Unable to use `Compact` on generic `T: HasCompact`

[kianenigma]

The below code demonstrates the problem. It is an attempt at doing a custom Encode implementation of a complex type which takes advantage of the fact that the internals of the complex type are all compact-encodable.

use parity_scale_codec::{Encode, Decode, HasCompact, Compact, CompactAs};

#[derive(Default)]
struct Solution<V, T, W> {
	votes1: Vec<(V, T)>,
	votes2: Vec<(V, (T, W), T)>,
	votes3: Vec<(V, [(T, W); 2usize], T)>,
}

#[derive(Default)]
struct BareSolution {
	votes1: Vec<(u32, u16)>,
	votes2: Vec<(u32, (u16, u64), u16)>,
	votes3: Vec<(u32, [(u16, u64); 2usize], u16)>,
}

// This works fine
impl Encode for BareSolution {
	fn encode(&self) -> Vec<u8> {
		let mut r = vec![];
		self.votes1
			.iter()
			.map(|(v, t)| (<Compact<u32>>::from(*v), <Compact<u16>>::from(*t)))
			.for_each(|(v, t)| {
				v.encode_to(&mut r);
				t.encode_to(&mut r);
			});
		r
	}
}

// This will not work.
impl<V: HasCompact + CompactAs + Copy, T: HasCompact + CompactAs + Copy, W: HasCompact + CompactAs + Copy> Encode for Solution<V, T, W> {
	fn encode(&self) -> Vec<u8> {
		let mut r = vec![];
		self.votes1
			.iter()
			.map(|(v, t)| (Compact::from(*v), Compact::from(*t)))
			.for_each(|(v, t)| {
				// will not work because the compiler fails to identify `impl Encode` for `V`
				// v.encode_to(&mut r);
				// will trigger recursive derive error.
				// <Compact<T> as Encode>::encode_to(&t, &mut r);
			});
		r
	}
}

---

Looking at the code, it seems to be that <Comact<T> as Encode> should work fine as long as T: HasCompact + CompactAs, based on:

parity-scale-codec/src/compact.rs

Lines 108 to 111 in a54a3dd

	impl<T> Encode for Compact<T>
	where
	for<'a> CompactRef<'a, T>: Encode,
	{

and

parity-scale-codec/src/compact.rs

Lines 135 to 138 in a54a3dd

	impl<'a, T> Encode for CompactRef<'a, T>
	where
	T: CompactAs,
	for<'b> CompactRef<'b, T::As>: Encode,

Albeit I might be missing something.

[gui1117]

You can use HasCompact without requiring other traits:

#[derive(Default)]
struct Solution<V, T> {
	votes1: Vec<(V, T)>,
}

impl<V: HasCompact, T: HasCompact> Encode for Solution<V, T> {
	fn encode(&self) -> Vec<u8> {
		let mut r = vec![];
		self.votes1
			.iter()
			.for_each(|(v, t)| {
				let compact_v = <<V as HasCompact>::Type as EncodeAsRef<'_, V>>::RefType::from(v);
				let compact_t = <<T as HasCompact>::Type as EncodeAsRef<'_, T>>::RefType::from(t);
				compact_v.encode_to(&mut r);
				compact_t.encode_to(&mut r);
			});
		r
	}
}

#[test]
fn test_solution() {
	let s = Solution {
		votes1: vec![(0u128, 0u128)]
	};
	assert_eq!(s.encode(), (Compact::<u128>(0u128), Compact::<u128>(0u128)).encode());
}

[gui1117]

I know traits are confusing but HasCompact can be used as is, the associated type Type is the compact type corresponding,
I guess this answers the issue

[kianenigma]

I actually came to this, but EncodeAsRef is not public and thus cannot be used. EncodeAsRef is pub, but it is not re-exported from the main crate lib file.

Perhaps we either export this, or some other easier way in the upcoming release.

[kianenigma]

Alternatively I was thinking, why not expose a fn compact_encode() in trait HasCompact? I would exactly do what you said, and is basically just a wrapper around Encode::encode, but much easier to use.

[gui1117]

Alternatively I was thinking, why not expose a fn compact_encode() in trait HasCompact? I would exactly do what you said, and is basically just a wrapper around Encode::encode, but much easier to use.

I think this would need more reorganization in the trait itself. Because the function compact_encode in HasCompact would be expected to be same logic as HasCompact::Type::EncodeAsRef.

I actually came to this, but EncodeAsRef is not public and thus cannot be used. EncodeAsRef is pub, but it is not re-exported from the main crate lib file.

Perhaps we either export this, or some other easier way in the upcoming release.

Maybe it has been changed since then, but as far as I see EncodeAsRef is public and reexported https://substrate.dev/rustdocs/v2.0.0-rc5/parity_scale_codec/trait.EncodeAsRef.html

[gui1117]

Actually the above example #218 (comment) compiles to me when I use parity-scale-codec has dependency, I think we can close this issue no ?

[kianenigma]

yeah your examples worked fine for me as well. It is just a bit unergonomic.