Embedding values

Rust values, types, and functions are easily embedded into Steel. Using the register_fn call, you can embed functions easily:

use steel_vm::engine::Engine;
use steel_vm::register_fn::RegisterFn;

fn external_function(arg1: usize, arg2: usize) -> usize {
    arg1 + arg2
}

fn option_function(arg1: Option<String>) -> Option<String> {
    arg1
}

fn result_function(arg1: Option<String>) -> Result<String, String> {
    if let Some(inner) = arg1 {
        Ok(inner)
    } else {
        Err("Got a none".to_string())
    }
}

pub fn main() {
    let mut vm = Engine::new();

    // Here we can register functions
    // Any function can accept parameters that implement `FromSteelVal` and
    // return values that implement `IntoSteelVal`
    vm.register_fn("external-function", external_function);

    // See the docs for more information about `FromSteelVal` and `IntoSteelVal`
    // but we can see even functions that accept/return Option<T> or Result<T,E>
    // can be registered
    vm.register_fn("option-function", option_function);

    // Result values will map directly to errors in the VM and bubble back up
    vm.register_fn("result-function", result_function);

    vm.run(
        r#"
        (define foo (external-function 10 25))
        (define bar (option-function "applesauce"))
        (define baz (result-function "bananas"))
    "#,
    )
    .unwrap();

    let foo = vm.extract::<usize>("foo").unwrap();
    println!("foo: {}", foo);
    assert_eq!(35, foo);

    // Can also extract a value by specifying the type on the variable
    let bar: String = vm.extract("bar").unwrap();
    println!("bar: {}", bar);
    assert_eq!("applesauce".to_string(), bar);

    let baz: String = vm.extract("baz").unwrap();
    println!("baz: {}", baz);
    assert_eq!("bananas".to_string(), baz);
}

We can also embed structs themselves:

use steel_vm::engine::Engine;
use steel_vm::register_fn::RegisterFn;

use steel_derive::Steel;

// In order to register a type with Steel,
// it must implement Clone, Debug, and Steel
#[derive(Clone, Debug, Steel, PartialEq)]
pub struct ExternalStruct {
    foo: usize,
    bar: String,
    baz: f64,
}

impl ExternalStruct {
    pub fn new(foo: usize, bar: String, baz: f64) -> Self {
        ExternalStruct { foo, bar, baz }
    }

    // Embedding functions that take self must take by value
    pub fn method_by_value(self) -> usize {
        self.foo
    }

    // Setters should update the value and return a new instance (functional set)
    pub fn set_foo(mut self, foo: usize) -> Self {
        self.foo = foo;
        self
    }
}

pub fn main() {
    let mut vm = Engine::new();

    // Registering a type gives access to a predicate for the type
    vm.register_type::<ExternalStruct>("ExternalStruct?");

    // Structs in steel typically have a constructor that is the name of the struct
    vm.register_fn("ExternalStruct", ExternalStruct::new);

    // register_fn can be chained
    vm.register_fn("method-by-value", ExternalStruct::method_by_value)
        .register_fn("set-foo", ExternalStruct::set_foo);

    let external_struct = ExternalStruct::new(1, "foo".to_string(), 12.4);

    // Registering an external value is fallible if the conversion fails for some reason
    // For instance, registering an Err(T) is fallible. However, most implementation outside of manual
    // ones should not fail
    vm.register_external_value("external-struct", external_struct)
        .unwrap();

    let output = vm
        .run(
            r#"
            (define new-external-struct (set-foo external-struct 100))
            (define get-output (method-by-value external-struct))
            (define second-new-external-struct (ExternalStruct 50 "bananas" 72.6))
            "last-result"
        "#,
        )
        .unwrap();

    let new_external_struct = vm.extract::<ExternalStruct>("new-external-struct").unwrap();
    println!("new_external_struct: {:?}", new_external_struct);
    assert_eq!(
        ExternalStruct::new(100, "foo".to_string(), 12.4),
        new_external_struct
    );

    // Can also extract a value by specifying the type on the variable
    let get_output: usize = vm.extract("get-output").unwrap();
    println!("get_output: {}", get_output);
    assert_eq!(1, get_output);

    let second_new_external_struct: ExternalStruct =
        vm.extract("second-new-external-struct").unwrap();
    println!(
        "second_new_external_struct: {:?}",
        second_new_external_struct
    );
    assert_eq!(
        ExternalStruct::new(50, "bananas".to_string(), 72.6),
        second_new_external_struct
    );

    // We also get the output of the VM as the value of every expression run
    // we can inspect the results just by printing like so
    println!("{:?}", output);
}