[v3] Implement new Concept Exercise: filter and compactMap

[wneumann]

This issue describes how to implement the filter concept exercise for the Swift track.

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read the following documents:

• The features of v3.
• Rationale for v3.
• What are concept exercises and how are they structured?

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

This concept exercise should convey a basic understanding of how to filter in Swift.

Learning Objectives

• What is filter?
• Sequence.filter
• What is compactMap?

Out of Scope

• Other higher-order functions

Concepts

• filter

Prerequisites

• map

Resources to refer to

Hints

• Sequence.filter
• Sequence.compactMap

After

• Sequence.filter
• Sequence.compactMap

Representer

TBD

Analyzer

TBD

Implementing

See the Swift track implementation guide for guidance.

Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

[wneumann]

Considering splitting out compactMap from map and merging it into this and making map a prerequisite, as compactMap is essentially a lazy map composed with a filter for non-nil values.

[wneumann]

Adding compactMap to this exercise.