Episode #11: Composition without Operators

Our series unabashedly promotes the use of custom operators, but they’re far from common in the Swift community. Our use of operators may even be the most controversial aspect of our series! We explored the “why” of operators in our first episode, and we continue to justify the operators we introduce with specific—maybe even rigorous—criteria, and while we may have you convinced, it’s another story to convince your coworkers! Your team may even adopt a style guide that prohibits it!

00:33

Operators shouldn’t be the bottleneck to introducing composition to your code. In this episode we’ll explore a few alternatives that may act as a more gentle introduction to the team and may even be the first step to getting your coworkers first-class tickets on the operator express!

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Exercises

Write concat for functions (inout A) -> Void.

Solution

func concat<A>(
  _ f: @escaping (inout A) -> Void,
  _ g: @escaping (inout A) -> Void,
  _ fs: ((inout A) -> Void)...
  ) -> (inout A) -> Void {

  return { a in
    f(&a)
    g(&a)
    fs.forEach { $0(&a) }
  }
}

Write concat for functions (A) -> A.

Solution

func concat<A>(
  _ f: @escaping (A) -> A,
  _ g: @escaping (A) -> A,
  _ fs: ((A) -> A)...)
  -> (A) -> A {

    return { a in
      ([f, g] + fs).reduce(a, { $1($0) })
    }
}

Write a function called compose for backward composition. Recreate some of the examples from our functional setters episodes (part 1 and part 2) using compose and pipe.

Solution

func compose<A, B, C>(_ f: @escaping (B) -> C, _ g: @escaping (A) -> B) -> (A) -> C {
  return { f(g($0)) }
}

with(
  ((1, true), "Swift"),
  pipe(
    backPipe(first, first)(incr),
    backPipe(first, second)(!),
    second { $0 + "!" }
  )
)

with(
  (42, ["Swift", "Objective-C"]),
  pipe(
    first(incr),
    backPipe(second, map)({ $0.uppercased() })
  )
)

References

Swift Overture

Brandon Williams & Stephen Celis • Monday Apr 9, 2018

We open sourced the Overture library to give everyone access to functional compositions, even if you can’t bring operators into your codebase.

https://github.com/pointfreeco/swift-overture

Downloads

Sample Code

0011-composition-without-operators