  Question Answering (QA) is a longstanding challenge in natural language
processing. Existing QA works mostly focus on specific question types,
knowledge domains, or reasoning skills. The specialty in QA research hinders
systems from modeling commonalities between tasks and generalization for wider
applications. To address this issue, we present ProQA, a unified QA paradigm
that solves various tasks through a single model. ProQA takes a unified
structural prompt as the bridge and improves the QA-centric ability by
structural prompt-based pre-training. Through a structurally designed
prompt-based input schema, ProQA concurrently models the knowledge
generalization for all QA tasks while keeping the knowledge customization for
every specific QA task. Furthermore, ProQA is pre-trained with structural
prompt-formatted large-scale synthesized corpus, which empowers the model with
the commonly-required QA ability. Experimental results on 11 QA benchmarks
demonstrate that ProQA consistently boosts performance on both full data
fine-tuning, few-shot learning, and zero-shot testing scenarios. Furthermore,
ProQA exhibits strong ability in both continual learning and transfer learning
by taking the advantages of the structural prompt.
