bevy/crates/bevy_reflect/src/func/mod.rs

//! Reflection-based dynamic functions.
//!
//! This module provides a way to pass around and call functions dynamically
//! using the [`DynamicFunction`] and [`DynamicFunctionMut`] types.
//!
//! Many simple functions and closures can be automatically converted to these types
//! using the [`IntoFunction`] and [`IntoFunctionMut`] traits, respectively.
//!
//! Once this dynamic representation is created, it can be called with a set of arguments provided
//! via an [`ArgList`].
//!
//! This returns a [`FunctionResult`] containing the [`Return`] value,
//! which can be used to extract a [`PartialReflect`] trait object.
//!
//! # Example
//!
//! ```
//! # use bevy_reflect::PartialReflect;
//! # use bevy_reflect::func::args::ArgList;
//! # use bevy_reflect::func::{DynamicFunction, FunctionResult, IntoFunction, Return};
//! fn add(a: i32, b: i32) -> i32 {
//!   a + b
//! }
//!
//! let mut func: DynamicFunction = add.into_function();
//! let args: ArgList = ArgList::default()
//!   // Pushing a known type with owned ownership
//!   .with_owned(25_i32)
//!   // Pushing a reflected type with owned ownership
//!   .with_boxed(Box::new(75_i32) as Box<dyn PartialReflect>);
//! let result: FunctionResult = func.call(args);
//! let value: Return = result.unwrap();
//! assert_eq!(value.unwrap_owned().try_downcast_ref::<i32>(), Some(&100));
//! ```
//!
//! # Types of Functions
//!
//! For simplicity, this module uses the umbrella term "function" to refer to any Rust callable:
//! code that can be invoked with a set of arguments to perform some action.
//!
//! In Rust, there are two main categories of callables: functions and closures.
//!
//! A "function" is a callable that does not capture its environment.
//! These are typically defined with the `fn` keyword, which are referred to as _named_ functions.
//! But they are also _anonymous_ functions, which are unnamed and defined with anonymous function syntax.
//!
//! ```rust
//! // This is a named function:
//! fn add(a: i32, b: i32) -> i32 {
//!   a + b
//! }
//!
//! // This is an anonymous function:
//! let add = |a: i32, b: i32| a + b;
//! ```
//!
//! Closures, on the other hand, are special functions that do capture their environment.
//! These are always defined with anonymous function syntax.
//!
//! ```
//! // A closure that captures an immutable reference to a variable
//! let c = 123;
//! let add = |a: i32, b: i32| a + b + c;
//!
//! // A closure that captures a mutable reference to a variable
//! let mut total = 0;
//! let add = |a: i32, b: i32| total += a + b;
//!
//! // A closure that takes ownership of its captured variables by moving them
//! let c = 123;
//! let add = move |a: i32, b: i32| a + b + c;
//! ```
//!
//! # Valid Signatures
//!
//! Many of the traits in this module have default blanket implementations over a specific set of function signatures.
//!
//! These signatures are:
//! - `(...) -> R`
//! - `for<'a> (&'a arg, ...) -> &'a R`
//! - `for<'a> (&'a mut arg, ...) -> &'a R`
//! - `for<'a> (&'a mut arg, ...) -> &'a mut R`
//!
//! Where `...` represents 0 to 15 arguments (inclusive) of the form `T`, `&T`, or `&mut T`.
//! The lifetime of any reference to the return type `R`, must be tied to a "receiver" argument
//! (i.e. the first argument in the signature, normally `self`).
//!
//! Each trait will also have its own requirements for what traits are required for both arguments and return types,
//! but a good rule-of-thumb is that all types should derive [`Reflect`].
//!
//! The reason for such a small subset of valid signatures is due to limitations in Rust—
//! namely the [lack of variadic generics] and certain [coherence issues].
//!
//! For other functions that don't conform to one of the above signatures,
//! [`DynamicFunction`] and [`DynamicFunctionMut`] can instead be created manually.
//!
//! # Generic Functions
//!
//! In Rust, generic functions are [monomorphized] by the compiler,
//! which means that a separate copy of the function is generated for each concrete set of type parameters.
//!
//! When converting a generic function to a [`DynamicFunction`] or [`DynamicFunctionMut`],
//! the function must be manually monomorphized with concrete types.
//! In other words, you cannot write `add<T>.into_function()`.
//! Instead, you will need to write `add::<i32>.into_function()`.
//!
//! This means that reflected functions cannot be generic themselves.
//! To get around this limitation, you can consider [overloading] your function with multiple concrete types.
//!
//! # Overloading Functions
//!
//! Both [`DynamicFunction`] and [`DynamicFunctionMut`] support [function overloading].
//!
//! Function overloading allows one function to handle multiple types of arguments.
//! This is useful for simulating generic functions by having an overload for each known concrete type.
//! Additionally, it can also simulate [variadic functions]: functions that can be called with a variable number of arguments.
//!
//! Internally, this works by storing multiple functions in a map,
//! where each function is associated with a specific argument signature.
//!
//! To learn more, see the docs on [`DynamicFunction::with_overload`].
//!
//! # Function Registration
//!
//! This module also provides a [`FunctionRegistry`] that can be used to register functions and closures
//! by name so that they may be retrieved and called dynamically.
//!
//! ```
//! # use bevy_reflect::func::{ArgList, FunctionRegistry};
//! fn add(a: i32, b: i32) -> i32 {
//!     a + b
//! }
//!
//! let mut registry = FunctionRegistry::default();
//!
//! // You can register functions and methods by their `core::any::type_name`:
//! registry.register(add).unwrap();
//!
//! // Or you can register them by a custom name:
//! registry.register_with_name("mul", |a: i32, b: i32| a * b).unwrap();
//!
//! // You can then retrieve and call these functions by name:
//! let reflect_add = registry.get(core::any::type_name_of_val(&add)).unwrap();
//! let value = reflect_add.call(ArgList::default().with_owned(10_i32).with_owned(5_i32)).unwrap();
//! assert_eq!(value.unwrap_owned().try_downcast_ref::<i32>(), Some(&15));
//!
//! let reflect_mul = registry.get("mul").unwrap();
//! let value = reflect_mul.call(ArgList::default().with_owned(10_i32).with_owned(5_i32)).unwrap();
//! assert_eq!(value.unwrap_owned().try_downcast_ref::<i32>(), Some(&50));
//! ```
//!
//! [`PartialReflect`]: crate::PartialReflect
//! [`Reflect`]: crate::Reflect
//! [lack of variadic generics]: https://poignardazur.github.io/2024/05/25/report-on-rustnl-variadics/
//! [coherence issues]: https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#coherence-leak-check
//! [monomorphized]: https://en.wikipedia.org/wiki/Monomorphization
//! [overloading]: #overloading-functions
//! [function overloading]: https://en.wikipedia.org/wiki/Function_overloading
//! [variadic functions]: https://en.wikipedia.org/wiki/Variadic_function

pub use args::{ArgError, ArgList, ArgValue};
pub use dynamic_function::*;
pub use dynamic_function_mut::*;
pub use error::*;
pub use function::*;
pub use info::*;
pub use into_function::*;
pub use into_function_mut::*;
pub use reflect_fn::*;
pub use reflect_fn_mut::*;
pub use registry::*;
pub use return_type::*;

pub mod args;
mod dynamic_function;
mod dynamic_function_internal;
mod dynamic_function_mut;
mod error;
mod function;
mod info;
mod into_function;
mod into_function_mut;
pub(crate) mod macros;
mod reflect_fn;
mod reflect_fn_mut;
mod registry;
mod return_type;
pub mod signature;

#[cfg(test)]
mod tests {
    use alloc::borrow::Cow;

    use super::*;
    use crate::func::args::ArgCount;
    use crate::{
        func::args::{ArgError, ArgList, Ownership},
        TypePath,
    };

    #[test]
    fn should_error_on_missing_args() {
        fn foo(_: i32) {}

        let func = foo.into_function();
        let args = ArgList::new();
        let result = func.call(args);
        assert_eq!(
            result.unwrap_err(),
            FunctionError::ArgCountMismatch {
                expected: ArgCount::new(1).unwrap(),
                received: 0
            }
        );
    }

    #[test]
    fn should_error_on_too_many_args() {
        fn foo() {}

        let func = foo.into_function();
        let args = ArgList::new().with_owned(123_i32);
        let result = func.call(args);
        assert_eq!(
            result.unwrap_err(),
            FunctionError::ArgCountMismatch {
                expected: ArgCount::new(0).unwrap(),
                received: 1
            }
        );
    }

    #[test]
    fn should_error_on_invalid_arg_type() {
        fn foo(_: i32) {}

        let func = foo.into_function();
        let args = ArgList::new().with_owned(123_u32);
        let result = func.call(args);
        assert_eq!(
            result.unwrap_err(),
            FunctionError::ArgError(ArgError::UnexpectedType {
                index: 0,
                expected: Cow::Borrowed(i32::type_path()),
                received: Cow::Borrowed(u32::type_path())
            })
        );
    }

    #[test]
    fn should_error_on_invalid_arg_ownership() {
        fn foo(_: &i32) {}

        let func = foo.into_function();
        let args = ArgList::new().with_owned(123_i32);
        let result = func.call(args);
        assert_eq!(
            result.unwrap_err(),
            FunctionError::ArgError(ArgError::InvalidOwnership {
                index: 0,
                expected: Ownership::Ref,
                received: Ownership::Owned
            })
        );
    }
}