One option is that you write in a high-level language where the top-level control code is single threaded, but you call APIs that perform multi-threaded operati

One option is that you write in a high-level language where the top-level control code is single threaded, but you call APIs that perform multi-threaded operations seamlessly. The prototypical example of this is Python with numpy/blas (or with deep learning libraries like TensorFlow).