    {-# LANGUAGE DeriveDataTypeable
               , EmptyDataDecls
               , FlexibleContexts
               , FlexibleInstances
               , FunctionalDependencies
               , MultiParamTypeClasses
               , TypeFamilies
               , UndecidableInstances
               , GADTs
     #-}
    -- | This is an internal module, anything exported by this module
    -- may change without a major version bump.  Please use only
    -- "Database.Esqueleto" if possible.
    module Database.Esqueleto.Internal.Language
      ( -- * The pretty face
        Esqueleto(..)
      , from
      , Value(..)
      , unValue
      , ValueList(..)
      , SomeValue(..)
      , ToSomeValues(..)
      , InnerJoin(..)
      , CrossJoin(..)
      , LeftOuterJoin(..)
      , RightOuterJoin(..)
      , FullOuterJoin(..)
      , OnClauseWithoutMatchingJoinException(..)
      , OrderBy
      , DistinctOn
      , Update
      , Insertion
      , LockingKind(..)
      , SqlString
        -- * The guts
      , JoinKind(..)
      , IsJoinKind(..)
      , PreprocessedFrom
      , From
      , FromPreprocess
      , when_
      , then_
      , else_
      ) where

    import Control.Applicative (Applicative(..), (<$>))
    import Control.Exception (Exception)
    import Data.Int (Int64)
    import Data.Typeable (Typeable)
    import Database.Esqueleto.Internal.PersistentImport
    import Text.Blaze.Html (Html)

    import qualified Data.ByteString as B
    import qualified Data.ByteString.Lazy as BL
    import qualified Data.Text as T
    import qualified Data.Text.Lazy as TL

    -- | Finally tagless representation of @esqueleto@'s EDSL.
    class (Functor query, Applicative query, Monad query) =>
          Esqueleto query expr backend | query -> expr backend, expr -> query backend where
      -- | (Internal) Start a 'from' query with an entity. 'from'
      -- does two kinds of magic using 'fromStart', 'fromJoin' and
      -- 'fromFinish':
      --
      --   1.  The simple but tedious magic of allowing tuples to be
      --   used.
      --
      --   2.  The more advanced magic of creating @JOIN@s.  The
      --   @JOIN@ is processed from right to left.  The rightmost
      --   entity of the @JOIN@ is created with 'fromStart'.  Each
      --   @JOIN@ step is then translated into a call to 'fromJoin'.
      --   In the end, 'fromFinish' is called to materialize the
      --   @JOIN@.
      fromStart
        :: ( PersistEntity a
           , PersistEntityBackend a ~ backend )
        => query (expr (PreprocessedFrom (expr (Entity a))))
      -- | (Internal) Same as 'fromStart', but entity may be missing.
      fromStartMaybe
        :: ( PersistEntity a
           , PersistEntityBackend a ~ backend )
        => query (expr (PreprocessedFrom (expr (Maybe (Entity a)))))
      -- | (Internal) Do a @JOIN@.
      fromJoin
        :: IsJoinKind join
        => expr (PreprocessedFrom a)
        -> expr (PreprocessedFrom b)
        -> query (expr (PreprocessedFrom (join a b)))
      -- | (Internal) Finish a @JOIN@.
      fromFinish
        :: expr (PreprocessedFrom a)
        -> query a

      -- | @WHERE@ clause: restrict the query's result.
      where_ :: expr (Value Bool) -> query ()

      -- | @ON@ clause: restrict the a @JOIN@'s result.  The @ON@
      -- clause will be applied to the /last/ @JOIN@ that does not
      -- have an @ON@ clause yet.  If there are no @JOIN@s without
      -- @ON@ clauses (either because you didn't do any @JOIN@, or
      -- because all @JOIN@s already have their own @ON@ clauses), a
      -- runtime exception 'OnClauseWithoutMatchingJoinException' is
      -- thrown.  @ON@ clauses are optional when doing @JOIN@s.
      --
      -- On the simple case of doing just one @JOIN@, for example
      --
      -- @
      -- select $
      -- 'from' $ \\(foo `'InnerJoin`` bar) -> do
      --   'on' (foo '^.' FooId '==.' bar '^.' BarFooId)
      --   ...
      -- @
      --
      -- there's no ambiguity and the rules above just mean that
      -- you're allowed to call 'on' only once (as in SQL).  If you
      -- have many joins, then the 'on's are applied on the /reverse/
      -- order that the @JOIN@s appear.  For example:
      --
      -- @
      -- select $
      -- 'from' $ \\(foo `'InnerJoin`` bar `'InnerJoin`` baz) -> do
      --   'on' (baz '^.' BazId '==.' bar '^.' BarBazId)
      --   'on' (foo '^.' FooId '==.' bar '^.' BarFooId)
      --   ...
      -- @
      --
      -- The order is /reversed/ in order to improve composability.
      -- For example, consider @query1@ and @query2@ below:
      --
      -- @
      -- let query1 =
      --       'from' $ \\(foo `'InnerJoin`` bar) -> do
      --         'on' (foo '^.' FooId '==.' bar '^.' BarFooId)
      --     query2 =
      --       'from' $ \\(mbaz `'LeftOuterJoin`` quux) -> do
      --         return (mbaz '?.' BazName, quux)
      --     test1 =      (,) \<$\> query1 \<*\> query2
      --     test2 = flip (,) \<$\> query2 \<*\> query1
      -- @
      --
      -- If the order was /not/ reversed, then @test2@ would be
      -- broken: @query1@'s 'on' would refer to @query2@'s
      -- 'LeftOuterJoin'.
      on :: expr (Value Bool) -> query ()

      -- | @GROUP BY@ clause. You can enclose multiple columns
      -- in a tuple.
      --
      -- @
      -- select $ 'from' \\(foo `'InnerJoin`` bar) -> do
      --   'on' (foo '^.' FooBarId '==.' bar '^.' BarId)
      --   'groupBy' (bar '^.' BarId, bar '^.' BarName)
      --   return (bar '^.' BarId, bar '^.' BarName, countRows)
      -- @
      --
      -- With groupBy you can sort by aggregate functions, like so
      -- (we used @let@ to restrict the more general 'countRows' to
      -- @SqlExpr (Value Int)@ to avoid ambiguity---the second use of
      -- 'countRows' has its type restricted by the @:: Int@ below):
      --
      -- @
      -- r \<- select $ 'from' \\(foo `'InnerJoin`` bar) -> do
      --   'on' (foo '^.' FooBarId '==.' bar '^.' BarId)
      --   'groupBy' $ bar '^.' BarName
      --   let countRows' = 'countRows'
      --   'orderBy' ['asc' countRows']
      --   return (bar '^.' BarName, countRows')
      -- forM_ r $ \\('Value' name, 'Value' count) -> do
      --   print name
      --   print (count :: Int)
      -- @
      groupBy :: (ToSomeValues expr a) => a -> query ()

      -- | @ORDER BY@ clause. See also 'asc' and 'desc'.
      --
      -- Multiple calls to 'orderBy' get concatenated on the final
      -- query, including 'distinctOnOrderBy'.
      orderBy :: [expr OrderBy] -> query ()

      -- | Ascending order of this field or expression.
      asc :: PersistField a => expr (Value a) -> expr OrderBy

      -- | Descending order of this field or expression.
      desc :: PersistField a => expr (Value a) -> expr OrderBy

      -- | @LIMIT@.  Limit the number of returned rows.
      limit :: Int64 -> query ()

      -- | @OFFSET@.  Usually used with 'limit'.
      offset :: Int64 -> query ()

      -- | @DISTINCT@.  Change the current @SELECT@ into @SELECT
      -- DISTINCT@.  For example:
      --
      -- @
      -- select $ distinct $
      --   'from' \\foo -> do
      --   ...
      -- @
      --
      -- Note that this also has the same effect:
      --
      -- @
      -- select $
      --   'from' \\foo -> do
      --   distinct (return ())
      --   ...
      -- @
      --
      -- /Since: 2.2.4/
      distinct :: query a -> query a

      -- | @DISTINCT ON@.  Change the current @SELECT@ into
      -- @SELECT DISTINCT ON (expressions)@.  For example:
      --
      -- @
      -- select $
      --   'from' \\foo ->
      --   'distinctOn' ['don' (foo ^. FooName), 'don' (foo ^. FooState)] $ do
      --   ...
      -- @
      --
      -- You can also chain different calls to 'distinctOn'.  The
      -- above is equivalent to:
      --
      -- @
      -- select $
      --   'from' \\foo ->
      --   'distinctOn' ['don' (foo ^. FooName)] $
      --   'distinctOn' ['don' (foo ^. FooState)] $ do
      --   ...
      -- @
      --
      -- Each call to 'distinctOn' adds more expressions.  Calls to
      -- 'distinctOn' override any calls to 'distinct'.
      --
      -- Note that PostgreSQL requires the expressions on @DISTINCT
      -- ON@ to be the first ones to appear on a @ORDER BY@.  This is
      -- not managed automatically by esqueleto, keeping its spirit
      -- of trying to be close to raw SQL.
      --
      -- Supported by PostgreSQL only.
      --
      -- /Since: 2.2.4/
      distinctOn :: [expr DistinctOn] -> query a -> query a

      -- | Erase an expression's type so that it's suitable to
      -- be used by 'distinctOn'.
      --
      -- /Since: 2.2.4/
      don :: expr (Value a) -> expr DistinctOn

      -- | A convenience function that calls both 'distinctOn' and
      -- 'orderBy'.  In other words,
      --
      -- @
      -- 'distinctOnOrderBy' [asc foo, desc bar, desc quux] $ do
      --   ...
      -- @
      --
      -- is the same as:
      --
      -- @
      -- 'distinctOn' [don foo, don  bar, don  quux] $ do
      --   'orderBy'  [asc foo, desc bar, desc quux]
      --   ...
      -- @
      --
      -- /Since: 2.2.4/
      distinctOnOrderBy :: [expr OrderBy] -> query a -> query a

      -- | @ORDER BY random()@ clause.
      --
      -- /Since: 1.3.10/
      rand :: expr OrderBy

      -- | @HAVING@.
      --
      -- /Since: 1.2.2/
      having :: expr (Value Bool) -> query ()

      -- | Add a locking clause to the query.  Please read
      -- 'LockingKind' documentation and your RDBMS manual.
      --
      -- If multiple calls to 'locking' are made on the same query,
      -- the last one is used.
      --
      -- /Since: 2.2.7/
      locking :: LockingKind -> query ()

      -- | Execute a subquery @SELECT@ in an expression.  Returns a
      -- simple value so should be used only when the @SELECT@ query
      -- is guaranteed to return just one row.
      sub_select :: PersistField a => query (expr (Value a)) -> expr (Value a)

      -- | Same as 'sub_select' but using @SELECT DISTINCT@.
      sub_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (Value a)

      -- | Project a field of an entity.
      (^.) :: (PersistEntity val, PersistField typ) =>
              expr (Entity val) -> EntityField val typ -> expr (Value typ)

      -- | Project a field of an entity that may be null.
      (?.) :: (PersistEntity val, PersistField typ) =>
              expr (Maybe (Entity val)) -> EntityField val typ -> expr (Value (Maybe typ))

      -- | Lift a constant value from Haskell-land to the query.
      val  :: PersistField typ => typ -> expr (Value typ)

      -- | @IS NULL@ comparison.
      isNothing :: PersistField typ => expr (Value (Maybe typ)) -> expr (Value Bool)

      -- | Analogous to 'Just', promotes a value of type @typ@ into
      -- one of type @Maybe typ@.  It should hold that @'val' . Just
      -- === just . 'val'@.
      just :: expr (Value typ) -> expr (Value (Maybe typ))

      -- | @NULL@ value.
      nothing :: expr (Value (Maybe typ))

      -- | Join nested 'Maybe's in a 'Value' into one. This is useful when
      -- calling aggregate functions on nullable fields.
      joinV :: expr (Value (Maybe (Maybe typ))) -> expr (Value (Maybe typ))

      -- | @COUNT(*)@ value.
      countRows :: Num a => expr (Value a)

      -- | @COUNT@.
      count :: Num a => expr (Value typ) -> expr (Value a)

      -- | @COUNT(DISTINCT x)@.
      --
      -- /Since: 2.4.1/
      countDistinct :: Num a => expr (Value typ) -> expr (Value a)

      not_ :: expr (Value Bool) -> expr (Value Bool)

      (==.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
      (>=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
      (>.)  :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
      (<=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
      (<.)  :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
      (!=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

      (&&.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)
      (||.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)

      (+.)  :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
      (-.)  :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
      (/.)  :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
      (*.)  :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)

      random_  :: (PersistField a, Num a) => expr (Value a)
      round_   :: (PersistField a, Num a, PersistField b, Num b) => expr (Value a) -> expr (Value b)
      ceiling_ :: (PersistField a, Num a, PersistField b, Num b) => expr (Value a) -> expr (Value b)
      floor_   :: (PersistField a, Num a, PersistField b, Num b) => expr (Value a) -> expr (Value b)

      sum_     :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value (Maybe b))
      min_     :: (PersistField a) => expr (Value a) -> expr (Value (Maybe a))
      max_     :: (PersistField a) => expr (Value a) -> expr (Value (Maybe a))
      avg_     :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value (Maybe b))

      -- | Allow a number of one type to be used as one of another
      -- type via an implicit cast.  An explicit cast is not made,
      -- this function changes only the types on the Haskell side.
      --
      -- /Caveat/: Trying to use @castNum@ from @Double@ to @Int@
      -- will not result in an integer, the original fractional
      -- number will still be used!  Use 'round_', 'ceiling_' or
      -- 'floor_' instead.
      --
      -- /Safety/: This operation is mostly safe due to the 'Num'
      -- constraint between the types and the fact that RDBMSs
      -- usually allow numbers of different types to be used
      -- interchangeably.  However, there may still be issues with
      -- the query not being accepted by the RDBMS or @persistent@
      -- not being able to parse it.
      --
      -- /Since: 2.2.9/
      castNum :: (Num a, Num b) => expr (Value a) -> expr (Value b)
      -- | Same as 'castNum', but for nullable values.
      --
      -- /Since: 2.2.9/
      castNumM :: (Num a, Num b) => expr (Value (Maybe a)) -> expr (Value (Maybe b))

      -- | @COALESCE@ function. Evaluates the arguments in order and
      -- returns the value of the first non-NULL expression, or NULL
      -- (Nothing) otherwise. Some RDBMSs (such as SQLite) require
      -- at least two arguments; please refer to the appropriate
      -- documentation.
      --
      -- /Since: 1.4.3/
      coalesce :: PersistField a => [expr (Value (Maybe a))] -> expr (Value (Maybe a))

      -- | Like @coalesce@, but takes a non-nullable expression
      -- placed at the end of the expression list, which guarantees
      -- a non-NULL result.
      --
      -- /Since: 1.4.3/
      coalesceDefault :: PersistField a => [expr (Value (Maybe a))] -> expr (Value a) -> expr (Value a)

      -- | @LOWER@ function.
      lower_ :: SqlString s => expr (Value s) -> expr (Value s)
      -- | @LIKE@ operator.
      like :: SqlString s => expr (Value s) -> expr (Value s) -> expr (Value Bool)
      -- | @ILIKE@ operator (case-insensitive @LIKE@).
      --
      -- Supported by PostgreSQL only.
      --
      -- /Since: 2.2.3/
      ilike :: SqlString s => expr (Value s) -> expr (Value s) -> expr (Value Bool)
      -- | The string @'%'@.  May be useful while using 'like' and
      -- concatenation ('concat_' or '++.', depending on your
      -- database).  Note that you always to type the parenthesis,
      -- for example:
      --
      -- @
      -- name `'like`` (%) ++. 'val' \"John\" ++. (%)
      -- @
      (%) :: SqlString s => expr (Value s)
      -- | The @CONCAT@ function with a variable number of
      -- parameters.  Supported by MySQL and PostgreSQL.
      concat_ :: SqlString s => [expr (Value s)] -> expr (Value s)
      -- | The @||@ string concatenation operator (named after
      -- Haskell's '++' in order to avoid naming clash with '||.').
      -- Supported by SQLite and PostgreSQL.
      (++.) :: SqlString s => expr (Value s) -> expr (Value s) -> expr (Value s)
      -- | Cast a string type into 'Text'.  This function
      -- is very useful if you want to use @newtype@s, or if you want
      -- to apply functions such as 'like' to strings of different
      -- types.
      --
      -- /Safety:/ This is a slightly unsafe function, especially if
      -- you have defined your own instances of 'SqlString'.  Also,
      -- since 'Maybe' is an instance of 'SqlString', it's possible
      -- to turn a nullable value into a non-nullable one.  Avoid
      -- using this function if possible.
      castString :: (SqlString s, SqlString r) => expr (Value s) -> expr (Value r)

      -- | Execute a subquery @SELECT@ in an expression.  Returns a
      -- list of values.
      subList_select :: PersistField a => query (expr (Value a)) -> expr (ValueList a)

      -- | Same as 'sublist_select' but using @SELECT DISTINCT@.
      subList_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (ValueList a)

      -- | Lift a list of constant value from Haskell-land to the query.
      valList :: PersistField typ => [typ] -> expr (ValueList typ)

      -- | Same as 'just' but for 'ValueList'.  Most of the time you
      -- won't need it, though, because you can use 'just' from
      -- inside 'subList_select' or 'Just' from inside 'valList'.
      --
      -- /Since: 2.2.12/
      justList :: expr (ValueList typ) -> expr (ValueList (Maybe typ))

      -- | @IN@ operator.
      in_ :: PersistField typ => expr (Value typ) -> expr (ValueList typ) -> expr (Value Bool)

      -- | @NOT IN@ operator.
      notIn :: PersistField typ => expr (Value typ) -> expr (ValueList typ) -> expr (Value Bool)

      -- | @EXISTS@ operator.  For example:
      --
      -- @
      -- select $
      -- 'from' $ \\person -> do
      -- 'where_' $ 'exists' $
      --          'from' $ \\post -> do
      --          'where_' (post '^.' BlogPostAuthorId '==.' person '^.' PersonId)
      -- return person
      -- @
      exists :: query () -> expr (Value Bool)

      -- | @NOT EXISTS@ operator.
      notExists :: query () -> expr (Value Bool)

      -- | @SET@ clause used on @UPDATE@s.  Note that while it's not
      -- a type error to use this function on a @SELECT@, it will
      -- most certainly result in a runtime error.
      set :: PersistEntity val => expr (Entity val) -> [expr (Update val)] -> query ()

      (=.)  :: (PersistEntity val, PersistField typ) => EntityField val typ -> expr (Value typ) -> expr (Update val)
      (+=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
      (-=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
      (*=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
      (/=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)

      -- | Apply a 'PersistField' constructor to @expr Value@ arguments.
      (<#) :: (a -> b) -> expr (Value a) -> expr (Insertion b)

      -- | Apply extra @expr Value@ arguments to a 'PersistField' constructor
      (<&>) :: expr (Insertion (a -> b)) -> expr (Value a) -> expr (Insertion b)

      -- | @CASE@ statement.  For example:
      --
      -- @
      -- select $
      -- return $
      -- 'case_'
      --    [ 'when_'
      --        ('exists' $
      --        'from' $ \\p -> do
      --        'where_' (p '^.' PersonName '==.' 'val' \"Mike\"))
      --      'then_'
      --        ('sub_select' $
      --        'from' $ \\v -> do
      --        let sub =
      --                'from' $ \\c -> do
      --                'where_' (c '^.' PersonName '==.' 'val' \"Mike\")
      --                return (c '^.' PersonFavNum)
      --        'where_' (v '^.' PersonFavNum >. 'sub_select' sub)
      --        return $ 'count' (v '^.' PersonName) +. 'val' (1 :: Int)) ]
      --    ('else_' $ 'val' (-1))
      -- @
      --
      -- This query is a bit complicated, but basically it checks if a person
      -- named @\"Mike\"@ exists, and if that person does, run the subquery to find
      -- out how many people have a ranking (by Fav Num) higher than @\"Mike\"@.
      --
      -- __NOTE:__ There are a few things to be aware about this statement.
      --
      --    * This only implements the full CASE statement, it does not
      --      implement the \"simple\" CASE statement.
      --
      --
      --    * At least one 'when_' and 'then_' is mandatory otherwise it will
      --      emit an error.
      --
      --
      --    * The 'else_' is also mandatory, unlike the SQL statement in which
      --      if the @ELSE@ is omitted it will return a @NULL@. You can
      --      reproduce this via 'nothing'.
      --
      -- /Since: 2.1.2/
      case_ :: PersistField a => [(expr (Value Bool), expr (Value a))] -> expr (Value a) -> expr (Value a)

    {-# DEPRECATED sub_selectDistinct "Since 2.2.4: use 'sub_select' and 'distinct'." #-}
    {-# DEPRECATED subList_selectDistinct "Since 2.2.4: use 'subList_select' and 'distinct'." #-}

    -- Fixity declarations
    infixl 9 ^.
    infixl 7 *., /.
    infixl 6 +., -.
    infixr 5 ++.
    infix  4 ==., >=., >., <=., <., !=.
    infixr 3 &&., =., +=., -=., *=., /=.
    infixr 2 ||., `like`, `ilike`
    infixl 2 `InnerJoin`, `CrossJoin`, `LeftOuterJoin`, `RightOuterJoin`, `FullOuterJoin`

    -- | Syntax sugar for 'case_'.
    --
    -- /Since: 2.1.2/
    when_ :: expr (Value Bool) -> () -> expr a -> (expr (Value Bool), expr a)
    when_ cond _ expr = (cond, expr)

    -- | Syntax sugar for 'case_'.
    --
    -- /Since: 2.1.2/
    then_ :: ()
    then_ = ()

    -- | Syntax sugar for 'case_'.
    --
    -- /Since: 2.1.2/
    else_ :: expr a -> expr a
    else_ = id

    -- | A single value (as opposed to a whole entity).  You may use
    -- @('^.')@ or @('?.')@ to get a 'Value' from an 'Entity'.
    data Value a = Value a deriving (Eq, Ord, Show, Typeable)
    -- Note: because of GHC bug #6124 we use @data@ instead of @newtype@.
    -- <https://ghc.haskell.org/trac/ghc/ticket/6124>

    -- | /Since: 1.4.4/
    instance Functor Value where
      fmap f (Value a) = Value (f a)

    -- | Unwrap a 'Value'.
    --
    -- /Since: 1.4.1/
    unValue :: Value a -> a
    unValue (Value a) = a

    -- | A list of single values.  There's a limited set of functions
    -- able to work with this data type (such as 'subList_select',
    -- 'valList', 'in_' and 'exists').
    data ValueList a = ValueList a deriving (Eq, Ord, Show, Typeable)
    -- Note: because of GHC bug #6124 we use @data@ instead of @newtype@.
    -- <https://ghc.haskell.org/trac/ghc/ticket/6124>

    -- | A wrapper type for for any @expr (Value a)@ for all a.
    data SomeValue expr where
      SomeValue :: Esqueleto query expr backend => expr (Value a) -> SomeValue expr

    -- | A class of things that can be converted into a list of SomeValue. It has
    -- instances for tuples and is the reason why 'groupBy' can take tuples, like
    -- @'groupBy' (foo '^.' FooId, foo '^.' FooName, foo '^.' FooType)@.
    class ToSomeValues expr a where
      toSomeValues :: a -> [SomeValue expr]

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             ) => ToSomeValues expr (a, b) where
      toSomeValues (a,b) = toSomeValues a ++ toSomeValues b

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             , ToSomeValues expr c
             ) => ToSomeValues expr (a, b, c) where
      toSomeValues (a,b,c) = toSomeValues a ++ toSomeValues b ++ toSomeValues c

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             , ToSomeValues expr c
             , ToSomeValues expr d
             ) => ToSomeValues expr (a, b, c, d) where
      toSomeValues (a,b,c,d) = toSomeValues a ++ toSomeValues b ++ toSomeValues c ++
        toSomeValues d

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             , ToSomeValues expr c
             , ToSomeValues expr d
             , ToSomeValues expr e
             ) => ToSomeValues expr (a, b, c, d, e) where
      toSomeValues (a,b,c,d,e) = toSomeValues a ++ toSomeValues b ++
        toSomeValues c ++ toSomeValues d ++ toSomeValues e

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             , ToSomeValues expr c
             , ToSomeValues expr d
             , ToSomeValues expr e
             , ToSomeValues expr f
             ) => ToSomeValues expr (a, b, c, d, e, f) where
      toSomeValues (a,b,c,d,e,f) = toSomeValues a ++ toSomeValues b ++
        toSomeValues c ++ toSomeValues d ++ toSomeValues e ++ toSomeValues f

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             , ToSomeValues expr c
             , ToSomeValues expr d
             , ToSomeValues expr e
             , ToSomeValues expr f
             , ToSomeValues expr g
             ) => ToSomeValues expr (a, b, c, d, e, f, g) where
      toSomeValues (a,b,c,d,e,f,g) = toSomeValues a ++ toSomeValues b ++
        toSomeValues c ++ toSomeValues d ++ toSomeValues e ++ toSomeValues f ++
        toSomeValues g

    instance ( ToSomeValues expr a
             , ToSomeValues expr b
             , ToSomeValues expr c
             , ToSomeValues expr d
             , ToSomeValues expr e
             , ToSomeValues expr f
             , ToSomeValues expr g
             , ToSomeValues expr h
             ) => ToSomeValues expr (a, b, c, d, e, f, g, h) where
      toSomeValues (a,b,c,d,e,f,g,h) = toSomeValues a ++ toSomeValues b ++
        toSomeValues c ++ toSomeValues d ++ toSomeValues e ++ toSomeValues f ++
        toSomeValues g ++ toSomeValues h

    -- | Data type that represents an @INNER JOIN@ (see 'LeftOuterJoin' for an example).
    data InnerJoin a b = a `InnerJoin` b

    -- | Data type that represents a @CROSS JOIN@ (see 'LeftOuterJoin' for an example).
    data CrossJoin a b = a `CrossJoin` b

    -- | Data type that represents a @LEFT OUTER JOIN@. For example,
    --
    -- @
    -- select $
    -- 'from' $ \\(person `'LeftOuterJoin`` pet) ->
    --   ...
    -- @
    --
    -- is translated into
    --
    -- @
    -- SELECT ...
    -- FROM Person LEFT OUTER JOIN Pet
    -- ...
    -- @
    --
    -- See also: 'from'.
    data LeftOuterJoin a b = a `LeftOuterJoin` b

    -- | Data type that represents a @RIGHT OUTER JOIN@ (see 'LeftOuterJoin' for an example).
    data RightOuterJoin a b = a `RightOuterJoin` b

    -- | Data type that represents a @FULL OUTER JOIN@ (see 'LeftOuterJoin' for an example).
    data FullOuterJoin a b = a `FullOuterJoin` b

    -- | (Internal) A kind of @JOIN@.
    data JoinKind =
        InnerJoinKind      -- ^ @INNER JOIN@
      | CrossJoinKind      -- ^ @CROSS JOIN@
      | LeftOuterJoinKind  -- ^ @LEFT OUTER JOIN@
      | RightOuterJoinKind -- ^ @RIGHT OUTER JOIN@
      | FullOuterJoinKind  -- ^ @FULL OUTER JOIN@
        deriving Eq

    -- | (Internal) Functions that operate on types (that should be)
    -- of kind 'JoinKind'.
    class IsJoinKind join where
      -- | (Internal) @smartJoin a b@ is a @JOIN@ of the correct kind.
      smartJoin :: a -> b -> join a b
      -- | (Internal) Reify a @JoinKind@ from a @JOIN@.  This
      -- function is non-strict.
      reifyJoinKind :: join a b -> JoinKind
    instance IsJoinKind InnerJoin where
      smartJoin a b = a `InnerJoin` b
      reifyJoinKind _ = InnerJoinKind
    instance IsJoinKind CrossJoin where
      smartJoin a b = a `CrossJoin` b
      reifyJoinKind _ = CrossJoinKind
    instance IsJoinKind LeftOuterJoin where
      smartJoin a b = a `LeftOuterJoin` b
      reifyJoinKind _ = LeftOuterJoinKind
    instance IsJoinKind RightOuterJoin where
      smartJoin a b = a `RightOuterJoin` b
      reifyJoinKind _ = RightOuterJoinKind
    instance IsJoinKind FullOuterJoin where
      smartJoin a b = a `FullOuterJoin` b
      reifyJoinKind _ = FullOuterJoinKind

    -- | Exception thrown whenever 'on' is used to create an @ON@
    -- clause but no matching @JOIN@ is found.
    data OnClauseWithoutMatchingJoinException =
      OnClauseWithoutMatchingJoinException String
      deriving (Eq, Ord, Show, Typeable)
    instance Exception OnClauseWithoutMatchingJoinException where

    -- | (Internal) Phantom type used to process 'from' (see 'fromStart').
    data PreprocessedFrom a

    -- | Phantom type used by 'orderBy', 'asc' and 'desc'.
    data OrderBy

    -- | Phantom type used by 'distinctOn' and 'don'.
    data DistinctOn

    -- | Phantom type for a @SET@ operation on an entity of the given
    -- type (see 'set' and '(=.)').
    data Update typ

    -- | Phantom type used by 'insertSelect'.
    data Insertion a

    -- | Different kinds of locking clauses supported by 'locking'.
    --
    -- Note that each RDBMS has different locking support.  The
    -- constructors of this datatype specify only the /syntax/ of the
    -- locking mechanism, not its /semantics/.  For example, even
    -- though both MySQL and PostgreSQL support 'ForUpdate', there
    -- are no guarantees that they will behave the same.
    --
    -- /Since: 2.2.7/
    data LockingKind =
        ForUpdate
        -- ^ @FOR UPDATE@ syntax.  Supported by MySQL, Oracle and
        -- PostgreSQL.
        --
        -- /Since: 2.2.7/
      | ForShare
        -- ^ @FOR SHARE@ syntax.  Supported by PostgreSQL.
        --
        -- /Since: 2.2.7/
      | LockInShareMode
        -- ^ @LOCK IN SHARE MODE@ syntax.  Supported by MySQL.
        --
        -- /Since: 2.2.7/

    -- | Phantom class of data types that are treated as strings by the
    -- RDBMS.  It has no methods because it's only used to avoid type
    -- errors such as trying to concatenate integers.
    --
    -- If you have a custom data type or @newtype@, feel free to make
    -- it an instance of this class.
    --
    -- /Since: 2.4.0/
    class PersistField a => SqlString a where

    -- | /Since: 2.3.0/
    instance (a ~ Char) => SqlString [a] where

    -- | /Since: 2.3.0/
    instance SqlString T.Text where

    -- | /Since: 2.3.0/
    instance SqlString TL.Text where

    -- | /Since: 2.3.0/
    instance SqlString B.ByteString where

    -- | /Since: 2.3.0/
    instance SqlString Html where

    -- | /Since: 2.4.0/
    instance SqlString a => SqlString (Maybe a) where

    -- | @FROM@ clause: bring entities into scope.
    --
    -- This function internally uses two type classes in order to
    -- provide some flexibility of how you may call it.  Internally
    -- we refer to these type classes as the two different magics.
    --
    -- The innermost magic allows you to use @from@ with the
    -- following types:
    --
    --  * @expr (Entity val)@, which brings a single entity into
    --  scope.
    --
    --  * @expr (Maybe (Entity val))@, which brings a single entity
    --  that may be @NULL@ into scope.  Used for @OUTER JOIN@s.
    --
    --  * A @JOIN@ of any other two types allowed by the innermost
    --  magic, where a @JOIN@ may be an 'InnerJoin', a 'CrossJoin', a
    --  'LeftOuterJoin', a 'RightOuterJoin', or a 'FullOuterJoin'.
    --  The @JOINs@ have left fixity.
    --
    -- The outermost magic allows you to use @from@ on any tuples of
    -- types supported by innermost magic (and also tuples of tuples,
    -- and so on), up to 8-tuples.
    --
    -- Note that using @from@ for the same entity twice does work and
    -- corresponds to a self-join.  You don't even need to use two
    -- different calls to @from@, you may use a @JOIN@ or a tuple.
    --
    -- The following are valid examples of uses of @from@ (the types
    -- of the arguments of the lambda are inside square brackets):
    --
    -- @
    -- 'from' $ \\person -> ...
    -- 'from' $ \\(person, blogPost) -> ...
    -- 'from' $ \\(p `'LeftOuterJoin`` mb) -> ...
    -- 'from' $ \\(p1 `'InnerJoin`` f `'InnerJoin`` p2) -> ...
    -- 'from' $ \\((p1 `'InnerJoin`` f) `'InnerJoin`` p2) -> ...
    -- @
    --
    -- The types of the arguments to the lambdas above are,
    -- respectively:
    --
    -- @
    -- person
    --   :: ( Esqueleto query expr backend
    --      , PersistEntity Person
    --      , PersistEntityBackend Person ~ backend
    --      ) => expr (Entity Person)
    -- (person, blogPost)
    --   :: (...) => (expr (Entity Person), expr (Entity BlogPost))
    -- (p `'LeftOuterJoin`` mb)
    --   :: (...) => InnerJoin (expr (Entity Person)) (expr (Maybe (Entity BlogPost)))
    -- (p1 `'InnerJoin`` f `'InnerJoin`` p2)
    --   :: (...) => InnerJoin
    --                 (InnerJoin (expr (Entity Person))
    --                            (expr (Entity Follow)))
    --                 (expr (Entity Person))
    -- (p1 `'InnerJoin`` (f `'InnerJoin`` p2)) ::
    --   :: (...) => InnerJoin
    --                 (expr (Entity Person))
    --                 (InnerJoin (expr (Entity Follow))
    --                            (expr (Entity Person)))
    -- @
    --
    -- Note that some backends may not support all kinds of @JOIN@s.
    from :: From query expr backend a => (a -> query b) -> query b
    from = (from_ >>=)

    -- | (Internal) Class that implements the tuple 'from' magic (see
    -- 'fromStart').
    class Esqueleto query expr backend => From query expr backend a where
      from_ :: query a

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (expr (Entity val))
             ) => From query expr backend (expr (Entity val)) where
      from_ = fromPreprocess >>= fromFinish

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (expr (Maybe (Entity val)))
             ) => From query expr backend (expr (Maybe (Entity val))) where
      from_ = fromPreprocess >>= fromFinish

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (InnerJoin a b)
             ) => From query expr backend (InnerJoin a b) where
      from_ = fromPreprocess >>= fromFinish

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (CrossJoin a b)
             ) => From query expr backend (CrossJoin a b) where
      from_ = fromPreprocess >>= fromFinish

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (LeftOuterJoin a b)
             ) => From query expr backend (LeftOuterJoin a b) where
      from_ = fromPreprocess >>= fromFinish

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (RightOuterJoin a b)
             ) => From query expr backend (RightOuterJoin a b) where
      from_ = fromPreprocess >>= fromFinish

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend (FullOuterJoin a b)
             ) => From query expr backend (FullOuterJoin a b) where
      from_ = fromPreprocess >>= fromFinish

    instance ( From query expr backend a
             , From query expr backend b
             ) => From query expr backend (a, b) where
      from_ = (,) <$> from_ <*> from_

    instance ( From query expr backend a
             , From query expr backend b
             , From query expr backend c
             ) => From query expr backend (a, b, c) where
      from_ = (,,) <$> from_ <*> from_ <*> from_

    instance ( From query expr backend a
             , From query expr backend b
             , From query expr backend c
             , From query expr backend d
             ) => From query expr backend (a, b, c, d) where
      from_ = (,,,) <$> from_ <*> from_ <*> from_ <*> from_

    instance ( From query expr backend a
             , From query expr backend b
             , From query expr backend c
             , From query expr backend d
             , From query expr backend e
             ) => From query expr backend (a, b, c, d, e) where
      from_ = (,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_

    instance ( From query expr backend a
             , From query expr backend b
             , From query expr backend c
             , From query expr backend d
             , From query expr backend e
             , From query expr backend f
             ) => From query expr backend (a, b, c, d, e, f) where
      from_ = (,,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_

    instance ( From query expr backend a
             , From query expr backend b
             , From query expr backend c
             , From query expr backend d
             , From query expr backend e
             , From query expr backend f
             , From query expr backend g
             ) => From query expr backend (a, b, c, d, e, f, g) where
      from_ = (,,,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_

    instance ( From query expr backend a
             , From query expr backend b
             , From query expr backend c
             , From query expr backend d
             , From query expr backend e
             , From query expr backend f
             , From query expr backend g
             , From query expr backend h
             ) => From query expr backend (a, b, c, d, e, f, g, h) where
      from_ = (,,,,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_

    -- | (Internal) Class that implements the @JOIN@ 'from' magic
    -- (see 'fromStart').
    class Esqueleto query expr backend => FromPreprocess query expr backend a where
      fromPreprocess :: query (expr (PreprocessedFrom a))

    instance ( Esqueleto query expr backend
             , PersistEntity val
             , PersistEntityBackend val ~ backend
             ) => FromPreprocess query expr backend (expr (Entity val)) where
      fromPreprocess = fromStart

    instance ( Esqueleto query expr backend
             , PersistEntity val
             , PersistEntityBackend val ~ backend
             ) => FromPreprocess query expr backend (expr (Maybe (Entity val))) where
      fromPreprocess = fromStartMaybe

    instance ( Esqueleto query expr backend
             , FromPreprocess query expr backend a
             , FromPreprocess query expr backend b
             , IsJoinKind join
             ) => FromPreprocess query expr backend (join a b) where
      fromPreprocess = do
        a <- fromPreprocess
        b <- fromPreprocess
        fromJoin a b