09 — Arenas

Arenas are scoped allocators. Allocations made inside an arena block live as long as the arena and are reclaimed together when the block exits.

The program

fn turn(input: Str) -> Lowered!ParseErr {
  arena turn {
    let toks = tokenize(input)
    let ast = parse(toks)?
    lower(ast)
  }
}

fn turn_short(input: Str) -> Lowered!ParseErr {
  arena turn: lower(parse(tokenize(input))?)
}

What is interesting

• arena turn { ... } opens a named arena. The name (turn) is meaningful — agent message handlers get a default turn arena, and observability surfaces report arena lifetimes by name.
• Allocations inside the arena cannot escape unless explicitly promoted to the caller's region. The borrow checker enforces this with MT3010 arena_escape: an arena body whose tail directly names an arena-local non-Copy binding errors. See 14 — Ownership for the full rule.
• The compact form arena turn: expr is the spec's short-body syntax: the arena scope contains a single expression. The expression's value is the arena block's value.
• tokenize, parse, and lower here are illustrative free functions; in a real program they would live in modules under use.

Try it

mty check examples/12_arena.mty

Next

Continue to 10 — Capabilities.