# `Flezha.SDK.State`
[🔗](https://github.com/flezha/flezha_elixir_sdk/blob/v0.1.0/lib/flezha/sdk/state.ex#L1)

The `state` block and `defchange`: §3's change derivation, in Elixir.

Declared state is mutated only through declared changes, because the change
log IS how a cell is rebuilt. A `state` block declares the fields and their
defaults, and `defchange` writes each change once:

    use Flezha.SDK.Tasklet, name: "position"

    state do
      field :qty, :integer, default: 0
      field :fill_ids, :list, default: []
    end

    defchange record_fill(state, qty, id) do
      %{state | qty: state.qty + qty, fill_ids: [id | state.fill_ids]}
    end

The block generates `init_state/1` (the map of defaults, overridable) and
`state_from_json/1` (each field read by its string key, falling back to its
default). It generates no change.

`defchange name(state, args…) do body end` is the one way to change state,
in a module with or without a block: it is written once and generated into
both halves — the `apply_change(state, {:name, args…})` clause and the
recorder `name(state, ctx, args…)`, which journals the change through
`Flezha.SDK.Ctx.apply/3` and answers the new state — so a mutation spanning
several fields is one journal record. A module with no `defchange` exports
no `apply_change/2` and is changeless.

A module that `use`s `Flezha.SDK.Tasklet` or a relay macro and writes
`apply_change/2` by hand fails to compile, directing the author to
`defchange`; a hand-written `apply_change/2` belongs to the reference form,
`@behaviour Flezha.SDK.Tasklet`. A block beside a hand-written
`state_from_json/1` fails to compile too: the block owns it.

§3.1's keyed-map and deque modes are deferred for Elixir; this block carries
the field and `:list` modes only. Renaming or retyping a field is a
non-additive state edit and takes a schema-version bump, as §3.0 says for
every SDK.

# `defchange`
*macro* 

A hand-written change: one body, generated into the fold and the recorder, so
a mutation spanning several fields is one journal record.

# `field`
*macro* 

One state field: a name, one of `:integer`, `:float`, `:string`, `:boolean`,
`:list`, `:map` or `:term`, and a `default:`.

# `state`
*macro* 

Declare the fields a cell's state carries. See the module doc.

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*Consult [api-reference.md](api-reference.md) for complete listing*