Piggy

Piggy is a tiny little Haskell library that makes Database.PostgreSQL.Simple just a tiny bit easier to work with. It also facilitates easier unit testing.

It uses the Operational monad and a simple typeclass:

class Monad m => MonadPG m where
  interpret :: PGDSL a -> m a
  withTransaction :: m a -> m a
  withTransaction t = t

A default interpreter — interpg — that talks to a Postgres database is provided, as is PG, a default implementation of MonadPG that actually talks to a database using interpg.

instance MonadPG PG where 
  interpret = interpg
  withTransaction transact = withPostgresTransaction $ flip withPG transact 

In unit tests, it's relatively straightforward to replace actual database access with stubs.

getPatients :: MonadPG m => m [Patient]
getPatients = interpret $ singleton (Query_ "SELECT * FROM patients")

patients :: [Patient]
-- etc.

interstubs :: Monad m => PGDSL a -> m a
interstubs m = case view m of
  (Query_ "SELECT * FROM patients") :>>= k -> return patients >>= interstubs . k
  -- etc.

newtype PGStub a = PGStub a deriving (Functor, Applicative, Monad)

-- If `getPatients` is called with `PGStub`, it will be interpreted with
-- interstubs and no actual database access will occur. In fact, the
-- function is actually pure because it does not require `MonadIO`.
instance MonadPG PGStub where
  interpret = interstubs