Fix log_call! macro example in "Rust for Python Programmers" (ch12 closures and iterators)

[Chandan-Kalita]

Fixes an issue in the log_call! macro where it expands to multiple statements instead of a single expression, causing a type mismatch in usage.

Problem

The macro was defined as:

macro_rules! log_call {
    ($func_name:expr, $body:expr) => {
        println!("Calling {}", $func_name);
        $body
    };
}

When used in expression position:

fn process(data: &str) -> String {
    log_call!("process", data.to_uppercase())
}

it expands to:

println!(...);
data.to_uppercase()

This results in a compilation error:

expected String, found ()
because the macro does not expand to a single expression.

Fix

Wrap the macro body in a block expression:

macro_rules! log_call {
    ($func_name:expr, $body:expr) => {{
        println!("Calling {}", $func_name);
        $body
    }};
}

This ensures the macro expands to a single expression whose value is the result of $body.

Result

• Fixes compilation error in the example
• Demonstrates correct macro usage in expression position
• Improves clarity for learners