Why we wrote Lux?

Julia already has quite a few well established Neural Network Frameworks – Flux & KNet. However, certain design elements – Coupled Model and Parameters & Internal Mutations – associated with these frameworks make them less compiler and user friendly. Making changes to address these problems in the respective frameworks would be too disruptive for users. Here comes in Lux: a neural network framework built completely using pure functions to make it both compiler and autodiff friendly.

Design Principles

• Layers must be immutable – cannot store any parameter/state but rather store the information to construct them

• Layers are pure functions

• Layers return a Tuple containing the result and the updated state

• Given same inputs the outputs must be same – yes this must hold true even for stochastic functions. Randomness must be controlled using rngs passed in the state.

• Easily extensible

• Extensive Testing – All layers and features are tested across all supported AD backends across all supported hardware backends.

Why use Lux over Flux?

• Neural Networks for SciML: For SciML Applications (Neural ODEs, Deep Equilibrium Models) solvers typically expect a monolithic parameter vector. Flux enables this via its destructure mechanism, but destructure comes with various edge cases and limitations. Lux forces users to make an explicit distinction between state variables and parameter variables to avoid these issues. Also, it comes battery-included for distributed training.

• Sensible display of Custom Layers – Ever wanted to see Pytorch like Network printouts or wondered how to extend the pretty printing of Flux's layers? Lux handles all of that by default.

• Truly immutable models - No unexpected internal mutations since all layers are implemented as pure functions. All layers are also deterministic given the parameters and state: if a layer is supposed to be stochastic (say Dropout), the state must contain a seed which is then updated after the function call.

• Easy Parameter Manipulation – By separating parameter data and layer structures, Lux makes implementing WeightNorm, SpectralNorm, etc. downright trivial. Without this separation, it is much harder to pass such parameters around without mutations which AD systems don't like.

• Small Neural Networks on CPU – Lux is developed for training large neural networks. For smaller architectures, we recommend using SimpleChains.jl or even better use it in conjunction with Lux via ToSimpleChainsAdaptor.

• Reliability – We have learned from the mistakes of the past with Flux and everything in our core framework is extensively tested, along with downstream CI to ensure that everything works as expected.

Why not use Lux (and Julia for traditional Deep Learning in general) ?

• Lack of Large Models Support – Classical deep learning is not Lux's primary focus. For these, python frameworks like PyTorch and Jax are better suited.

• XLA Support – Lux doesn't compile to XLA which means no TPU support unfortunately.