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Dragging proofs out of pictures. (English) Zbl 1343.18007

Lindley, Sam (ed.) et al., A list of successes that can change the world. Essays dedicated to Philip Wadler on the occasion of his 60th birthday. Cham: Springer (ISBN 978-3-319-30935-4/pbk; 978-3-319-30936-1/ebook). Lecture Notes in Computer Science 9600, 152-168 (2016).
Summary: String diagrams provide category theory with a different and very distinctive visual flavour. We demonstrate that they are an effective tool for equational reasoning using a variety of examples evolving around the composition of monads. A deductive approach is followed, discovering necessary concepts as we go along. In particular, we show that the Yang-Baxter equation arises naturally when composing three monads. We are also concerned with the pragmatics of string diagrams. Diagrams are carefully arranged to provide geometric intution for the proof steps being followed. We introduce thick wires to distinguish composite functors and suggest a two-dimensional analogue of parenthesis to partition diagrams.
For the entire collection see [Zbl 1333.68014].

MSC:

18C15 Monads (= standard construction, triple or triad), algebras for monads, homology and derived functors for monads
18A10 Graphs, diagram schemes, precategories
18A15 Foundations, relations to logic and deductive systems
18D10 Monoidal, symmetric monoidal and braided categories (MSC2010)
Full Text: DOI

References:

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