This is Episode 15 of The Vibe Coder’s Guide to the Machine: computer science for a new generation.
Your shop sells one thing, and it costs 1999 cents. Orders arrive from elsewhere as JSON, and you ask an assistant for the small piece of code that turns an order into a total. Such a piece of code is usually called a handler: the code that runs when a message arrives and does whatever the message is for. The assistant looks at the sample order you pasted, {"id": 1001, "qty": 2}, and writes a handler whose working part is one line: take the order’s qty and multiply it by the price. It runs the sample. The answer is 3998, two times 1999, and the feature is done in less time than it took to describe.
In the previous episode, an order came back changed on its way into text and out again. The other direction is what a program does with a message once it arrives. The sample worked because it had the shape the handler assumed. It had a qty field, the field held a whole number, and there was nothing else in the order. Nobody wrote that shape down. It lived in one example and in a line of code that took the example at its word. A shape nobody wrote down has a particular weakness: a message can be perfectly valid JSON and still not be the order the handler was built for.
