Is it possible to set default values for type-parameterized fields of a struct?

Question

I have a trait that genericizes getting a (audio, etc) sample from a "signal":

pub trait Signal {
    /// When requested, all signals must produce a f64 sample.
    fn evaluate(&mut self) -> f64;
}

I wanted to implement this trait for plain-old f64s and make it just keep on returning the same value instead of making a whole different struct that implements Signal to do the same thing:

impl Signal for f64 {
    fn evaluate(&mut self) -> f64 {
        *self
    }
}

It turns out that this doesn't matter anyway; even if I make said struct, when I try to give a default value it doesn't work either way.

---

In one of my signal generators, I'd like to switch over from using a f64 type:

// Sine implements Signal
pub struct Sine {
    frequency: f64,
    // Other fields omitted
}

To a Signal type, to be able to have the frequency change over time:

pub struct Sine<F> where
    F: Signal,
{
    frequency: F,
    // Other fields omitted
}

The problem arises when I go to make a Sine::new() function that tries to default the parameterized type (NOTE that I needed to use parameterized types to capture a trait type instead of using references due to a previous issue with borrowing I had with rust-portaudio):

impl<F> Sine<F> where
    F: Signal,
{
    /// Creates a new Sine wave signal generator.
    pub fn new() -> Sine<F> {
        Sine {
            frequency: 444.0,
            // Other fields omitted
        }
    }
}

I get the following error:

error[E0308]: mismatched types
  --> src/dsp/generators/sine.rs:35:24
   |
35 |             frequency: 444.0,
   |                        ^^^^^ expected type parameter, found floating-point variable
   |
   = note: expected type `F`
              found type `{float}`

This same struct definition works fine as long as I have a new() where you explicitly pass in a type that implements Signal:

let sine_gen1 = generators::Sine::new(440.0);

but I would like to be able to default values instead of passing all of the arguments all of the time.

A few other things I've tried:

• 444.0 as F
  • error[E0605]: non-primitive cast: 'f64' as 'F' (error[E0605]: non-primitive cast: 'f64' as 'F')
• 440.0_f64 as Signal
  • error[E0620]: cast to unsized type: 'f64' as 'dsp::traits::Signal'
• Making a new struct (called Constant) that takes in a f64 in its own new() function and implements Signal, then making an instance of that struct in Sine::new() and trying to set it to frequency
  • I haven't found a combination of doing this that seems to work, plus it kind of feels too hacky to be correct.