For years the young monk Djishin had been apprenticed to old master Banzen. Although this was a privilege it brought Djishin little joy, for lately the master had been growing visibly irritated with the young monk. Yet the monk could not fathom the reason.

One morning Djishin said to his fellow apprentice Satou: I have committed Banzen’s teachings to heart and perform my duties according to his example, yet his disdain for me is palpable! And whenever I ask if I have done anything to incur his displeasure, he tells me No! I am at my wit’s end with the old man!

Satou replied: If you can no longer march uphill, then the way forward is not in front of you.

Djishin said: I cannot quit his service any more than I could bite my own legs off. Yet my patience has waned to the thinnest sliver. Soon, I fear, the master will push me too far, and I will surely rebel.

Satou replied: Then run from the path and don’t look back; for I cannot see your rebellion ending well for anyone, except perhaps the undertaker.

Not long after, Banzen was reviewing Satou’s code and noted that she was now using Java’s new Optional class practically everywhere. Banzen scolded the nun for her usual habit of playing with novelties instead of focusing her efforts on building a reliable implementation with customary patterns.

Overhearing this exchange, Djishin felt the last dregs of patience drain from his body. The monk spat: Optional is no passing fad; it is Java’s implementation of the Maybe monad, and any fool who truly cares about reliable implementations would do well to follow the nun’s example!

There followed an awful silence. No one moved. Djishin could feel his heart pounding in his ears.

Finally the master arched a bushy white eyebrow, saying: And what—my impertinent, former apprentice—is a Monad?

With a shaky voice, Djishin explained the nature of monads and how he had learned of them. When he finished, Banzen strode over to the monk, raised his staff, and flung both arms around the boy.

At last! exclaimed the master. I have been wondering whether I’d trained a monk or a mockingbird. Now I can say to the other masters: on this day I learned two things, for at last Djishin told me something that I did not know.