Enum ExecutionStep 

pub enum ExecutionStep {
    ExecuteSlot(usize, DonePredicate),
    AppendChild(usize),
    Done,
}

Scheduler step indicator returned alongside an array in ExecutionResult.

Instead of recursively executing children, encodings return an ExecutionStep that tells the scheduler what to do next. This enables the scheduler to manage execution iteratively using an explicit work stack plus an optional builder.

Semantics

Each variant describes a different execution strategy with distinct cost profiles:

• Done: The current activation has finished its work. If no builder is active, the returned array is the result. If a builder is active, the scheduler ignores the placeholder array and finishes the builder instead. The scheduler may continue executing if the target form (e.g. canonical) has not yet been reached.

• ExecuteSlot: The encoding needs one of its children decoded before it can make further progress. The scheduler detaches that child, pushes the parent onto the explicit stack, executes the child until the DonePredicate matches, puts it back, and re-enters the parent. This is a cooperative yield: the encoding does a bounded amount of work per step while the loop tracks the parent-child relationship explicitly.

• AppendChild: The encoding needs one child executed to canonical form and then appended into a builder owned by the current activation. The scheduler detaches that child, lazily creates current_builder if needed, appends the child into it, and keeps the parent as current_array for the next iteration. While the builder is active, parent-kernel rewrites are skipped because the parent is partially consumed. Important: in the single-step executor (Executable for ArrayRef), returning AppendChild still causes the executor to drive the entire array to completion via execute_into_builder in one call — this can do significantly more work than a single ExecuteSlot step.

Variants

ExecuteSlot(usize, DonePredicate)

Request that the scheduler execute the slot at the given index, using the provided DonePredicate to determine when the slot is “done”, then replace the slot in this array and re-enter execution.

Use ExecutionResult::execute_slot instead of constructing this variant directly.

AppendChild(usize)

Detach the slot at the given index, append that child into the current activation’s canonical builder, and keep the returned parent as current_array.

Done finalizes that builder and turns it into the result of the activation.

Note: In the single-step executor (Executable for ArrayRef), this variant drives the entire parent to completion in one call via execute_into_builder, which may perform substantially more work than a single ExecuteSlot step.

Done

Execution is complete. If no builder is active, the array in the accompanying ExecutionResult is the result. Otherwise, the scheduler finalizes the active builder and uses that finished array instead.

The scheduler will continue executing if it has not yet reached the target form.

Trait Implementations

impl Debug for ExecutionStep

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.