Selda

What is Selda?

Selda is a Haskell library for interacting with SQL-based relational databases. It was inspired by LINQ and Opaleye.

Features

Monadic interface.
Portable: backends for SQLite and PostgreSQL.
Generic: easy integration with your existing Haskell types.
Creating, dropping and querying tables using type-safe database schemas.
Typed query language with products, filtering, joins and aggregation.
Inserting, updating and deleting rows from tables.
Conditional insert/update.
Transactions, uniqueness constraints and foreign keys.
Type-safe, backend-specific functionality, such as JSON lookups.
Seamless prepared statements.
Lightweight and modular: few dependencies, and non-essential features are optional or split into add-on packages.

Getting started

Install the selda package from Hackage, as well as at least one of the backends:

$ cabal update
$ cabal install selda selda-sqlite selda-postgresql

Then, read the tutorial. The API documentation will probably also come in handy.

Requirements

Selda requires GHC 8.0+, as well as SQLite 3.7.11+ or PostgreSQL 9.4+. To build the SQLite backend, you need a C compiler installed. To build the PostgreSQL backend, you need the libpq development libraries installed (libpq-dev on Debian-based Linux distributions).

Hacking

Contributing

All forms of contributions are welcome!

If you have a bug to report, please try to include as much information as possible, preferably including:

A brief description (one or two sentences) of the bug.
The version of Selda+backend where the bug was found.
A step-by-step guide to reproduce the bug.
The expected result from following these steps.
What actually happens when following the steps.
Which component contains the bug (selda, selda-sqlite or selda-postgresql), if you're reasonably sure about where the bug is.

Bonus points for a small code example that illustrates the problem.

If you want to contribute code, please consult the following checklist before sending a pull request:

Does the code build with a recent version of GHC?
Do all the tests pass?
Have you added any tests covering your code?

If you want to contribute code but don't really know where to begin, issues tagged good first issue are a good start.

Setting up the build environment

From the repository root:

Install libpq-dev from your package manager. This is required to build the PostgreSQL backend.
Make sure you're running a cabal version that supports v2-style commands.
Familiarise yourself with the various targets in the makefile. The dependencies between Selda, the backends and the tests are slightly complex, so straight-up cabal is too quirky for day to day hacking.

PostgreSQL backend testing with Docker

To test the PostgreSQL backend, use the provided pgtest-compose.yml docker-compose file:

sudo docker-compose -f pgtest-compose.yml up -d
make pgtest
sudo docker-compose -f pgtest-compose.yml down

TODOs

Features that would be nice to have but are not yet implemented.

Monadic if/else
Streaming
MySQL/MariaDB backend
MSSQL backend

jchia / selda