[Haskell] ANNOUNCE: Groundhog, a database connectivity library

Boris Lykah lykahb at gmail.com
Fri Jun 17 18:31:13 CEST 2011

Hi, everyone! I am happy to announce a new persistence library, Groundhog.


Groundhog does mapping between datatypes and database like Persistent
in Haskell or Hibernate in Java and makes dealing with database very
easy. The API is inspired by Persistent, a great framework by Michael
Snoyman, from which I used some ideas and code. At first I wanted to
enhance Persistent, but many useful ideas don't fit into its design.
Groundhog offers features I wanted to see in a modern Haskell database

My intent was to create a package which you can plug into your
existing project and it would just work. So it does not require
defining datatypes using its  own mechanisms (quasiquotation, XML,
etc). You can use your own types defined anywhere. The restrictions on
type structure are very mild. Groundhog uses data families and GADTs
and they did not play nicely together until GHC 7, so GHC 6.12.x and
earlier is not supported.

Currently there is support only for Sqlite but I hope to add more
backends soon. Sqlite backend is based on direct-sqlite package by Dan
Knapp. I modified it to improve performance and provide better error
messages. Now it is bundled with groundhog-sqlite but I hope to merge
it with direct-sqlite.

On simple datatypes performance is ~2.5 times higher compared to
Persistent. Some of this gain is achieved because of Sqlite specific
optimisations, but I expect to see high performance on other backends
as well. In fact, it could be even faster. I sacrificed ~30% of
backend-independent performance for flexibility when I chose DbPersist
to be a monad transformer instead of sticking with IO and replaced
direct mentions of DbPersist with Monad m in PersistBackend.

* Persists datatypes defined in an ordinary way
* Supports fields of user-defined types
* Supports polymorphic datatypes and datatypes with several constructors
* Basic support for lists and tuples
* Type safety
* Migration from an empty schema
* Powerful expression DSL for use in queries
* Execution of arbitrary queries
* High performance

Plans (in priority order):
* Add more backends, particularly PostgreSQL
* Allow migration when data definition changes
* Add Persistent-like quasiquotation syntax

Here is an example from the documentation:

{-# LANGUAGE GADTs, TypeFamilies, TemplateHaskell #-}
import Control.Monad.IO.Class(liftIO)
import Database.Groundhog.TH
import Database.Groundhog.Sqlite

data Customer a = Customer {customerName :: String, details :: a} deriving Show
data Item = ProductItem {productName :: String, quantity :: Int,
customer :: Customer String}
          | ServiceItem {serviceName :: String, deliveryAddress ::
String, servicePrice :: Int}
     deriving Show

deriveEntity ''Customer $ Just $ do
  setConstructor 'Customer $ do
    setConstraints [("NameConstraint", ["customerName"])]
deriveEntity ''Item Nothing

main = withSqliteConn ":memory:" $ runSqliteConn $ do
  -- Customer is also migrated because Item contains it
  runMigration silentMigrationLogger $ migrate (undefined :: Item)
  let john = Customer "John Doe" "Phone: 01234567"
  johnKey <- insert john
  -- John is inserted only once because of the name constraint
  insert $ ProductItem "Apples" 5 john
  insert $ ProductItem "Melon" 2 john
  insert $ ServiceItem "Taxi" "Elm Street" 50
  insert $ ProductItem "Melon" 6 (Customer "Jack Smith" "Don't let him
pay by check")
  -- bonus melon for all large melon orders
  update [QuantityField =. toArith QuantityField + 1]
(ProductNameField ==. "Melon" &&. QuantityField >. (5 :: Int))
  productsForJohn <- select (CustomerField ==. johnKey) [] 0 0
  liftIO $ putStrLn $ "Products for John: " ++ show productsForJohn
  -- check bonus
  melon <- select (ProductNameField ==. "Melon") [Desc QuantityField] 0 0
  liftIO $ putStrLn $ "Melon orders: " ++ show melon

Currently Hackage cannot build it due to technical issues (as Dons
assumed several packages it depends on are not exposed), but you can
install it with cabal.

It is still very early beta and it may have some bugs. I am very
interested to hear your feedback.

Boris Lykah

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