Rust 49 🦀 Closure Syntax, Type Inference, and Capturing Environment
A closure is an anonymous function that can capture the variables from the scope where it is defined. It is the fundamental tool for the higher-order functions, the callbacks, and the concise inline logic. The |x| x + 1 is a closure that takes the x and returns the x + 1, and unlike the function, it can use the variables from the enclosing scope without the explicit parameter. The let add = |a, b| a + b is the closure bound to the variable, and the add(2, 3) calls it the same way the function is called. The closures are the values, and they implement the Fn, the FnMut, and the FnOnce traits .
The three traits are the heart of the closure system. The Fn borrows the captured variables immutably and can be called multiple times. The FnMut borrows them mutably and can be called multiple times. The FnOnce takes the ownership and can be called only once. The compiler infers the trait from the closure body, and the function that accepts the closure uses the trait bound to describe the requirement. The Fn is the most permissive, and the FnOnce is the most restrictive: every Fn is the FnMut, and every FnMut is the FnOnce, but not the reverse .
The type inference is the convenience and the subtlety. The closure parameters are usually inferred from the first call, and the closure return type is inferred from the body. The explicit annotation is the option, but the inference is the default. The capturing is the automatic: the closure captures the variables it uses, and the compiler determines whether the capture is by the immutable borrow, the mutable borrow, or the move. The move keyword forces the capture by the value, which is the requirement for the threads and the 'static bounds .
This chapter covers three areas. First, why the closures exist — the anonymous-function problem and the environment-capture. Second, the syntax and the type inference — the |params| body, the inference, the explicit types, and the return type. Third, the capturing and the three traits — the Fn, the FnMut, the FnOnce, the move, and the interaction with the borrow checker. The chapter ends with a complete example session, a quick reference, best practices, common pitfalls, real-world examples, and diagrams showing the capture modes.
Key point: A closure is an anonymous function that captures the variables from its scope. The syntax is |params| body, and the types are inferred from the context. The three traits are the Fn (the immutable borrow, multiple calls), the FnMut (the mutable borrow, multiple calls), and the FnOnce (the ownership, one call). The move keyword forces the capture by the value, and the compiler picks the least restrictive trait that works.
Why the closures exist
The anonymous-function problem. A function is the named item, and the name is the documentation. But many functions are used once, inline, and the name is the noise. The map, the filter, the and_then, and the unwrap_or_else take the function as the argument, and the inline logic is the natural. The closure is the anonymous function, and the |x| x * 2 is the inline logic without the name. The closures are the values, and they can be stored, passed, and returned.
The environment-capture problem. A function cannot access the variables from the enclosing scope. The fn add(x: i32) -> i32 { x + n } does not compile, because the n is not in the function’s scope. The closure captures the n, and the |x| x + n works. The capturing is the point, and it is what separates the closure from the function.
The callback problem. The callback is the function passed to the library, and the library calls it later. The iter.map(|x| x * 2) passes the closure to the map, and the map calls it for each element. The callback is the closure, and the closures are the foundation of the iterator API and the async runtime.
The three-traits problem. The closure can capture the environment in the three ways: the immutable borrow, the mutable borrow, and the ownership. The Fn, the FnMut, and the FnOnce are the traits that describe the three, and the compiler infers which one the closure implements. The function that accepts the closure uses the trait bound, and the bound determines what the closure can do.
The move problem. The thread requires the closure to be the 'static, which means the closure must own the captured variables. The move keyword forces the capture by the value, and the closure owns the variables. The pattern is the requirement for the thread::spawn and the async tasks.
The trade-off. The closures are the concise and the ergonomic, and the compiler infers the types and the traits. The cost is the opaque types: the closure has the unique anonymous type that cannot be named, and the impl Fn or the Box<dyn Fn> is the way to return it. The trade-off is between the convenience of the inference and the explicitness of the named function.
a. The syntax and the type inference
The closure syntax is the |params| body. The parameters are the |a, b|, and the body is the expression or the block.
let add = |a, b| a + b;
let double = |x| x * 2;
let greet = |name: &str| println!("Hello, {}", name);
println!("{}", add(2, 3)); // 5
println!("{}", double(5)); // 10
greet("Alice");The parameter types are inferred from the usage, and the return type is inferred from the body. The add(2, 3) is the first call, and the a and the b are the i32.
The explicit types. The types can be annotated:
let add = |a: i32, b: i32| -> i32 { a + b };The explicit types are the documentation, and the return type is the -> i32. The types are the requirement when the inference is not the possible.
The multi-line body. The body can be the block:
let process = |x: i32| {
let doubled = x * 2;
let squared = doubled * doubled;
squared
};The block is the multiple statements, and the last expression is the return value.
The no-parameter. The closure with the no parameters:
let greet = || println!("Hello");
greet();The || is the empty parameter list.
The _ parameter. The unused parameter:
let ignore = |_: i32| println!("Ignoring the argument");The type inference from the context. The inference comes from the context, not only from the call:
let numbers = vec![1, 2, 3];
let doubled: Vec<i32> = numbers.iter().map(|x| x * 2).collect();The map expects the FnMut(&i32) -> i32, and the x is the &i32. The x * 2 is the i32, and the collect infers the Vec<i32>.
The inference problem. The closure without the context does not infer the types:
// ❌ The type annotations needed
// let add = |a, b| a + b;
// let result = add(2, 3);
// ✅ The explicit types
let add = |a: i32, b: i32| a + b;
let result = add(2, 3);The add without the call does not have the context, and the compiler requires the annotations. The first call usually provides the context, and the annotation is the fallback.
b. The three traits
The closures implement the Fn, the FnMut, and the FnOnce. The trait describes how the closure captures the environment and how many times it can be called.
The Fn. The closure borrows the captured variables immutably and can be called multiple times.
let name = String::from("Alice");
let greet = || println!("Hello, {}", name);
greet();
greet();
println!("{}", name); // the name is still availableThe greet borrows the name immutably, and the name is still usable after the calls. The closure is the Fn.
The FnMut. The closure borrows the captured variables mutably and can be called multiple times.
let mut count = 0;
let mut increment = || {
count += 1;
println!("Count: {}", count);
};
increment();
increment();
// println!("{}", count); // the count is mutably borrowedThe increment borrows the count mutably, and the count is not usable while the closure is alive. The closure is the FnMut.
The FnOnce. The closure takes the ownership of the captured variables and can be called only once.
let name = String::from("Alice");
let consume = || {
let owned = name;
println!("Consuming {}", owned);
};
consume();
// consume(); // the error: the closure is the FnOnce
// println!("{}", name); // the error: the name is movedThe consume takes the name by the value, and the closure is the FnOnce. The second call and the use of the name are the errors.
The trait hierarchy. The Fn is the FnMut, and the FnMut is the FnOnce. The function that requires the Fn accepts the Fn only, and the function that requires the FnOnce accepts all three.
fn call_fn<F: Fn()>(f: F) {
f();
f();
}
fn call_fn_mut<F: FnMut()>(mut f: F) {
f();
f();
}
fn call_fn_once<F: FnOnce()>(f: F) {
f();
}The call_fn requires the multiple calls, and the call_fn_once requires the one call. The call_fn accepts the Fn only, and the call_fn_once accepts the Fn, the FnMut, and the FnOnce.
The move keyword. The move forces the capture by the value:
let name = String::from("Alice");
let greet = move || println!("Hello, {}", name);
greet();
// println!("{}", name); // the error: the name is movedThe move moves the name into the closure, and the closure owns it. The greet is the Fn because the body only reads the name, but the name is moved.
The move with the thread. The thread requires the 'static, and the move is the requirement:
use std::thread;
let name = String::from("Alice");
let handle = thread::spawn(move || {
println!("Hello, {}", name);
});
handle.join().unwrap();The move moves the name into the closure, and the closure owns it. The thread can outlive the main scope because the closure owns the data.
c. The patterns and the pitfalls
The impl Fn return. The function that returns the closure uses the impl Fn:
fn make_adder(x: i32) -> impl Fn(i32) -> i32 {
move |y| x + y
}
let add_five = make_adder(5);
println!("{}", add_five(10)); // 15The closure captures the x by the move, and the impl Fn(i32) -> i32 is the return type. The concrete type is the unnameable, and the impl Trait is the way to return it.
The Box<dyn Fn> return. The function that returns the different closures uses the Box<dyn Fn>:
fn make_greeter(style: &str) -> Box<dyn Fn(&str)> {
match style {
"formal" => Box::new(|name| println!("Good day, {}", name)),
_ => Box::new(|name| println!("Hey, {}", name)),
}
}The Box<dyn Fn(&str)> allows the two different closures, and the Box is the heap allocation.
The closure in the struct. The struct with the closure field needs the generic or the box:
struct Handler<F: Fn(i32) -> i32> {
callback: F,
}
struct HandlerBox {
callback: Box<dyn Fn(i32) -> i32>,
}The generic is the static dispatch, and the Box<dyn Fn> is the dynamic.
The borrow checker and the closure. The closure that captures the mutable borrow while the outer code uses the variable is the borrow error:
let mut count = 0;
let mut increment = || count += 1;
increment();
println!("{}", count); // the error: the count is mutably borrowedThe fix is the scope:
let mut count = 0;
{
let mut increment = || count += 1;
increment();
}
println!("{}", count); // the count is 1The closure’s scope is the block, and the borrow ends at the block’s end.
The move with the loop. The move in the loop captures the loop variable by the value:
let mut handles = vec![];
for i in 0..5 {
let handle = thread::spawn(move || {
println!("Thread {}", i);
});
handles.push(handle);
}
for handle in handles {
handle.join().unwrap();
}The move captures the i by the value for each iteration, and the threads print the 0 to the 4.
The closure and the lifetime. The closure that captures the reference and the move:
let name = String::from("Alice");
let greet = move || println!("Hello, {}", name);
// The name is moved into the closure.The move moves the name even though the closure only reads it, because the move is the explicit ownership.
Complete Example Session
// ============================================
// PART 1: THE BASIC CLOSURE
// ============================================
fn main() {
let add = |a, b| a + b;
let double = |x| x * 2;
println!("{}", add(2, 3)); // 5
println!("{}", double(5)); // 10
}
// ============================================
// PART 2: THE EXPLICIT TYPES
// ============================================
fn main() {
let add = |a: i32, b: i32| -> i32 { a + b };
let greet = |name: &str| println!("Hello, {}", name);
println!("{}", add(2, 3));
greet("Alice");
}
// ============================================
// PART 3: THE MULTI-LINE BODY
// ============================================
fn main() {
let process = |x: i32| {
let doubled = x * 2;
let squared = doubled * doubled;
squared
};
println!("{}", process(3)); // 36
}
// ============================================
// PART 4: THE TYPE INFERENCE
// ============================================
fn main() {
let numbers = vec![1, 2, 3, 4, 5];
let doubled: Vec<i32> = numbers
.iter()
.map(|x| x * 2)
.collect();
let evens: Vec<&i32> = numbers
.iter()
.filter(|x| *x % 2 == 0)
.collect();
println!("{:?}", doubled); // [2, 4, 6, 8, 10]
println!("{:?}", evens); // [2, 4]
}
// ============================================
// PART 5: THE FN
// ============================================
fn main() {
let name = String::from("Alice");
let greet = || println!("Hello, {}", name);
greet();
greet();
println!("{}", name); // the name is still available
}
// The greet borrows the name immutably.
// The closure is the Fn.
// ============================================
// PART 6: THE FNMUT
// ============================================
fn main() {
let mut count = 0;
let mut increment = || {
count += 1;
println!("Count: {}", count);
};
increment();
increment();
increment();
}
// The increment borrows the count mutably.
// The closure is the FnMut.
// ============================================
// PART 7: THE FNONCE
// ============================================
fn main() {
let name = String::from("Alice");
let consume = || {
let owned = name;
println!("Consuming {}", owned);
};
consume();
// consume(); // the error: the closure is the FnOnce
}
// The consume takes the name by the value.
// The closure is the FnOnce.
// ============================================
// PART 8: THE MOVE
// ============================================
use std::thread;
fn main() {
let name = String::from("Alice");
let handle = thread::spawn(move || {
println!("Hello, {}", name);
});
handle.join().unwrap();
}
// The move moves the name into the closure.
// The thread owns the data.
// ============================================
// PART 9: THE IMPL FN RETURN
// ============================================
fn make_adder(x: i32) -> impl Fn(i32) -> i32 {
move |y| x + y
}
fn make_multiplier(x: i32) -> impl Fn(i32) -> i32 {
move |y| x * y
}
fn main() {
let add_five = make_adder(5);
let double = make_multiplier(2);
println!("{}", add_five(10)); // 15
println!("{}", double(10)); // 20
}
// ============================================
// PART 10: THE COMPLETE EXAMPLE
// ============================================
fn main() {
let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
// The Fn: the immutable borrow
let threshold = 5;
let above_threshold: Vec<i32> = numbers
.iter()
.filter(|x| **x > threshold)
.copied()
.collect();
println!("Above {}: {:?}", threshold, above_threshold);
// The FnMut: the mutable borrow
let mut sum = 0;
numbers.iter().for_each(|x| {
sum += x;
});
println!("Sum: {}", sum);
// The FnOnce: the ownership
let owned: Vec<String> = numbers
.iter()
.map(|x| x.to_string())
.collect();
let consume = move || {
let total = owned.len();
println!("Consuming {} items", total);
};
consume();
// owned is moved into the closure.
// The impl Fn return
let make_adder = |x: i32| move |y: i32| x + y;
let add_ten = make_adder(10);
println!("10 + 5 = {}", add_ten(5));
// The Box<dyn Fn>
let operations: Vec<Box<dyn Fn(i32) -> i32>> = vec![
Box::new(|x| x + 1),
Box::new(|x| x * 2),
Box::new(|x| x - 1),
];
let value = 5;
for op in &operations {
println!("Result: {}", op(value));
}
}The ten parts show the basic closure, the explicit types, the multi-line body, the type inference, the Fn, the FnMut, the FnOnce, the move, the impl Fn return, and the complete example.
Quick Reference
The Closure Syntax
| Syntax | Meaning |
|---|---|
|x| x + 1 | The single parameter |
|a, b| a + b | The multiple parameters |
|| println!("hi") | The no parameters |
|x: i32| -> i32 { x + 1 } | The explicit types |
|x| { let y = x * 2; y + 1 } | The multi-line body |
The Three Traits
| Trait | The capture | The calls |
|---|---|---|
Fn | The immutable borrow | The multiple |
FnMut | The mutable borrow | The multiple |
FnOnce | The ownership | The one |
The Trait Hierarchy
| The trait | The accepts |
|---|---|
Fn | The Fn only |
FnMut | The Fn and the FnMut |
FnOnce | The Fn, the FnMut, and the FnOnce |
The Capture Modes
| The mode | The example | The trait |
|---|---|---|
| The immutable borrow | |x| x + n | The Fn |
| The mutable borrow | || count += 1 | The FnMut |
| The move | move || println!("{}", name) | The Fn / the FnOnce |
The Return Types
| The return | The use case |
|---|---|
impl Fn(i32) -> i32 | The single closure |
Box<dyn Fn(i32) -> i32> | The different closures |
impl FnMut() | The mutable |
impl FnOnce() | The consuming |
The Common Patterns
| The pattern | The example |
|---|---|
| The callback | iter.map(|x| x * 2) |
| The predicate | iter.filter(|x| x > 0) |
| The thread | thread::spawn(move || { ... }) |
| The factory | fn make(x) -> impl Fn(i32) -> i32 { move |y| x + y } |
| The boxed | Box::new(|x| x + 1) |
Best Practices
✅ Do This:
// Use the closures for the inline logic
numbers.iter().map(|x| x * 2).collect(); // ✅
// Use the move for the threads
thread::spawn(move || { ... }); // ✅
// Use the impl Fn for the single closure return
fn make_adder(x: i32) -> impl Fn(i32) -> i32 { move |y| x + y } // ✅
// Use the Box<dyn Fn> for the different closures
let ops: Vec<Box<dyn Fn(i32) -> i32>> = vec![...]; // ✅
// Use the explicit types when the inference fails
let add = |a: i32, b: i32| a + b; // ✅
// Use the scope to limit the borrow
{ let mut inc = || count += 1; inc(); } // ✅❌ Don’t Do This:
// Don't call the FnOnce twice
consume(); consume(); // the error // ❌
// Don't use the moved variable after the closure
let greet = move || println!("{}", name); greet(); println!("{}", name); // ❌
// Don't ignore the borrow error
let mut inc = || count += 1; inc(); println!("{}", count); // the error // ❌
// Don't return the different closures without the Box
fn make(x: bool) -> impl Fn() { if x { || {} } else { || {} } } // the error // ❌
// Don't forget the move in the thread
thread::spawn(|| { ... }); // the borrow error // ❌
// Don't annotate the types when the inference works
let double = |x: i32| -> i32 { x * 2 }; // the unnecessary // ✅ (works but verbose)Common Pitfalls
| Pitfall | Why It Happens | Fix |
|---|---|---|
The FnOnce called twice | The ownership | Use the Fn or the FnMut |
| The moved value used | The move | Clone or restructure |
| The borrow error | The mutable borrow | Limit the scope |
| The type inference fails | The no context | Add the explicit types |
The impl Fn with the different closures | The unnameable | Use the Box<dyn Fn> |
| The thread borrow error | The missing move | Add the move |
The closure is not 'static | The borrowed data | Use the move or the owned data |
Real-World Examples
1. The Basic Closure
let add = |a, b| a + b;2. The Map
numbers.iter().map(|x| x * 2).collect()3. The Filter
numbers.iter().filter(|x| **x > 5).collect()4. The Fold
numbers.iter().fold(0, |acc, x| acc + x)5. The FnMut
let mut count = 0;
let mut inc = || count += 1;6. The Move
let greet = move || println!("{}", name);7. The Thread
thread::spawn(move || { ... })8. The impl Fn
fn make_adder(x: i32) -> impl Fn(i32) -> i32 { move |y| x + y }9. The Box dyn Fn
let op: Box<dyn Fn(i32) -> i32> = Box::new(|x| x + 1);10. The Complete
let make = |x: i32| move |y: i32| x + y;
let add_ten = make(10);
println!("{}", add_ten(5)); // 15Visual
The Three Traits
┌──────────────────────────────────────────────────────────────┐
│ THE THREE TRAITS │
│ │
│ THE Fn: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let name = String::from("Alice"); │ │
│ │ let greet = || println!("{}", name); │ │
│ │ │ │
│ │ The immutable borrow of the name. │ │
│ │ The multiple calls allowed. │ │
│ │ The name is still available. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ THE FnMut: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let mut count = 0; │ │
│ │ let mut inc = || count += 1; │ │
│ │ │ │
│ │ The mutable borrow of the count. │ │
│ │ The multiple calls allowed. │ │
│ │ The count is borrowed. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ THE FnOnce: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let name = String::from("Alice"); │ │
│ │ let consume = || { let x = name; }; │ │
│ │ │ │
│ │ The ownership of the name. │ │
│ │ The one call allowed. │ │
│ │ The name is moved. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
└──────────────────────────────────────────────────────────────┘The Trait Hierarchy
┌──────────────────────────────────────────────────────────────┐
│ THE TRAIT HIERARCHY │
│ │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ Fn │ │
│ │ the immutable borrow, the multiple calls │ │
│ └──────────────────────────────────────────────────────┘ │
│ ▲ │
│ │ the Fn is the FnMut │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ FnMut │ │
│ │ the mutable borrow, the multiple calls │ │
│ └──────────────────────────────────────────────────────┘ │
│ ▲ │
│ │ the FnMut is the FnOnce │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ FnOnce │ │
│ │ the ownership, the one call │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ The function that requires the Fn accepts the Fn only. │
│ The function that requires the FnOnce accepts all three. │
│ │
└──────────────────────────────────────────────────────────────┘The Capture Modes
┌──────────────────────────────────────────────────────────────┐
│ THE CAPTURE MODES │
│ │
│ THE IMMUTABLE BORROW: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let n = 5; │ │
│ │ let f = |x| x + n; │ │
│ │ → the n is borrowed immutably │ │
│ │ → the trait is the Fn │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ THE MUTABLE BORROW: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let mut count = 0; │ │
│ │ let f = || count += 1; │ │
│ │ → the count is borrowed mutably │ │
│ │ → the trait is the FnMut │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ THE MOVE: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let name = String::from("Alice"); │ │
│ │ let f = move || println!("{}", name); │ │
│ │ → the name is moved into the closure │ │
│ │ → the trait is the Fn (the read) or the FnOnce │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
└──────────────────────────────────────────────────────────────┘The impl Fn Return
┌──────────────────────────────────────────────────────────────┐
│ THE impl Fn RETURN │
│ │
│ fn make_adder(x: i32) -> impl Fn(i32) -> i32 { │
│ move |y| x + y │
│ } │
│ │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ The closure captures the x by the move. │ │
│ │ The return type is the impl Fn(i32) -> i32. │ │
│ │ The concrete type is the unnameable. │ │
│ │ │ │
│ │ The caller: │ │
│ │ let add_five = make_adder(5); │ │
│ │ add_five(10); // 15 │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ The impl Trait hides the concrete closure type. │
│ │
└──────────────────────────────────────────────────────────────┘Summary
| Item | Value |
|---|---|
| The closure | The anonymous function |
| The syntax | |params| body |
The Fn | The immutable borrow, the multiple calls |
The FnMut | The mutable borrow, the multiple calls |
The FnOnce | The ownership, the one call |
The move | The capture by the value |
The impl Fn | The single closure return |
The Box<dyn Fn> | The different closures |
| The type inference | The inferred from the context |
The impl Fn return | The unnameable type |
Key takeaways:
- A closure is an anonymous function that captures the environment. The syntax is the
|params| body, and the closure can use the variables from the enclosing scope without the explicit parameter. The closures are the values, and they can be stored, passed, and returned . - The three traits are the
Fn, theFnMut, and theFnOnce. TheFnborrows the captured variables immutably, theFnMutmutably, and theFnOncetakes the ownership. The compiler infers the trait from the closure body, and the function that accepts the closure uses the trait bound . - The trait hierarchy is the
Fn→ theFnMut→ theFnOnce. EveryFnis theFnMut, and everyFnMutis theFnOnce. The function that requires theFnaccepts theFnonly, and the function that requires theFnOnceaccepts all three . - The type inference is the convenience and the subtlety. The parameter types are inferred from the first call, and the return type is inferred from the body. The explicit annotation is the fallback when the inference fails .
- The
movekeyword forces the capture by the value. Themovemoves the captured variables into the closure, and the closure owns them. The pattern is the requirement for the threads and the'staticbounds . - The
impl Fnreturn type hides the concrete closure type. The closure has the unique anonymous type that cannot be named, and theimpl Fn(i32) -> i32is the way to return it. TheBox<dyn Fn>is the alternative when the different closures are returned . - The borrow checker and the closures interact. The closure that captures the mutable borrow while the outer code uses the variable is the borrow error. The fix is the scope or the
move. - The closures are the foundation of the iterator API. The
map, thefilter, thefold, and theand_thentake the closures, and the closures are the inline logic. The closures are the concise and the ergonomic, and the compiler infers the types and the traits .
Remember: A closure is an anonymous function that captures the variables from its scope. The syntax is the |params| body, and the types are inferred from the context. The three traits are the Fn, the FnMut, and the FnOnce, and the compiler picks the least restrictive trait that works. The move keyword forces the capture by the value, and the impl Fn return type hides the concrete closure type. The result is the concise, the ergonomic, and the powerful inline logic, which is the foundation of the functional Rust.
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