Returning custom error from Vec binary search

Doordashcon · January 11, 2022, 4:37pm

How do I return a custom error from Vec binary search when the item is not present?

let who = ["account 1", "account 2", "account 3", "account 4", "account 5", "account 6"];
let insert = who.binary_search(&who).err().ok_or(Error::<T>::AlreadyVetoed)?;

error message

 cannot use the `?` operator in a method that returns `()`

alice · January 11, 2022, 4:45pm

You can't return an error in a function that doesn't have a Result as its return value.

Doordashcon · January 11, 2022, 4:47pm

It was done here

github.com

Doordashcon/substrate/blob/b54c21f354fd973e6a35a53c37b317ae293dbfc8/frame/democracy/src/lib.rs#L877

    
      
          
          
			if let Some((e_proposal_hash, _)) = <NextExternal<T>>::get() {
          				ensure!(proposal_hash == e_proposal_hash, Error::<T>::ProposalMissing);
          			} else {
          				Err(Error::<T>::NoProposal)?;
          			}
          
          
			let mut existing_vetoers =
          				<Blacklist<T>>::get(&proposal_hash).map(|pair| pair.1).unwrap_or_else(Vec::new);
          			let insert_position =
          				existing_vetoers.binary_search(&who).err().ok_or(Error::<T>::AlreadyVetoed)?;
          
          
			existing_vetoers.insert(insert_position, who.clone());
          			let until = <frame_system::Pallet<T>>::block_number() + T::CooloffPeriod::get();
          			<Blacklist<T>>::insert(&proposal_hash, (until, existing_vetoers));
          
          
			Self::deposit_event(Event::<T>::Vetoed { who, proposal_hash, until });
          			<NextExternal<T>>::kill();
          			Ok(())
          		}

Cerber-Ursi · January 11, 2022, 5:25pm

The function you've linked to returns DispatchResult, which is defined here as std::result::Result<(), DispatchError>.

tuffy · January 11, 2022, 5:30pm

Slice's binary search returns a Result<usize, usize> with the error being where in the slice a new entry might fit. So wouldn't it be possible to use Result::map_err to transform it into your desired custom error?

kornel · January 12, 2022, 1:29am

Are you sure it's the right code? The error message you've quoted doesn't seem to apply to the code you've linked to.

Check if you don't have the code inside a closure. That's a common source of this error, e.g.:

fn fallible(…) -> Result<…> {
   foo.map(|x| bar(x)?);
   …
}

won't work, even if foo is in a function returning result, because the bar call is in a closure that has its own return type.