qubit-function 0.18.2

Functional programming traits and Box/Rc/Arc adapters for Rust, inspired by Java functional interfaces
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// =============================================================================
//    Copyright (c) 2025 - 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================

//! # Function Identity Macro
//!
//! Generates identity method implementation for function types.
//!
//! This macro generates an `impl` block that implements the `identity()` method
//! for function types that have identical input and output types (T -> T).
//!
//! # Parameters
//!
//! * `$struct_name` - The struct name (e.g., `BoxFunction`, `RcFunction`,
//!   `ArcFunction`)
//! * `$t:ident` - The generic type parameter name (usually `T`)
//!
//! # Usage
//!
//! ```text
//! impl_function_identity!(BoxFunction<T>);
//! impl_function_identity!(RcFunction<T>);
//! impl_function_identity!(ArcFunction<T>);
//! impl_function_identity!(BoxBiFunction<T, U>);
//! impl_function_identity!(ArcBiFunction<T, U>);
//! ```
//!
//! # Generated Implementation
//!
//! For single-parameter functions, the macro generates:
//!
//! ```text
//! impl<T> BoxFunction<T, T> where T: Clone {
//!     pub fn identity() -> BoxFunction<T, T>;
//! }
//! ```
//!
//! For two-parameter functions, the macro generates:
//!
//! ```text
//! impl<T, U> ArcBiFunction<T, U, T> where T: Clone {
//!     pub fn identity() -> ArcBiFunction<T, U, T>;
//! }
//! ```

/// Generates identity method implementation for function types.
///
/// This macro generates an `impl` block that implements the `identity()` method
/// for function types that have identical input and output types (T -> T).
///
/// # Parameters
///
/// * `$struct_name<$input_type, $output_type>` - The struct name with two
///   generic type parameters
///   - Both generic parameters must be the same type identifier (e.g.,
///     `BoxFunction<T, T>`)
///   - Note: The macro caller must ensure $input_type and $output_type are
///     identical
///
/// # Usage
///
/// ```text
/// impl_function_identity_method!(BoxFunction<T, T>, Function);
/// impl_function_identity_method!(RcFunction<T, T>, Function);
/// impl_function_identity_method!(ArcFunction<T, T>, Function);
/// impl_function_identity_method!(BoxFunctionOnce<T, T>, FunctionOnce);
/// impl_function_identity_method!(BoxMutatingFunction<T, T>, mutating, MutatingFunction);
/// impl_function_identity_method!(BoxStatefulFunction<T, T>, StatefulFunction);
/// ```
macro_rules! impl_function_identity_method {
    ($struct_name:ident < $t:ident , $r:ident >, $trait_name:ident) => {
        // Note: The caller must ensure $t and $r are the same identifier
        impl<$t> $struct_name<$t, $t> {
            /// Creates a function that clones its input and returns the clone.
            ///
            /// The function requires `T: Clone` and does not modify the input.
            ///
            /// # Type Parameters
            ///
            /// - `T`: The input and output type, which must be cloneable.
            ///
            /// # Returns
            ///
            /// A function that clones and returns its borrowed input.
            ///
            /// # Examples
            #[doc = concat!("```rust\nuse qubit_function as qf;\nuse qf::", stringify!($struct_name), ";\nuse qf::", stringify!($trait_name), ";\n\nlet mut identity = ", stringify!($struct_name), "::<i32, i32>::identity();\nassert_eq!(identity.apply(&42), 42);\n```")]
            #[inline]
            pub fn identity() -> $struct_name<$t, $t>
            where
                $t: Clone,
            {
                $struct_name::new(|x: &$t| x.clone())
            }
        }
    };

    // Special case for mutating functions that take &mut T
    ($struct_name:ident < $t:ident , $r:ident >, mutating, $trait_name:ident) => {
        // Note: The caller must ensure $t and $r are the same identifier
        impl<$t> $struct_name<$t, $t> {
            /// Creates a function that clones its input and returns the clone.
            ///
            /// The function requires `T: Clone` and leaves the mutable input unchanged.
            ///
            /// # Type Parameters
            ///
            /// - `T`: The input and output type, which must be cloneable.
            ///
            /// # Returns
            ///
            /// A function that clones and returns its mutably borrowed input
            /// without modifying it.
            ///
            /// # Examples
            #[doc = concat!("```rust\nuse qubit_function as qf;\nuse qf::", stringify!($struct_name), ";\nuse qf::", stringify!($trait_name), ";\n\nlet mut identity = ", stringify!($struct_name), "::<i32, i32>::identity();\nlet mut value = 42;\nassert_eq!(identity.apply(&mut value), 42);\n```")]
            #[inline]
            pub fn identity() -> $struct_name<$t, $t>
            where
                $t: Clone,
            {
                $struct_name::new(|x: &mut $t| x.clone())
            }
        }
    };
}

pub(crate) use impl_function_identity_method;