rudihorn/haskell-lenses

Demo Schema

The examples contain a simple schema for the example tables from the Bohannon PODS 2016 paper located in schema.sql. These are two tables albums and tracks. By default the database expects a database called links with user links and password links.

Running examples

The easiest way to run examples is to use ghci. Calling ./run.sh will start GHCI with the required extensions needed for lenses. The example code is located in Scratch.hs and can be loaded using the :l Scratch pragma.

Example functions

test_get : lens -> records queries a lens.

test_put : lens -> records -> unit updates a lens with the supplied records.

test_put_debug : lens -> records -> unit returns how it would update the database, but does not actually make changes.

Filling database

The database can be filled with information

test_put albums unchangedAlbums
test_put tracks unchangedTracks

PODS example

The PODS example is defined with the following lenses and primitive tables:

type Albums = '[ '("album", String), '("quantity", Int)]

albums = prim @"albums" @Albums
  @'[ '["album"] --> '["quantity"]]

type Tracks = '[
  '("track", String),
  '("date", Int),
  '("rating", Int),
  '("album", String)]

tracks = prim @"tracks" @Tracks
  @'[ '["track"] --> '["date", "rating"]]

tracks1 = join tracks albums
tracks2 = dropl @'[ '("date", 'P.Int 2020)] @'["track"] tracks1
tracks3 = select (#quantity !> di 2) tracks2

The view can be fetched using:

test_get tracks3

And the example update can be performed by:

test_put tracks3 examplePut