;; Accepts lists
(transduce (list 1 2 3 4 5) (mapping (lambda (x) (+ x 1))) (into-list)) ;; => '(2 3 4 5 6)

;; Accepts vectors
(transduce (vector 1 2 3 4 5) (mapping (lambda (x) (+ x 1))) (into-vector)) ;; '#(2 3 4 5 6)

;; Even accepts streams!
(define (integers n)
    (stream-cons n (lambda () (integers (+ 1 n)))))

(transduce (integers 0) (taking 5) (into-list)) ;; => '(0 1 2 3 4)

转换器还接受一个归约函数。上面我们使用了into-list和into-vector，但下面我们可以使用任何任意的归约器。

;; (-> transducer reducing-function initial-value iterable)
(transduce (list 0 1 2 3) (mapping (lambda (x) (+ x 1))) (into-reducer + 0)) ;; => 10

组合仅仅是将迭代器函数组合起来，并让我们避免中间分配。组合是从左到右工作的——它通过函数链每个值，然后累积到输出类型中。请看以下内容：

(define xf 
    (compose 
        (mapping add1)
        (filtering odd?)
        (taking 5)))

(transduce (range 0 100) xf (into-list)) ;; => '(1 3 5 7 9)

语法选择

Steel对定义变量和函数的方式有一些观点。除了我从Racket那里借用的缩写函数语法外，这些选择相当随意。我非常鼓励使用defn和fn，因为这样可以少输入一些字符。

;; All of the following are equivalent
(define (foo x) (+ x 1))
(define foo (lambda (x) (+ x 1)))
(defn (foo x) (+ x 1))
(defn foo (lambda (x) (+ x 1)))

;; All of the following are equivalent
(lambda (x) (+ x 1))
(λ (x) (+ x 1))
(fn (x) (+ x 1))

模块

为了支持日益增长的代码库，Steel支持跨多个文件的模块。Steel文件可以provide值（附有契约）并从其他文件require模块。

;; main.scm
(require "provide.scm")

(even->odd 10)


;; provide.scm
(provide 
    (contract/out even->odd (->/c even? odd?))
    no-contract
    flat-value)

(define (even->odd x) 
    (+ x 1))

(define (accept-number x) (+ x 10))

(define (no-contract) "cool cool cool")
(define flat-value 15)

(displayln "Calling even->odd with some bad inputs but its okay")
(displayln (even->odd 1))

在这里，如果我们运行main，它将包含provide的内容，并且只有提供的值可以从main访问。契约附加在契约边界处，因此在使用provide模块时，您可以违反契约，但模块外部将应用契约。

关于模块的一些说明

• 不允许循环依赖
• 模块只编译一次，并在多个文件中使用。如果A需要B和C，而B需要C，则C将只编译一次，并在A和B之间共享。
• 模块在更改时将被重新编译，并且任何相关文件也将在必要时重新编译。

性能

初步基准测试显示以下结果（在我的机器上）

在虚拟机中嵌入Rust值的示例

Rust值、类型和函数可以轻松嵌入到Steel中。使用register_fn调用，您可以轻松地嵌入函数。

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);
}

我们还可以嵌入结构体本身

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 by value
    pub fn method_by_value(self) -> usize {
        self.foo
    }

    pub fn method_by_reference(&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("method-by-reference", ExternalStruct::method_by_reference)
        .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);
}

请参阅示例文件夹，了解更多有关嵌入值和与外部世界交互的示例。

许可证

根据以下任一许可证授权：

• Apache许可证2.0版本（LICENSE-APACHE 或 https://apache.ac.cn/licenses/LICENSE-2.0）
• MIT许可证（LICENSE-MIT 或 http://opensource.org/licenses/MIT）

任选其一。

贡献

除非你明确声明，否则根据Apache-2.0许可证定义的，你有意提交并包含在工作中的任何贡献，将按照上述方式双重许可，不附加任何额外的条款或条件。

请参阅 CONTRIBUTING.md。