three-layer

This package is aimed at being a modern, production-level, batteries-included starting template for writing web servers with Haskell on backend and Elm on frontend. It follows the Three Layer Cake. architecture pattern.

Haskell libraries used in here:

relude: alternative prelude; here base-noprelude trick is used.
co-log: composable contravariant comonadic logging library.
postgresql-simple: mid-level PostgreSQL client library for database interaction.
servant: family of libraries for defining webservices Rest API on type-level.
elm-street: bridge between Elm and Haskell - generating Elm data types, JSON encoders and decoders automatically from Haskell types.
proto-lens: Protobuf messages for integration with the mobile application.
ekg: application performance monitoring.
bcrypt: password hashing functions.
jwt: user authentication via JWT.
hspec and hedgehog: testing libraries.

Detailed approach description

This section contains more detailed description of the chosen architecture and our particular implementation of it.

Application environment

Data type for the runtime environment for the whole application is defined in the Lib/App/Env.hs module. It contains various fields required for the application processing, like database pool, JWT secret, logger, etc. It also has instance of custom Has typeclass which tells how to extract different parts of the application. This is done to achieve the following purposes:

Specify in the constraints what parts of the environment you need.
Introduce more modularity when multiple different environments are implemented.

Environment initialisation is happening in the Lib.hs module.

Application errors

Module Lib/App/Error.hs contains exhaustive list of all errors that application can throw. This module provides convenient layer between human-readable error names and HTTP error codes. It also contains useful utilities for throwing errors and for formatting CallStack of errors.

Application monad

Main application monad can be found in the Lib/App/Monad.hs module.

Database

This template uses PostgreSQL database and contains helper wrappers around functions from the postgresql-simple library to integrate smoother with our own monad. See Lib/Db/Functions.hs for more details.

Effects

All new effects (like sending an email. storing the file, etc.) should be added to the Lib/Effects/ directory.