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https://github.com/Holmusk/three-layer

:three: :cake: Architecture of the Haskell web applications
https://github.com/Holmusk/three-layer

backend haskell servant three-layer-architecture web web-application

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:three: :cake: Architecture of the Haskell web applications

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# 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](http://www.parsonsmatt.org/2018/03/22/three_layer_haskell_cake.html).

architecture pattern.



Haskell libraries used in here:

* [`relude`](https://github.com/kowainik/relude): alternative prelude; here

  `base-noprelude` trick is used.

* [`co-log`](https://github.com/kowainik/co-log):

  [composable contravariant comonadic logging library](https://kowainik.github.io/posts/2018-09-25-co-log).

* [`postgresql-simple`](http://hackage.haskell.org/package/postgresql-simple):

  mid-level PostgreSQL client library for database interaction.

* [`servant`](http://hackage.haskell.org/package/servant): family of libraries

  for defining webservices Rest API on type-level.

* [`elm-street`](https://github.com/Holmusk/elm-street): bridge between Elm and

  Haskell - generating Elm data types, JSON encoders and decoders automatically

  from Haskell types.

* [`proto-lens`](http://hackage.haskell.org/package/proto-lens): Protobuf

  messages for integration with the mobile application.

* [`ekg`](http://hackage.haskell.org/package/ekg): application performance monitoring.

* [`bcrypt`](http://hackage.haskell.org/package/bcrypt): password hashing functions.

* [`jwt`](http://hackage.haskell.org/package/jwt): user authentication via JWT.

* [`hspec`](http://hackage.haskell.org/package/hspec) and [`hedgehog`](http://hackage.haskell.org/package/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`](src/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:



1. Specify in the constraints what parts of the environment you need.

2. Introduce more modularity when multiple different environments are implemented.



Environment initialisation is happening in the [`Lib.hs`](src/Lib.hs) module.



### Application errors



Module [`Lib/App/Error.hs`](src/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`](src/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`](src/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/`](src/Lib/Effects) directory.