When you submit a query, mutation or a subscription to your GraphQL server one of the following things might happen:
  1. the request was successful,
  2. your request encountered an error,
  3. your request encountered an error but could still produce a result.
In both the first and the third case, your GraphQL server is going to conform to your schema - certain fields may have null values instead of the expected data, but the shape of the response will match the expected one. In the second case, on the other hand, GraphQL encountered a “terminating error” meaning that it could not produce a result that would conform to your schema (e.g. a non-nullable field was null). In this case, GraphQL will not return a result at all. Since SwiftGraphQL uses the schema as the single source of truth, handling the first and the third case is easy. Handling the second case, however, requires some additional work.

​
Operation Result Errors vs Throwables

Because Swift is a compiled, statically typed language, we need to be explicit about every type conversion happening in our code. To make sure developer experience doesn’t suffer, SwiftGraphQL produces a terminating error when it encounters a diviation from the schema and produces a DecodedOperationResult in other cases. 
let query = Selection.Query<String> {
	try $0.hello()
}

// General result callback.
client.query(query)
	.sink { completion in
		switch (completion) {
		case .failure(let error):
			// Handle a terminating error.
		case .finished:
			()
		}

	} receiveValue: { (result: DecodedOperationResult) in
		// Handle a successful or partial result.
	}
You can read more about error handling in The Swift Programming Language.

​
Combined Errors

CombinedError pattern is heavily inspired by urql client. I suggest you check their documentation for any further explanations.
When we use a GraphQL API there are two kinds of errors we may encounter: Network Errors and GraphQL Errors from the API. Since it’s common to encounter either of them, there’s a CombinedError class that can hold and abstract either.
It’s worth noting that an error can coexist and be returned in a successful request alongside data. This is because in GraphQL a query can have partially failed but still contain some data.