Composable Randomness

Episode #30 • Sep 17, 2018 • Subscriber-Only

Randomness is a topic that may not seem so functional, but it gives us a wonderful opportunity to explore composition. After a survey of what randomness looks like in Swift today, we’ll build a complex set of random APIs from just a single unit.

Composable Randomness
Randomness in Swift: numbers
Randomness in Swift: elements
Randomness in Swift: more
Rethinking randomness
What’s the point?

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Today we’re going to start a series of episodes on a topic that might not seem very functional: randomness.

Swift 4.2 introduces much-needed library support for randomness, which was designed from the ground up to be easier and safer to use than the existing available APIs. We’re going to take a look at some of the problems it was designed to solve and explore how we may have solved them in an alternate, functional API rooted in composition.

We’ll start by taking quick look at the original problems.

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  1. Create a function called frequency that takes an array of pairs, [(Int, Gen<A>)], to create a Gen<A> such that (2, gen) is twice as likely to be run than a (1, gen).

  2. Extend Gen with an optional computed property that returns a generator that returns nil a quarter of the time. What other generators can you compose this from?

  3. Extend Gen with a filter method that returns a generator that filters out random entries that don’t match the predicate. What kinds of problems may this function have?

  4. Create a string generator of type Gen<String> that randomly produces a randomly-sized string of any unicode character. What smaller generators do you composed it from?

  5. Redefine element(of:) to work with any Collection. Can it also be redefined in terms of Sequence?

  6. Create a subsequence generator to return a randomly-sized, randomly-offset subsequence of an array. Can it be redefined in terms of Collection?

  7. The Gen type has map defined it, which, as we’ve seen in the past, allows us to consider what zip might look like. Define zip2 on Gen:

    func zip2<A, B>(_ ga: Gen<A>, _ gb: Gen<B>) -> Gen<(A, B)>
  8. Define zip2(with:):

    func zip2<A, B, C>(with f: (A, B) -> C) -> (Gen<A>, Gen<B>) -> Gen<C>
  9. With zip2 and zip2(with:) defined, define higher-order zip3 and zip3(with:) and explore some uses. What functionality does zip provide our Gen type?


Random Zalgo Generator

Brandon Williams • Tuesday Nov 20, 2018

We apply the ideas of composable randomness to build a random Zalgo generator, which is a way to apply gitchy artifacts to a string by adding strange unicode characters to it. It shows that we can start with very simple, small pieces and then compose them together to create a really complicated machine.