Endo Semigroup conformance.

endo-semigroup.swift

infix operator <>: SemigroupPrecedence
precedencegroup SemigroupPrecedence {
	higherThan: MultiplicationPrecedence
	associativity: left
}

protocol Semigroup {
	static func <> (lhs: Self, rhs: Self) -> Self
}

struct Endo<A> {
	let call: (A) -> A
	func callAsFunction(_ a: A) -> A { call(a) }
}

extension Endo: Semigroup {
	static func <> (lhs: Endo<A>, rhs: Endo<A>) -> Endo<A> {
		Endo { rhs(lhs($0)) } // You might see folks use
		// `lhs.call >>> rhs.call` here à la Point-Free’s Prelude:
		// https://github.com/pointfreeco/swift-prelude/blob/0ec0a56bb99911647d2081f99a6f6c9699a6a646/Sources/Prelude/Endo.swift#L9-L13
	}
}