Hasql Database Migration Algorithm

gistfile1.hs

-- |
-- Migrate from any version to any other higher version.
migrate :: (Word, Word) -> EitherT Text (Tx s) ()
migrate =
  \case
    (0, 1) -> -- Execute statements, which create the DB in initial state.
    (1, 2) -> -- Execute statements, which alter the DB to migrate from version 1 to 2.
    (2, 3) -> -- Execute statements, which alter the DB to migrate from version 2 to 3.
    (3, 4) -> throwError $ "Trying to migrate to an inexistent version"
    (from, to) 
      | from == to -> return ()
      | from < to  -> migrate (from, succ from) >> migrate (succ from, to)
      | from > to  -> throwError $ "Trying to migrate to an older version"