Sorry, I copied the autofill.. Here's the definition...
func get(request: RESTRequest, listener: RESTOperation.ResultListener?) -> RESTOperation
And here's "RestOperation.ResultListener...
open class AmplifyOperation<Request: AmplifyOperationRequest, Success, Failure: AmplifyError>: AsynchronousOperation {
/// The concrete Request associated with this operation
public typealias Request = Request
/// The concrete Success type associated with this operation
public typealias Success = Success
/// The concrete Failure type associated with this operation
public typealias Failure = Failure
/// Convenience typealias defining the `Result`s dispatched by this operation
public typealias OperationResult = Result<Success, Failure>
/// Convenience typealias for the `listener` callback submitted during Operation creation
public typealias ResultListener = (OperationResult) -> Void
Hoping that helps clarify things? I didn't post all the code as it's not mine...