Passing function as parameter in Rust

Passing function type check in Rust

Passing a function to another function is not a new thing in programming. We usually pass the address of the function( in C like languages &function name ).
but do we ensure that passed function performing the intended functionality or at least the parameter and return types are matching with what we intended to pass.

Rust asks passed function signature

Rust explicitly ask for the type of the pass function signature . If it’s not matching the rust program won’t compile.
that means - no one can inject anonymous functions to our function for some extend.

Rust passed function

In the below example , you would notice an extra type in the function signature. Anything in ‘’<>" are generic type in Rust. But what is that generic type means ?
If you don't know the type of the function parameter, you can specify it as generic.
It represent the passed function or closure type, that will be declared in the “where” clause.
P is a function type with signature Fn(i32) -> bool means it a closure or function which can accept an integer parameter and return a bool type value.
fn foo(x:i32,mult:P) -> i32
where P: Fn(i32) -> bool
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b1a66f49cc31c9176d894ddd42c4422b
from the main function we call foo as below
foo(220,mult1); which is a valid call because the ‘‘mult1’’ function signature matches with generic type

///foo function definition - it has 2 parameter 
/// an integer value and a passed function
///return type of the function is integer
fn foo(x:i32,mult:P) -> i32
    /// passed function type  
    where P: Fn(i32) -> bool {
  
    ///calling the passed function and getting the return value
    let y = mult(x);
    
    ///some calculation around based on the value 'y'
    if y {
        x
    }else{
        x - 1
    }
}
///the signature of this function matches with foo's passed function signature 'P'
fn mult1(x:i32) -> bool {
    x> 32
}
///the signature of this function not matches with foo's passed function signature 'P'
///as it return integer
fn mult2(x:i32) -> i32{
  x
}

fn main() {
    ///call to foo with passed function 'mult1'
    let q = foo(220,mult1);
    println!("Hello, world! = {}",q);   
}

When I change the same program to call foo with passed function as mult2,
foo(220,mult2);
got the below error message while compiling

error[E0271]: type mismatch resolving ` i32 {mult2} as std::ops::FnOnce<(i32,)>>::Output == bool`
  --> src/main.rs:30:13
   |
2  | fn foo(x:i32,mult:P) -> i32
   |    ---
3  |  
4  |   where P: Fn(i32) -> bool {
   |                       ---- required by this bound in `foo`
...
30 |     let q = foo(220,mult2);
   |             ^^^ expected `bool`, found `i32`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.
error: could not compile `funexp1`.

To learn more, run the command again with --verbose.

the error clearly states below, which is awesome!!

where P: Fn(i32) -> bool {
   |                       ---- required by this bound in `foo`