An adjunction is a way to relate two objects a and c, but not directly: C(a,c) Instead, we get two maps, (L)eft and (R)ight, which allow to relate them: C(L(a),c) A(a,R(c)) 1/n The means that there's a pair of arrows, one in each direction, that these two hom-sets are isomorphic. The first example given is currying: a function in two arguments is equivalent to a function of one argument returning another function in the remaining argument. 2/n We can write this like so: C(a✕b,c) C(a,c^b) where L = _✕b, and R = _^b.

In my previous blog post, Programming with Universal Constructions, I mentioned in passing that one-to-one mappings between sets of morphisms are often a manifestation of adjunctions between functo…

The notion of an adjunction. Definition via unit and counit natural transformations and the triangle identities.