The following parable is borrowed from Call-by-value is dual to call-by-name by Philip Wadler.

Once upon a time, the devil approached a man and made an offer: “Either (a) I will give you one billion dollars, or (b) I will grant you any wish if you pay me one billion dollars. Of course, I get to choose whether I offer (a) or (b).” The man was wary. Did he need to sign over his soul? No, said the devil, all the man need do is accept the offer. The man pondered. If he was offered (b) it was unlikely that he would ever be able to buy the wish, but what was the harm in having the opportunity available? “I accept,” said the man at last. “Do I get (a) or (b)?” The devil paused. “I choose (b).”

The man was disappointed but not surprised. That was that, he thought. But the offer gnawed at him. Imagine what he could do with his wish! Many years passed, and the man began to accumulate money. To get the money he sometimes did bad things, and dimly he realized that this must be what the devil had in mind.

Eventually he had his billion dollars, and the devil appeared again. “Here is a billion dollars,” said the man, handing over a valise containing the money. “Grant me my wish!” The devil took possession of the valise. Then he said, “Oh, did I say (b) before? I’m so sorry. I meant (a). It is my great pleasure to give you one billion dollars.” And the devil handed back to the man the same valise that the man had just handed to him.


  • Philip Wadler. 2003. Call-by-value is dual to call-by-name. doi