servant-persistent

Servant is an awesome Haskell library for writing web APIs. It uses the type system in a way that can only be described as magic to generate type safe routes as well as clients.

Persistent is another awesome Haskell library for querying databases. It manages migrations, your schema, and querying to make data transactions mostly painless.

For some reason, no one had created an example on how to use these guys together. I put together this minimal example to show an example implementation, along with some resource management and basic error handling.

I wrote a blog post that goes into a bit more detail, as well as having some exercises to work on.

Requirements:

Haskell

You can use stack to get started:

1. stack build
2. stack exec perservant

Alternatively, cabal can be used:

1. cabal sandbox init
2. cabal install --dependencies-only && cabal configure && cabal build
3. cabal run

Database:

You will need PostgreSQL installed and listening on port 5432. The default configuration uses a database name perservant with username/password test:test.

These steps work on Ubuntu:

$ apt install postgres libpq-dev
$ sudo -u postgres createuser -se test
$ sudo -u postgres psql -c "alter role test with password 'test'"
$ sudo -u postgres psql -c "create database perservant"

These following steps worked on Arch Linux:

# install postgres
$ sudo pacman -S postgres

# The installation process should have created the postgres system user for us.
# Become that user in order to initialize the DB.  This is required before
# running the postgres service.
$ sudo -i -u postgres

# As the postgres user, initialize the database.
[postgres]$ initdb --locale en_US.UTF-8 -E UTF8 -D '/var/lib/postgres/data'
# Exit to go back to your normal user.
[postgres]$ exit

# As your normal user start the postgres service.
$ sudo systemctl start postgres.service

# When that starts successfully, then we need to become the postgres system
# user again to create the "test" user and perservant database.
$ sudo -i -u postgres
[postgres]$ createuser --interactive
Enter name of role to add: test
Shall the new role be a superuser? (y/n) y
[postgres]$ createdb perservant -U test
# Exit to go back to your normal user.
[postgres]$ exit

# As your normal user you can log in and play around with the DB:
$ psql -d perservant -U test
psql (9.4.4)
Type "help" for help.

perservant=#

The API:

• GET /users returns a list of all users in the database
• GET /users/:name returns the first user whose name is :name, and returns 404 if the user doesn't show up.
• POST /users with JSON like { "name": "String", "email": "String" } to create a User.

Playing with the API from the command line

Once the compiled perservant binary is running, you can use curl like below to play with the API from the command line.

# create a new user
$ curl --verbose --request POST --header "Content-Type: application/json" \
    --data '{"name": "foo", "email": "[email protected]"}' \
	http://localhost:8081/users

# get all users in database
$ curl --verbose --request GET --header "Content-Type: application/json" \
	http://localhost:8081/users

# get certain user in database
$ curl --verbose --request GET --header "Content-Type: application/json" \
	http://localhost:8081/users/foo

src/Main.hs

main starts off by pulling some settings from the environment, creating a connection pool, running the migrations, and finally running the app.

src/Api.hs

This source contains the actual API definition.

src/Config.hs

Contains the runDb, makePool, and Config definitions.

src/Models.hs

Contains fairly typical Persistent schema definitions.