system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are monads in programming?
gpt
That's easy. Imagine that you have a large pizza delivery organization. There are many people who are trained to pack pizza boxes, transport them, etc,etc, there are many stages of delivery. And all this works perfectly. Imagine also that all this happens in a strange world where people can only pay in cash. They leave their money on the counter in the morning and get their orders delivered in the evening to their houses. And there are no phones.

Suddenly you encounter a problem: sometimes there is no pizza available. In this case you need to return the money. You can build an alternative structure with people transporting money back to customers, but it can be expensive and troublesome. Eventually you find a brilliant solution. You just put money into an empty pizza box along with the note explaining the problem. Your delivery system continues to operate as usual but now it has an additional functionality packed into the same boxes.

Of course it comes at additional cost for you. At least you are sending empty boxes every time you need to return the money.  Also some people in your delivery network may need to be retrained, for example there could be some additional weighting station that checks the weight of each box. You need to tell them that there could be unusually light boxes.