Why folks use <> for same-type composition

19 Jul 2020 ⇐ Notes archive

(This is an entry in my technical notebook. There will likely be typos, mistakes, or wider logical leaps — the intent here is to “let others look over my shoulder while I figure things out.”)

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A wondering I had back when I started studying FP in earnest was why folks used <> for same-type composition instead of the more general >>> operator (which stitches (A) -> Bs with (B) -> Cs).

And yesterday while re-watching Brandon Williams’ “Monoids, predicates and sorting functions” talk, I found a likely answer (timestamped link).

Functions in the form (A) -> A — endomorphisms — come with a semigroup (and monoidal) structure and the diamond operator is a nod to that. The operator is subtly interchangeable with >>> because Endo’s¹ conformance leans on function composition under the hood.

(Gist permalink.)

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