions. Let us now define a layer, which is basically a composition of two linear layers.

struct ComposedLinear{L1, L2} <: Lux.AbstractExplicitContainerLayer{(:linear_1, :linear_2)}

function (cl::ComposedLinear)(x::AbstractMatrix, ps, st::NamedTuple)
    # To access the parameters and states for `linear_1` we do `ps.linear_1` and
    # `st.linear_1`. Similarly for `linear_2`
    y, st_l1 = cl.linear_1(x, ps.linear_1, st.linear_1)
    y, st_l2 = cl.linear_2(y, ps.linear_2, st.linear_2)
    # Finally, we need to return the new state which has the exact structure as `st`
    return y, (linear_1 = st_l1, linear_2 = st_l2)

Here, you will notice we have passed (:linear_1, :linear_2) to the supertype. It essentially informs the type that, <obj>.linear_1 and <obj>.linear_2 are Lux layers and we need to construct parameters and states for those. Let's construct these and see:

model = ComposedLinear(Linear(2, 4), Linear(4, 2))

ps, st = Lux.setup(rng, model)

println("Parameters: ", ps)
println("States: ", st)

println("Parameter Length: ", Lux.parameterlength(model), "; State Length: ",

x = randn(rng, Float32, 2, 1)

Lux.apply(model, x, ps, st) # or `model(x, ps, st)`
(Float32[1.3410565; 0.78000563;;], (linear_1 = NamedTuple(), linear_2 = NamedTuple()))

Parameter Interface¤

We accept any parameter type as long as we can fetch the parameters using getproperty(obj, :parameter_name). This allows us to simulaneously support NamedTuples and ComponentArrays. Let us go through a concrete example of what it means. Consider Dense which expects two parameters named weight and bias.


If you are defining your own parameter type, it is your responsibility to make sure that it works with the AutoDiff System you are using.

using Lux, Random

d = Dense(2, 3)
rng = Random.default_rng()
Random.seed!(rng, 0)

ps_default, st = Lux.setup(rng, d)

x = randn(rng, Float32, 2, 1)

println("Result with `NamedTuple` parameters: ", first(d(x, ps_default, st)))
Result with `NamedTuple` parameters: Float32[1.135916; 0.7668784; -1.0876652;;]

Let, us define a custom paramter type with fields myweight and mybias but if we try to access weight we get back myweight, similar for bias.


This is for demonstrative purposes, don't try this at home!

struct DenseLayerParameters{W, B}

function Base.getproperty(ps::DenseLayerParameters, x::Symbol)
    if x == :weight
        return getfield(ps, :myweight)
    elseif x == :bias
        return getfield(ps, :mybias)
    return getfield(ps, x)

ps = DenseLayerParameters(ps_default.weight, ps_default.bias)

println("Result with `DenseLayerParameters` parameters: ", first(d(x, ps, st)))
Result with `DenseLayerParameters` parameters: Float32[1.135916; 0.7668784; -1.0876652;;]

The takeaway from this shouldn't be – lets define weird parameter types. Simply because you can do weird things like this doesn't mean you should, since it only leads to bugs.

Instead this shows the flexibility you have for how your parameters can be structured.

State Interface¤

States are always type constrained to be NamedTuple. The structure of the input state must match that of the output state, i.e. keys(st_in) == keys(st_out). This doesn't imply that types of the input and output state match. To generate efficient code, we often do dispatch on the state, for example, Dropout, BatchNorm, etc.