Juncture Derive

Proc-macro derive implementation for the Juncture State trait. This crate provides #[derive(State)] which generates typed State/Update pairs with per-field reducer semantics.

Generated Code

For a struct like:

#[derive(State)]
struct MyState {
    #[reducer(append)]
    messages: Vec<String>,
    #[reducer(replace)]
    count: usize,
}

The macro generates:

MyStateUpdate - Update struct with all fields as Option<T>
Field index constants - MyState::FIELD_MESSAGES: usize = 0, etc.
State trait impl - apply() with per-field reducer dispatch

Reducer Types

Reducer	Behavior
`replace`	Default. Single writer per superstep
`append`	Vec extend
`ephemeral`	Reset to Default after each superstep
`last_write_wins`	Multiple writers, last wins
`untracked`	Not persisted across checkpoints
`replace_after_finish`	Available only after `finish()` call
`any`	All writers should provide equal values
`custom = path::to::func`	Custom merge function

Container Attributes

#[state_version(N)] - Set schema version (default 1)
#[migrate_from(N, path::to::func)] - Migration from version N
#[subset_of(ParentState)] - Generate StateSubset<ParentState> impl

Constraints

Structs must have named fields (no tuple or unit structs)
Maximum 64 fields (u64 bitmask limit)
Only one derive per struct

License