The comprehension construction

In this paper we construct an analogue of Lurie’s “unstraightening” construction that we refer to as the comprehension construction. Its input is a cocartesian fibration p: E ↠ B between ∞-categories together with a third ∞-category A. The comprehension construction then defines a map from the quasi-category of functors from A to B to the large quasi-category of cocartesian fibrations over A that acts on f : A → B by forming the pullback of p along f. To illustrate the versatility of this construction, we define the covariant and contravariant Yoneda embeddings as special cases of the comprehension functor. We then prove that the hom-wise action of the comprehension functor coincides with an “external action” of the hom-spaces of B on the fibres of p and use this to prove that the Yoneda embedding is fully faithful, providing an explicit equivalence between a quasi-category and the homotopy coherent nerve of a Kan-complex enriched category.