Introduce ToInt type operator to return integer values without explicit type annotations (resolve #133)

[jerry73204]

The ToInt type operator return arbitrary integer types, including {i,u}{8,16,32,64} types. Below is the example usage of ToInt. It is useful when passing concrete values to methods with distinct argument types from typed numbers without runtime casting, or when you build a type operator upon arbitrary integer types.

use typenum::{type_operators::ToInt, consts::*};

fn accept_i8(n: i8) { /* ... */ }
fn accept_usize(n: usize) { /* ... */ }

type Value = U7;

fn main() {
    accept_i8(Value::to_int());
    accept_usize(Value::to_int());
}

EDIT 1:
This PR resolves #133.

EDIT 2:
Fix typo

[hgomersall]

This is a useful addition in my opinion. I'd love to see it merged!

[paholg]

Hey, sorry I have been unresponsive. I did not have the mental energy to look at this repo for a while there.

---

I'm all for this, but what do you think of implementing std::convert::From instead of introducing a new trait?

[hgomersall]

@paholg would it not need to be TryFrom? On reflection, I can't see how the current code handles overflow...

[paholg]

@paholg would it not need to be TryFrom? On reflection, I can't see how the current code handles overflow...

One could add less than / greater than where bounds, causing compilation failure on overflow. This would be my preferred solution:

impl<U> core::convert::From<PInt<U>> for i32
where
    U: Unsigned + NonZero + IsLess<U4294967296>,
    Le<U, U4294967296>: Same<True>,
{
    fn from(_: PInt<U>) -> i32 {
        <PInt<U> as Integer>::I32
    }
}

Edit: Fixed example.

[hgomersall]

That's nice - is the trait bound only invoked when the trait method is used?

[paholg]

That's nice - is the trait bound only invoked when the trait method is used?

Yeah. Think about it; the compiler can only evaluate the bounds for U when it has some concrete U, which is when you're calling the function / using the trait 🙂.

[paholg]

Note my initial example didn't actually work, as that bound was satisfied by all integers. I think there's a cleaner way to do it, but the updated example works.

[paholg]

Sorry I've been absent.

If you'd still like this merged, any thought on implementing std::convert::From instead?

[jerry73204]

std::convert::From needs a instanced value to work. Though the type is already zero sized, the writing is a bit verbose.

let three = i64::from(U3::new());

The .to_int() gives the advantage to get the value in desired integer type without instantiating the value.

let three: i64 = U3::to_int();

[paholg]

But From gives you Into for free:

let three: u64 = U3::new().into()

I guess that's still more verbose, and it may be weird for users to be forced to instantiate.

Okay, you've convinced me.