Implement new Concept Exercise: options

This issue describes how to implement the options concept exercise for the F# 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 up on the following documents:

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

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

The goal of this exercise is to teach the student the basics of the Concept of Options in F#.

Learning objectives

• Know of the existing of the Option<T> type and when to use it.
• Know how to define an option value.
• Know how to use pattern matching with options.
• Know of some basic functions in the Option module (like Option.map) to modify options.
• Know how options differ from null.
• Know what generic types are.

Out of scope

• Custom computation expression to deal with options.
• Custom operators to deal with options applicatively (like <*>).
• Converting to and from other types (like arrays of lists).

Concepts

The Concepts this exercise unlocks are:

• options: know of the existing of the Option<T> type and when to use it; know how to define an optional value; know of some basic functions in the Option module (like Option.map) to modify options; know how options differ from null; know how to use pattern matching with options.
• generic-types: know what generic types are.

Prerequisites

This exercise's prerequisites Concepts are:

• discriminated-unions: know that an option is implemented as a DU and know how to do pattern matching on a DU.
• higher-order-functions: how to use higher-order functions.

Any data types used in this exercise (e.g. strings) should also be added as prerequisites.

Resources to refer to

Hints

• Options: shows how options in F# work and how to use them.
• Options introduction: nice introduction to using options.

After

• Options: shows how options in F# work and how to use them.
• Options introduction: nice introduction to using options.
• C# null vs F# Option: introdution to options in F#, but from the viewpoint of a C# user.

Representer

This exercise does not require any specific representation logic to be added to the representer.

Analyzer

This exercise does not require any specific logic to be added to the analyzer.

Implementing

To implement this exercise, please follow these instructions.

Help

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