Category Theory for Programmers
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Free online book by Bartosz Milewski introducing category theory through Haskell and C++ examples, starting from types and functions and building to functors, natural transformations, adjunctions and monads. Readers learn to recognize categorical structures in everyday functional programming code.
More resources on Functors
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
nLab: Functor
Reference entry from the nLab wiki defining functors between categories, with variants such as contravariant, full, faithful and essentially surjective functors, plus examples and properties. Useful for checking precise definitions and seeing how functors connect to wider higher-category theory.
Category Theory for Programmers
Learn functors and other category theory concepts applied to programming! This course by Bartosz Milewski makes abstract math practical.
nLab
Collaborative wiki for mathematics, physics and philosophy written from a category-theoretic viewpoint, with cross-linked entries on categories, functors, adjunctions, topos theory, higher category theory and homotopy theory. Readers can look up precise definitions, examples and references for advanced structural mathematics.