An example service built on Tagless Final

Medium article

A small example demonstrating how to make use of Tagless Final in order to delay the decision of choosing a concrete effect (like Future or Task) till the end and in order to provide easily swappable implementation of effects.

Here, we delay the implementation of OrderRepository to the "end of the world". The OrderService that depends on the abstract OrderRepository makes a requirement that OrderRepository needs to have an effect that can support sequencing via the Monad typeclass constraint (F[_]: Monad) since it needs to sequence actions (i.e. in order to perform an update, it needs to sequence get and put).

There are three interpreters:

• An in-memory implementation backed by the Id monad
• A Postgres implementation backed by Slick and Monix Task
• A DynamoDB implementation backed by Scanamo and scala Futures

Setup instructions for PostgreSQL

Use docker-compose up to bring up a Dockerized PostgreSQL.

Log in to the default postgres database (username = docker, password = docker)

CREATE DATABASE calvin;
GRANT ALL PRIVILEGES ON DATABASE calvin TO docker;

Log into the calvin database (username = docker, password = docker). Make sure you use calvin.<no-schema>

CREATE SCHEMA IF NOT EXISTS example;

The PostgresApp will automatically take care of creating the table in the provided database and schema.

Setup instructions for DynamoDB

Download DynamoDB local. Start it up using:

java -Djava.library.path=./DynamoDBLocal_lib -jar DynamoDBLocal.jar -sharedDb

Visit the shell at localhost:8000/shell and create the following table:

const params = {
    TableName: 'orders_by_id',
    KeySchema: [
        {
            AttributeName: 'id',
            KeyType: 'HASH'
        }
    ],
    AttributeDefinitions: [
        {
            AttributeName: 'id',
            AttributeType: 'S' // (S | N | B) for string, number, binary
        }
    ],
    ProvisionedThroughput: {
        ReadCapacityUnits: 10,
        WriteCapacityUnits: 10
    }
};
dynamodb.createTable(params, function(err, data) {
    if (err) ppJson(err); // an error occurred
    else ppJson(data); // successful response
});

The DynamoApp will connect to the local database and create records in that table