Using trait constraints

Back in the section on Generic data structures and functions in Chapter 5, Higher Order Functions and Error-Handling, we made a function sqroot to calculate the square root of a 32-bit floating point number:

// see code in Chapter 5/code/sqrt_match.rs 
Use std::f32; 
 
fn sqroot(r: f32) -> Result<f32, String> { 
if r < 0.0 {  
return Err("Number cannot be negative!".to_string());  
} 
   Ok(f32::sqrt(r)) 
} 

What if we wanted to calculate the square root of an f64 type number? It would be very unpractical to make a different version of the function for each type. A first attempt would be to just replace an f32 type with a generic type <T>:

// see code in Chapter 6/code/trait_constraints.rsfn sqroot<T>(r: T) -> Result<T, String> { 
   if r < 0.0 {  
      return Err("Number cannot be negative!".to_string());  
   } 
    Ok(T::sqrt(r)) 
} 

But Rust does not agree because it doesn't know anything about T, signaling multiple errors:

   ...