newtype SqlPersistT

[parsonsmatt]

SqlPersistT is currently:

type SqlPersistT = ReaderT SqlBackend

It will become:

newtype SqlPersistT m a = SqlPersistT
  { unsafeRunSqlPersistT :: ReaderT SqlBackend m a
  }

This gives us the opportunity to provide better documentation and error messages for the type, along with better instances.

[parsonsmatt]

Unfortunately, another problem with this is that it's relatively common to refer to the backend abstractly - BackendCompatible SqlBackend backend => ReaderT backend m a is a relatively common thing to see.

newtype SqlPersistT backend m a = SqlPersistT { unSqlPersistT :: ReaderT backend m a } seems like a slightly odd thing to have.