Why do miniKanren names always end with `o`?

Question

All miniKanren relations end with the letter o. What is the motivation for this? I see that the Clojure core.logic library also does this.

Answer 1

In the Preface of The Reasoned Schemer, they explain it thus:

A relation, a function that returns a goal as its value, ends its name with a superscript 'o' (e.g., car^o and null^o).

So, it's a notation to denote a relation.

Answer 2

It's because the authors of The Reasoned Schemer wanted the notation of miniKanren relations to be evocative of ordinary Scheme predicates which end in ? (e.g., null?, pair?) by convention. The superscript ^o, if you squint enough (and have heard this story before), looks like a modified ?.