Bar.hs

{-# LANGUAGE
    EmptyDataDecls
  , FlexibleContexts
  , GADTs
  , OverloadedStrings
  , QuasiQuotes
  , TemplateHaskell
  , TypeFamilies
  #-}
module Bar where

import Database.Persist.TH
import Database.Persist

import Settings
import qualified Foo as F
share [mkPersist mySettings, mkMigrate "migrateAll"] [persistLowerCase|
Bar
  fooId F.FooId
|]

error message

Bar.hs:17:1:
    Could not deduce (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
                        F.FooId)
      arising from a use of `persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType'
    from the context (monad-control-0.3.2.1:Control.Monad.Trans.Control.MonadBaseControl
                        IO m,
                      transformers-0.3.0.0:Control.Monad.IO.Class.MonadIO m,
                      monad-logger-0.3.2.0:Control.Monad.Logger.MonadLogger m)
      bound by the type signature for
                 migrateAll :: (monad-control-0.3.2.1:Control.Monad.Trans.Control.MonadBaseControl
                                  IO m,
                                transformers-0.3.0.0:Control.Monad.IO.Class.MonadIO m,
                                monad-logger-0.3.2.0:Control.Monad.Logger.MonadLogger m) =>
                               persistent-1.2.3.0:Database.Persist.Sql.Types.Migration
                                 (persistent-1.2.3.0:Database.Persist.Sql.Types.SqlPersistT m)
      at Bar.hs:(17,1)-(20,2)
    Possible fix:
      add (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
             F.FooId) to the context of
        the type signature for
          migrateAll :: (monad-control-0.3.2.1:Control.Monad.Trans.Control.MonadBaseControl
                           IO m,
                         transformers-0.3.0.0:Control.Monad.IO.Class.MonadIO m,
                         monad-logger-0.3.2.0:Control.Monad.Logger.MonadLogger m) =>
                        persistent-1.2.3.0:Database.Persist.Sql.Types.Migration
                          (persistent-1.2.3.0:Database.Persist.Sql.Types.SqlPersistT m)
      or add an instance declaration for
         (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
            F.FooId)
    In the fourth argument of `FieldDef', namely
      `persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
         (Nothing :: Maybe F.FooId)'
    In the expression:
      FieldDef
        (HaskellName (Database.Persist.TH.pack' "fooId"))
        (DBName (Database.Persist.TH.pack' "foo_id"))
        (FTTypeCon
           (Just (Database.Persist.TH.pack' "F"))
           (Database.Persist.TH.pack' "FooId"))
        (persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
           (Nothing :: Maybe F.FooId))
        []
        True
        Nothing
    In the fifth argument of `EntityDef', namely
      `[FieldDef
          (HaskellName (Database.Persist.TH.pack' "fooId"))
          (DBName (Database.Persist.TH.pack' "foo_id"))
          (FTTypeCon
             (Just (Database.Persist.TH.pack' "F"))
             (Database.Persist.TH.pack' "FooId"))
          (persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
             (Nothing :: Maybe F.FooId))
          []
          True
          Nothing]'

Bar.hs:17:1:
    Could not deduce (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
                        F.FooId)
      arising from a use of `persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType'
    from the context (typ ~ F.FooId)
      bound by a pattern with constructor
                 FooId :: forall typ. typ ~ F.FooId => EntityField Bar typ,
               in an equation for `persistFieldDef'
      at Bar.hs:(17,1)-(20,2)
    Possible fix:
      add (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
             F.FooId) to the context of
        the data constructor `FooId'
        or the type signature for
             persistFieldDef :: EntityField Bar typ -> FieldDef SqlType
      or add an instance declaration for
         (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
            F.FooId)
    In the fourth argument of `FieldDef', namely
      `persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
         (Nothing :: Maybe F.FooId)'
    In the expression:
      FieldDef
        (HaskellName (Database.Persist.TH.pack' "fooId"))
        (DBName (Database.Persist.TH.pack' "foo_id"))
        (FTTypeCon
           (Just (Database.Persist.TH.pack' "F"))
           (Database.Persist.TH.pack' "FooId"))
        (persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
           (Nothing :: Maybe F.FooId))
        []
        True
        Nothing
    In an equation for `persistFieldDef':
        persistFieldDef FooId
          = FieldDef
              (HaskellName (Database.Persist.TH.pack' "fooId"))
              (DBName (Database.Persist.TH.pack' "foo_id"))
              (FTTypeCon
                 (Just (Database.Persist.TH.pack' "F"))
                 (Database.Persist.TH.pack' "FooId"))
              (persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
                 (Nothing :: Maybe F.FooId))
              []
              True
              Nothing

Bar.hs:17:1:
    Could not deduce (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
                        F.FooId)
      arising from a use of `persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType'
    from the context (Monad m)
      bound by the type signature for
                 entityDef :: Monad m => m Bar -> EntityDef SqlType
      at Bar.hs:(17,1)-(20,2)
    Possible fix:
      add (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
             F.FooId) to the context of
        the type signature for
          entityDef :: Monad m => m Bar -> EntityDef SqlType
      or add an instance declaration for
         (persistent-1.2.3.0:Database.Persist.Sql.Class.PersistFieldSql
            F.FooId)
    In the fourth argument of `FieldDef', namely
      `persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
         (Nothing :: Maybe F.FooId)'
    In the expression:
      FieldDef
        (HaskellName (Database.Persist.TH.pack' "fooId"))
        (DBName (Database.Persist.TH.pack' "foo_id"))
        (FTTypeCon
           (Just (Database.Persist.TH.pack' "F"))
           (Database.Persist.TH.pack' "FooId"))
        (persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
           (Nothing :: Maybe F.FooId))
        []
        True
        Nothing
    In the fifth argument of `EntityDef', namely
      `[FieldDef
          (HaskellName (Database.Persist.TH.pack' "fooId"))
          (DBName (Database.Persist.TH.pack' "foo_id"))
          (FTTypeCon
             (Just (Database.Persist.TH.pack' "F"))
             (Database.Persist.TH.pack' "FooId"))
          (persistent-1.2.3.0:Database.Persist.Sql.Class.sqlType
             (Nothing :: Maybe F.FooId))
          []
          True
          Nothing]'

Foo.hs

  {-# LANGUAGE
    EmptyDataDecls
  , FlexibleContexts
  , GADTs
  , OverloadedStrings
  , QuasiQuotes
  , TemplateHaskell
  , TypeFamilies
  #-}
module Foo where

import Database.Persist.TH
import Settings

share [mkPersist mySettings, mkMigrate "migrateAll"] [persistLowerCase|
Foo
|]

Settings.hs

module Settings where

import Database.Persist.TH

mySettings :: MkPersistSettings
mySettings = sqlSettings { mpsGeneric = False, mpsPrefixFields = False }