Enum Step 

pub enum Step {
    Done(Result<Value, String>),
    Wait(Promise, Box<dyn FnOnce(PromiseState) -> Step>),
    Yield(Value, Box<dyn FnOnce(Value) -> Step>),
    Continue(Box<dyn FnOnce() -> Step>),
}

Variants

Done(Result<Value, String>)

Wait(Promise, Box<dyn FnOnce(PromiseState) -> Step>)

Yield(Value, Box<dyn FnOnce(Value) -> Step>)

Continue(Box<dyn FnOnce() -> Step>)

Defers the next synchronous continuation to the fiber driver. Without this trampoline, a document with many top-level forms keeps one Rust stack frame per completed form (and can exhaust the smaller WASM stack).