flatmap_and_compactmap_combined.swift

/*
Given an array with optional sub-arrays, how can I flatten the array?

Example:

Given [[1, 2], nil, [3]]
Expect [1, 2, 3]

Solution:

First compact-map, then flat-map.
*/

let array = [[1, 2], nil, [3]]
let flattened = array
   .compactMap { $0 } // Remove nil sub-arrays
   .flatMap { $0 } // Flatten the sub-arrays

/*
Note:

If I try to do the flat-map first, Swift thinks I'm mistakenly using `flatMap` where I meant to use `compactMap`;
`flatMap` then removes the nil sub-arrays, and the subsequent `compactMap` has no effect.
*/