Storage Machine exercises

Introduction

Storage Machine is a small but realistic and rather complete back-end of a Web-application. It does not include the front-end part so you will use Postman or similar to interact with it.

The goal is to see an example of a complete system made using F# and get acquainted with the Onion architecture. You will do this by example functionalities and (architectural and FP) patterns already implemented in the back-end and extend it with new ones, by applying the same patterns.

Resources

• As always, F# language reference.
• Storage Machine uses Giraffe -- a library for creating Web-applications using F# as a thin layer on top of ASP.NET Core in FP style, unlike heavily OO-based ASP.NET Core model.
• For JSON serialization the Thoth.Json library is used because it provides FP-oriented combinator style of JSON (de)serialization.

Exercise 0 (warming up)

Fill in missing parts to complete the functionality of retrieving an overview of products in stock.

Requirements and hints:

• The Stock component already contains two other complete and working functionalities. Study the corresponding use-case functions binOverview and stockOverview in the Application layer.
• Study how other layers support these two working use-cases.
• Discover how to trigger these use-cases from outside and use Postman to see the results.
• Fill in all "holes" marked with "Exercise 0" required to complete implementation of productsInStock in the Application layer.
• Compare the response in Postman with simulated (hard-coded) data in the Data Access layer.

Exercise A

Extend the Stock component with functionality to store a new bin containing zero or one products.

Requirements and hints:

• Adding a bin should be possible using a POST request.
• After adding a bin, it should be visible in the bin and/or stock overview already implemented in by the same Stock component.
• Extend the data access interface in the Application layer.
• Define the use-case function in the Application layer.
• Is there anything missing in the Model layer or is it complete for this functionality?
• Extend the data access implementation object in the Data Access layer part of the Stock component.
• Make a Decoder for bins.
• Module PostExample in the Service layer contains an example of decoding (simple) JSON values from the body of a POST request and returing various responses.
• Existing functionality in the Stock component contains examples of serializing JSON responses.
• Make an HttpHandler for the handling the POST request and combine it with existing handlers of the Stock component.

Exercise B

Implement "product repacking" functionality in the Repacking component.

Requirements and hints:

• Bins can be nested in other bins, this is already modeled in the Model layer of the Repacking component.
• Products stored in bins ("bin trees") need to get protective packaging. Each individual product needs to be wrapped in a package, but the overall shape of the bin tree must remain the same. Storage Maching can magically mechanically do this, if you implement this repacking in F#.
• The Repacking component contains working functionality for viewing bin trees and counting products in a bin tree. Experiment with these.
• After repacking, the number of products in a bin tree should remain the same. Use this as a sanity check for your repacking implementation.
• Adjust the model to express the packaging requirement.
• Implement repacking as a bin tree transformation function.
• Add the repacking step to an existing use-case function in the Application layer of the Repacking component.

Exercise C

Improve the model of product repacking.

• Current model in module BinTree allows a bin tree to be a single product. Strictly speaking, this does not make sense. Define the type for bin trees such that this is not possible anymore.
• Adjust the model further to make applying packaging to products more than once impossible.