Persistent problems in LTS-11.15+

a-persistent.hs

#!/usr/bin/env stack
-- stack script --resolver lts-10.8
-- vim: ft=haskell
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}

module Main where

import ClassyPrelude

import Database.Persist
import Database.Persist.TH
import Database.Persist.Sql

share [mkPersist sqlSettings, mkMigrate "migration"] [persistLowerCase|
User
  name Text
|]

getUsers :: MonadIO m => SqlReadT m [Entity User]
getUsers = selectList [] []

main :: IO ()
main = undefined

b-persistent-11.15.hs

#!/usr/bin/env stack
-- stack script --resolver lts-11.15
-- vim: ft=haskell
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}

module Main where

import ClassyPrelude

import Database.Persist
import Database.Persist.TH
import Database.Persist.Sql

share [mkPersist sqlSettings, mkMigrate "migration"] [persistLowerCase|
User
  name Text
|]

-- • Couldn't match type ‘BaseBackend backend’ with ‘SqlBackend’
--     arising from a use of ‘selectList’
-- • In the expression: selectList [] []
--   In an equation for ‘getUsers’: getUsers = selectList [] []
--
--    |
-- 27 | getUsers = selectList [] []
--    |            ^^^^^^^^^^^^^^^^
--
-- getUsers :: MonadIO m => SqlReadT m [Entity User]
-- getUsers = selectList [] []

-- The lts-11.15 definitions:
--
-- type SqlBackendCanRead backend
--   = ( BackendCompatible SqlBackend backend <-- TAKE NOTE
--     , PersistQueryRead backend
--     , PersistStoreRead backend
--     , PersistUniqueRead backend
--     )

-- type SqlReadT m a
--   = forall backend. SqlBackendCanRead' backend
--   => ReaderT backend m a

-- However, if I do this:
--
type SqlBackendCanRead' backend
  = ( BaseBackend backend ~ SqlBackend -- <-- Only difference
    , PersistQueryRead backend
    , PersistStoreRead backend
    , PersistUniqueRead backend
    )

type SqlReadT' m a
  = forall backend. SqlBackendCanRead' backend
  => ReaderT backend m a

-- And now this works!
getUsers :: MonadIO m => SqlReadT' m [Entity User]
getUsers = selectList [] []

main :: IO ()
main = undefined