A protocol that teaches defn how to traverse data structures.

When you invoke a defn, its arguments must implement a Nx.LazyContainer and return a data structure that implements Nx.Container. Inside defn, you can work with any container data structure, such as:

1. numbers/tensors
2. tuples
3. maps of any key
4. any struct that implements Nx.Container

In other words, LazyContainer is how you convert data structures that are not meant to work inside defn into a Nx.Container. And a Nx.Container is a data structure that can be manipulated inside defn itself.

The easiest way to implement Nx.Container is by deriving it. For example:

@derive {Nx.Container,
         containers: [:field_name, :other_field]}
defstruct [:field_name, :other_fields, ...]

The :containers option is required and it must specify a list of fields that contains tensors (or other containers). Inside defn, the container fields will be automatically converted to tensor expressions. All other fields will be reset to their default value, unless you explicitly declare them to be kept:

@derive {Nx.Container,
         containers: [:field_name, :other_field],
         keep: [:another_field]}
defstruct [:field_name, :other_fields, ...]

Note Nx.LazyContainer is automatically provided for all data structures that implement Nx.Container.

Careful!: If you keep a field, its value will be part of the Nx.Defn compiler cache key (i.e. therefore if you give a struct with two different values for a kept field, Nx.Defn will have to compile and cache it twice). You must only keep fields that you are certain to be used inside defn during compilation time.

Serialization

If you @derive {Nx.Container, ...}, it will automatically define a serialization function with the container and keep fields you declare. If you expect a struct to be serialized, then you must be careful to evolve its schema over time in a compatible way. In particular, removing fields will lead to crashes. If you change the type of a field value, previously serialized structs may still hold the old type. And if you add new fields, previously serialized structs won't have such fields and therefore be deserialized with its default value.