Trait FnTransformerOnceOps 

pub trait FnTransformerOnceOps<T, R>: FnOnce(T) -> R + Sized {
    // Provided methods
    fn and_then<S, G>(self, after: G) -> BoxTransformerOnce<T, S>
       where Self: 'static,
             S: 'static,
             G: TransformerOnce<R, S> + 'static,
             T: 'static,
             R: 'static { ... }
    fn when<P>(self, predicate: P) -> BoxConditionalTransformerOnce<T, R>
       where Self: 'static,
             P: Predicate<T> + 'static,
             T: 'static,
             R: 'static { ... }
}

Extension trait for closures implementing FnOnce(T) -> R

Provides composition methods (and_then, compose, when) for one-time use closures and function pointers without requiring explicit wrapping in BoxTransformerOnce.

This trait is automatically implemented for all closures and function pointers that implement FnOnce(T) -> R.

Design Rationale

While closures automatically implement TransformerOnce<T, R> through blanket implementation, they don’t have access to instance methods like and_then, compose, and when. This extension trait provides those methods, returning BoxTransformerOnce for maximum flexibility.

Examples

Chain composition with and_then

use qubit_function::{TransformerOnce, FnTransformerOnceOps};

let parse = |s: String| s.parse::<i32>().unwrap_or(0);
let double = |x: i32| x * 2;

let composed = parse.and_then(double);
assert_eq!(composed.apply("21".to_string()), 42);

Forward composition with and_then

use qubit_function::{TransformerOnce, FnTransformerOnceOps};

let double = |x: i32| x * 2;
let parse = |s: String| s.parse::<i32>().unwrap_or(0);

let composed = parse.and_then(double);
assert_eq!(composed.apply("21".to_string()), 42);

Conditional transformation with when

use qubit_function::{TransformerOnce, FnTransformerOnceOps};

let double = |x: i32| x * 2;
let conditional = double.when(|x: &i32| *x > 0).or_else(|x: i32| -x);

assert_eq!(conditional.apply(5), 10);

Author

Haixing Hu

Provided Methods

fn and_then<S, G>(self, after: G) -> BoxTransformerOnce<T, S>

where Self: 'static, S: 'static, G: TransformerOnce<R, S> + 'static, T: 'static, R: 'static,

Chain composition - applies self first, then after

Creates a new transformer that applies this transformer first, then applies the after transformer to the result. Consumes self and returns a BoxTransformerOnce.

Type Parameters

• S - The output type of the after transformer
• G - The type of the after transformer (must implement TransformerOnce<R, S>)

Parameters

• after - The transformer to apply after self. Note: This parameter is passed by value and will transfer ownership. Since this is a FnOnce transformer, the parameter will be consumed. Can be:
  • A closure: |x: R| -> S
  • A function pointer: fn(R) -> S
  • A BoxTransformerOnce<R, S>
  • Any type implementing TransformerOnce<R, S>

Returns

A new BoxTransformerOnce<T, S> representing the composition

Examples

use qubit_function::{TransformerOnce, FnTransformerOnceOps,
    BoxTransformerOnce};

let parse = |s: String| s.parse::<i32>().unwrap_or(0);
let double = BoxTransformerOnce::new(|x: i32| x * 2);

// double is moved and consumed
let composed = parse.and_then(double);
assert_eq!(composed.apply("21".to_string()), 42);
// double.apply(5); // Would not compile - moved

Examples found in repository

examples/transformers/fn_transformer_once_ops_demo.rs (line 26)

fn main() {
    println!("=== FnTransformerOnceOps Example ===\n");

    // 1. Basic and_then composition
    println!("1. Basic and_then composition:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let double = |x: i32| x * 2;
    let composed = parse.and_then(double);
    println!(
        "   parse.and_then(double).apply(\"21\") = {}",
        composed.apply("21".to_string())
    );
    println!();

    // 2. Chained and_then composition
    println!("2. Chained and_then composition:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let add_one = |x: i32| x + 1;
    let double = |x: i32| x * 2;
    let chained = parse.and_then(add_one).and_then(double);
    println!(
        "   parse.and_then(add_one).and_then(double).apply(\"5\") = {}",
        chained.apply("5".to_string())
    ); // (5 + 1) * 2 = 12
    println!();

    // 3. More and_then composition
    println!("3. More and_then composition:");
    let double = |x: i32| x * 2;
    let to_string = |x: i32| x.to_string();
    let composed = double.and_then(to_string);
    println!(
        "   double.and_then(to_string).apply(21) = {}",
        composed.apply(21)
    ); // (21 * 2).to_string() = "42"
    println!();

    // 4. Conditional transformation when
    println!("4. Conditional transformation when:");
    let double = |x: i32| x * 2;
    let conditional = double.when(|x: &i32| *x > 0).or_else(|x: i32| -x);
    println!("   double.when(x > 0).or_else(negate):");
    println!("     transform(5) = {}", conditional.apply(5)); // 10

    let double2 = |x: i32| x * 2;
    let conditional2 = double2.when(|x: &i32| *x > 0).or_else(|x: i32| -x);
    println!("     transform(-5) = {}", conditional2.apply(-5)); // 5
    println!();

    // 5. Complex composition
    println!("5. Complex composition:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let double = |x: i32| x * 2;
    let triple = |x: i32| x * 3;
    let to_string = |x: i32| x.to_string();

    let complex = parse
        .and_then(double.when(|x: &i32| *x > 5).or_else(triple))
        .and_then(to_string);

    println!("   parse.and_then(double.when(x > 5).or_else(triple)).and_then(to_string):");
    println!("     transform(\"3\") = {}", complex.apply("3".to_string())); // 3 <= 5, so 3 * 3 = 9

    let parse2 = |s: String| s.parse::<i32>().unwrap_or(0);
    let double2 = |x: i32| x * 2;
    let triple2 = |x: i32| x * 3;
    let to_string2 = |x: i32| x.to_string();
    let complex2 = parse2
        .and_then(double2.when(|x: &i32| *x > 5).or_else(triple2))
        .and_then(to_string2);
    println!(
        "     transform(\"10\") = {}",
        complex2.apply("10".to_string())
    ); // 10 > 5, so 10 * 2 = 20
    println!();

    // 6. Type conversion
    println!("6. Type conversion:");
    let to_string = |x: i32| x.to_string();
    let get_length = |s: String| s.len();
    let length_transformer = to_string.and_then(get_length);
    println!(
        "   to_string.and_then(get_length).apply(12345) = {}",
        length_transformer.apply(12345)
    ); // 5
    println!();

    // 7. Closures that capture environment
    println!("7. Closures that capture environment:");
    let multiplier = 3;
    let multiply = move |x: i32| x * multiplier;
    let add_ten = |x: i32| x + 10;
    let with_capture = multiply.and_then(add_ten);
    println!(
        "   multiply(3).and_then(add_ten).apply(5) = {}",
        with_capture.apply(5)
    ); // 5 * 3 + 10 = 25
    println!();

    // 8. Function pointers
    println!("8. Function pointers:");
    fn parse_fn(s: String) -> i32 {
        s.parse().unwrap_or(0)
    }
    fn double_fn(x: i32) -> i32 {
        x * 2
    }
    let fn_composed = parse_fn.and_then(double_fn);
    println!(
        "   parse_fn.and_then(double_fn).apply(\"21\") = {}",
        fn_composed.apply("21".to_string())
    ); // 42
    println!();

    // 9. String operations that consume ownership
    println!("9. String operations that consume ownership:");
    let owned = String::from("hello");
    let append = move |s: String| format!("{} {}", s, owned);
    let uppercase = |s: String| s.to_uppercase();
    let composed = append.and_then(uppercase);
    println!(
        "   append.and_then(uppercase).apply(\"world\") = {}",
        composed.apply("world".to_string())
    ); // "WORLD HELLO"
    println!();

    // 10. Parsing and validation
    println!("10. Parsing and validation:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let validate = |x: i32| if x > 0 { x } else { 1 };
    let composed = parse.and_then(validate);
    println!(
        "   parse.and_then(validate).apply(\"42\") = {}",
        composed.apply("42".to_string())
    ); // 42

    let parse2 = |s: String| s.parse::<i32>().unwrap_or(0);
    let validate2 = |x: i32| if x > 0 { x } else { 1 };
    let composed2 = parse2.and_then(validate2);
    println!(
        "   parse.and_then(validate).apply(\"-5\") = {}",
        composed2.apply("-5".to_string())
    ); // 1
    println!();

    println!("=== Example completed ===");
}

fn when<P>(self, predicate: P) -> BoxConditionalTransformerOnce<T, R>

where Self: 'static, P: Predicate<T> + 'static, T: 'static, R: 'static,

Creates a conditional transformer

Returns a transformer that only executes when a predicate is satisfied. You must call or_else() to provide an alternative transformer for when the condition is not satisfied.

Parameters

• predicate - The condition to check. Note: This parameter is passed by value and will transfer ownership. If you need to preserve the original predicate, clone it first (if it implements Clone). Can be:
  • A closure: |x: &T| -> bool
  • A function pointer: fn(&T) -> bool
  • A BoxPredicate<T>
  • An RcPredicate<T>
  • An ArcPredicate<T>
  • Any type implementing Predicate<T>

Returns

Returns BoxConditionalTransformerOnce<T, R>

Examples

Basic usage with or_else

use qubit_function::{TransformerOnce, FnTransformerOnceOps};

let double = |x: i32| x * 2;
let conditional = double.when(|x: &i32| *x > 0).or_else(|x: i32| -x);

assert_eq!(conditional.apply(5), 10);

Preserving predicate with a second closure

use qubit_function::{Predicate, TransformerOnce, FnTransformerOnceOps};

let double = |x: i32| x * 2;
let is_positive = |x: &i32| *x > 0;
let is_positive_for_validation = |x: &i32| *x > 0;
let conditional = double.when(is_positive)
    .or_else(|x: i32| -x);

assert_eq!(conditional.apply(5), 10);

// Original predicate still usable
assert!(is_positive_for_validation(&3));

Examples found in repository

examples/transformers/fn_transformer_once_ops_demo.rs (line 59)

fn main() {
    println!("=== FnTransformerOnceOps Example ===\n");

    // 1. Basic and_then composition
    println!("1. Basic and_then composition:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let double = |x: i32| x * 2;
    let composed = parse.and_then(double);
    println!(
        "   parse.and_then(double).apply(\"21\") = {}",
        composed.apply("21".to_string())
    );
    println!();

    // 2. Chained and_then composition
    println!("2. Chained and_then composition:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let add_one = |x: i32| x + 1;
    let double = |x: i32| x * 2;
    let chained = parse.and_then(add_one).and_then(double);
    println!(
        "   parse.and_then(add_one).and_then(double).apply(\"5\") = {}",
        chained.apply("5".to_string())
    ); // (5 + 1) * 2 = 12
    println!();

    // 3. More and_then composition
    println!("3. More and_then composition:");
    let double = |x: i32| x * 2;
    let to_string = |x: i32| x.to_string();
    let composed = double.and_then(to_string);
    println!(
        "   double.and_then(to_string).apply(21) = {}",
        composed.apply(21)
    ); // (21 * 2).to_string() = "42"
    println!();

    // 4. Conditional transformation when
    println!("4. Conditional transformation when:");
    let double = |x: i32| x * 2;
    let conditional = double.when(|x: &i32| *x > 0).or_else(|x: i32| -x);
    println!("   double.when(x > 0).or_else(negate):");
    println!("     transform(5) = {}", conditional.apply(5)); // 10

    let double2 = |x: i32| x * 2;
    let conditional2 = double2.when(|x: &i32| *x > 0).or_else(|x: i32| -x);
    println!("     transform(-5) = {}", conditional2.apply(-5)); // 5
    println!();

    // 5. Complex composition
    println!("5. Complex composition:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let double = |x: i32| x * 2;
    let triple = |x: i32| x * 3;
    let to_string = |x: i32| x.to_string();

    let complex = parse
        .and_then(double.when(|x: &i32| *x > 5).or_else(triple))
        .and_then(to_string);

    println!("   parse.and_then(double.when(x > 5).or_else(triple)).and_then(to_string):");
    println!("     transform(\"3\") = {}", complex.apply("3".to_string())); // 3 <= 5, so 3 * 3 = 9

    let parse2 = |s: String| s.parse::<i32>().unwrap_or(0);
    let double2 = |x: i32| x * 2;
    let triple2 = |x: i32| x * 3;
    let to_string2 = |x: i32| x.to_string();
    let complex2 = parse2
        .and_then(double2.when(|x: &i32| *x > 5).or_else(triple2))
        .and_then(to_string2);
    println!(
        "     transform(\"10\") = {}",
        complex2.apply("10".to_string())
    ); // 10 > 5, so 10 * 2 = 20
    println!();

    // 6. Type conversion
    println!("6. Type conversion:");
    let to_string = |x: i32| x.to_string();
    let get_length = |s: String| s.len();
    let length_transformer = to_string.and_then(get_length);
    println!(
        "   to_string.and_then(get_length).apply(12345) = {}",
        length_transformer.apply(12345)
    ); // 5
    println!();

    // 7. Closures that capture environment
    println!("7. Closures that capture environment:");
    let multiplier = 3;
    let multiply = move |x: i32| x * multiplier;
    let add_ten = |x: i32| x + 10;
    let with_capture = multiply.and_then(add_ten);
    println!(
        "   multiply(3).and_then(add_ten).apply(5) = {}",
        with_capture.apply(5)
    ); // 5 * 3 + 10 = 25
    println!();

    // 8. Function pointers
    println!("8. Function pointers:");
    fn parse_fn(s: String) -> i32 {
        s.parse().unwrap_or(0)
    }
    fn double_fn(x: i32) -> i32 {
        x * 2
    }
    let fn_composed = parse_fn.and_then(double_fn);
    println!(
        "   parse_fn.and_then(double_fn).apply(\"21\") = {}",
        fn_composed.apply("21".to_string())
    ); // 42
    println!();

    // 9. String operations that consume ownership
    println!("9. String operations that consume ownership:");
    let owned = String::from("hello");
    let append = move |s: String| format!("{} {}", s, owned);
    let uppercase = |s: String| s.to_uppercase();
    let composed = append.and_then(uppercase);
    println!(
        "   append.and_then(uppercase).apply(\"world\") = {}",
        composed.apply("world".to_string())
    ); // "WORLD HELLO"
    println!();

    // 10. Parsing and validation
    println!("10. Parsing and validation:");
    let parse = |s: String| s.parse::<i32>().unwrap_or(0);
    let validate = |x: i32| if x > 0 { x } else { 1 };
    let composed = parse.and_then(validate);
    println!(
        "   parse.and_then(validate).apply(\"42\") = {}",
        composed.apply("42".to_string())
    ); // 42

    let parse2 = |s: String| s.parse::<i32>().unwrap_or(0);
    let validate2 = |x: i32| if x > 0 { x } else { 1 };
    let composed2 = parse2.and_then(validate2);
    println!(
        "   parse.and_then(validate).apply(\"-5\") = {}",
        composed2.apply("-5".to_string())
    ); // 1
    println!();

    println!("=== Example completed ===");
}

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl<T, R, F> FnTransformerOnceOps<T, R> for F

where F: FnOnce(T) -> R,

Blanket implementation of FnTransformerOnceOps for all FnOnce closures

Automatically implements FnTransformerOnceOps<T, R> for any type that implements FnOnce(T) -> R.

Author

Haixing Hu