Basic Category Theory
《Basic Category Theory》 · 第 21 页 · Definition 1.2.1
A functor \(F:\mathcal A\to\mathcal B\) assigns an object \(F(A)\) to each object \(A\) and a map \(F(f):F(A)\to F(A')\) to each map \(f:A\to A'\), preserving composition and identities.