Replacing Loops with Mapped Ranges

During the last weeks, I tried to use map() even when the conversion wasn’t straightforward to learn how it affects reading code. I found that some applications of map() were dumb while I liked how others turned out to read. The functional programming paradigm is about writing code in a declarative way, which has its benefits.

Take this, for example:

extension Int {    
    func times<T>(f: () -> T) -> [T] {
        var result = [T]()
        for _ in 0..<self {
        return result

10.times { "foo" }
// => ["foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo", "foo"]

Looping over the range works well and is the way of the future to use for-loops from A to B, but a few months ago I thought this was clever already.

The method has 3 parts:

  1. Declare a mutable array
  2. Use (mutate) the array
  3. Return the array

When I see mutable arrays used like this, I see map, though:

extension Int {    
    func times<T>(f: () -> T) -> [T] {
        return (0..<self).map { _ in f() }

The result is a bit more terse. But unlike the mutable array from above, you do not wonder what will happen to the array, why it is mutable (var instead of let) etc. Instead, you see that there’s a range and that each element in the sequence is transformed (“mapped”) to something else.

The return (0..<self).map part already tells most of the story: you can expect an array with as many elements as self indicates. To guess what the contents will look like, you’ll have to take a look at the block.

Compared this to the example from above. There, you knew an array is involved. Then there was a loop. Then the array was changed with every iteration. You had to read 3/4 of the method body to fathom what the result will be.

I am afraid to go too far down the rabbit hole of functional thinking, thus rendering my code harder to comprehend. This example is not that bad I think. But when I prefer mapping the N elements in a range to produce an array of size N, I think I already lost some people in my team. What’s idiomatic Swift and thus to be expected from co-workers and what’s too much?

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