Struct HostFn 

pub struct HostFn { /* private fields */ }

A host function: a native callback the embedder hands the runtime as a value, behind the Rc a Value::HostFn carries.

It is what a builtin is, minus the compile-time enum: a name, an arity, and code. The evaluator gathers arguments against arity and calls f once they are all there, so a host callback is strict and curried like everything else, and the host writes an ordinary Rust closure rather than an application protocol.

The Ctx reaches f so a callback that is handed an Elly callable can call it back (apply_with) under the same id source; without it a closure the callback minted would collide with one the program already holds.

Arity is at least 1. A zero-argument callback could never fire — nothing applies a value to no arguments — so a host that wants a constant supplies the value itself.

Implementations

impl HostFn

pub fn new( name: impl Into<Text>, arity: usize, f: impl Fn(&[Value], &Ctx) -> Result<Value, Raised> + 'static, ) -> HostFn

A host function called as name, taking arity arguments.

The name is for rendering and diagnostics only — identity is the allocation, so two HostFns of one name are two functions.

Panics on arity == 0, which is not a function the runtime could ever call (see the type’s docs).

pub fn name(&self) -> &Text

The name the function renders under.

pub fn arity(&self) -> usize

How many arguments it is called with.

Trait Implementations

impl Debug for HostFn

The callback is not Debug, so the derive cannot apply; the shape a Value::HostFn shows is its name and arity, which is what identifies it in a dump anyway.

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

Formats the value using the given formatter.