The young monk Djishin approached master Kaimu and said, “Tell me about the Temple of the White Iron Sky, where you were first apprenticed.”

Kaimu said, “It is a strange and perilous place. Knowledge hangs in the air as thick as the mist, yet if you linger there for an hour you will emerge knowing less than when you went in.”

Djishin thought, I must see how such a thing can be true. Thus he set out.

By his maps, the temple lay nestled in the folds of a forested hill below high mountains; a signpost on the main road pointed to an untended path. Djishin followed the path through a great AND Gate and found that the climbing land beyond was indeed shrouded in mist, billowing in great roils from a grove of weeping-cypress trees further up the hillside. The mist curled around the base of a pale tower at the center the temple grounds. The path—now flagged with damp stones—led Djishin to its steps.

“What is this place, fellow?” he asked a white-clad monk seated upon the steps.

The monk looked up from the Haskell code he was studying. “Outsiders call it the Tower of the Elephant’s Tusk, though we know it by another name. But answer this question: Are you a software engineer?”

“In heart and in deed, I am,” answered Djishin.

“If you would be one in thought as well, then follow the path to the cypress grove, and converse with the nuns that meditate there,” said the white-clad monk. “The tower is not yet for you.”

Within the grove the mist thickened to a warm and bitter-tasting fog; from somewhere up ahead came the sound of bubbling water. The trees parted, and Djishin found himself in a clearing where four nuns in white robes sat contemplating a monolith of glistening black basalt. On its face were inscriptions such as the monk had never seen:

(>>=) :: m a -> (a -> m b) -> m b return :: a -> m a

“What is this stone, great ladies?” asked Djishin.

“We call it the Monad,” said the first nun.

“Why do you venerate it so?” asked Djishin.

“Through it, we may touch the impure without being corrupted,” said the second nun. “We can fell a Maybe-tree with a Maybe-ax and always hear a Maybe-sound when it crashes down—even if the sound is Nothing at all, when the ax isn’t real or there’s no tree to fall.”

“Can you... explain that explanation?” asked Djishin.

“It empowers us to code without error,” said the third nun. “For we only deem a function safe if its input alone determines its output. But when a function bleeds side-effects, then from each drop a vine may spring to pierce our application with its thorns. The Monad tells us how to weave bandages to staunch the flow. We wrap the function up, we bind the Real World down; we pass it in, shake it, and pull it back out, and that’s what it’s all about.”

“And this inscription, this is code?” asked Djishin, wondering at the monolith.

The nuns looked at each other. “Someday all will know it as such, as readily as 20 GOTO 10. But answer this question: are you a software engineer?”

“I am no longer certain,” answered Djishin, whose confusion was complete.

At which point the young monk was enlightened.

Qi’s commentary

Ironically, the Monad always has at least one side-effect: it makes me feel like an idiot.

Qi’s poem

The screen darkens.

Semicolons scatter to the four corners of the window.

A storm may beat the high hill down—

Yet when the world changes, storms yield to a greater enigma.