Trait Backend 

pub trait Backend: Send + Sync {
    // Required methods
    fn name(&self) -> &'static str;
    fn constant(&self, code: &Core) -> Result<Value, ExecError>;
    fn function(&self, code: &Core) -> Result<Callable, ExecError>;

    // Provided methods
    fn intercepting(
        &self,
        _by: Arc<dyn Interceptor>,
    ) -> Option<Arc<dyn Backend>> { ... }
    fn stack_bytes(&self) -> usize { ... }
}

A way to execute Core.

Required Methods

fn name(&self) -> &'static str

What to call this in a diagnostic or on a dashboard. Two backends running differentially need to be distinguishable in the report that says they disagreed.

fn constant(&self, code: &Core) -> Result<Value, ExecError>

Reduce a closed expression to a value — the fold’s initial accumulator.

fn function(&self, code: &Core) -> Result<Callable, ExecError>

Prepare an expression denoting a function for calling.

Called once per role at startup, so a backend that compiles is free to do the expensive thing here rather than on every event.

Provided Methods

fn intercepting(&self, _by: Arc<dyn Interceptor>) -> Option<Arc<dyn Backend>>

The same program, executed with an Interceptor consulted at every call of a top-level definition. None — the default — means this backend cannot do it.

Defaulted rather than required because it is not part of executing a program: a backend that only ever runs an application in production has no reason to carry it, and the seam should not grow a method every host must implement to serve one command.

fn stack_bytes(&self) -> usize

How much host stack a thread must have before it calls into this backend.

Zero — the default — means “whatever the caller has”, which is the honest answer for a backend that compiles to a machine-code loop and never nests host frames on the program’s recursion. A tree-walker does nest, and needs to say so: docs/31 §31.3 records what leaving it unsaid cost, which was a SIGSEGV where a diagnostic belonged.

It is part of the seam rather than of one crate because the runtime is what spawns threads and the runtime may not name a backend crate (docs/19 §19.9). Asking the backend it was handed is how it finds out without one.

Implementors