The Legend of Selda

Haskell has many database libraries, and there's a recent trend in leveraging the type system to giving increasingly safe database access. Despite the goofy name, Selda is a very robust database DSL, and works equally well for PostgreSQL or SQLite.

The flip side of having so much power in a database library is that the types can get pretty out of hand sometimes. This is a trade-off that we make in a number of places throughout the application, and is highly beneficial 98% of the time. Selda uses a lot of constraint aliases to merge multiple constraints into one. GHC or Intero will give you hints with the underlying constraints. We'll touch of a few things to keep in mind below.

MonadSelda

Fission.Storage.Types
type SeldaPool = Database.Pool (SeldaConnection SQLite)
​
newtype Pool = Pool { getPool :: SeldaPool }
  deriving Show

Fission.Internal.Orphanage
instance Has DB.Pool cfg => MonadSelda (RIO cfg) where
 type Backend (RIO cfg) = SQLite
​
 withConnection action = do
    DB.Pool pool <- Config.get
    withResource pool action