Language/Haskell/TH/Syntax.hs

    {-# LANGUAGE CPP, DeriveDataTypeable,
                 DeriveGeneric, FlexibleInstances, DefaultSignatures,
                 RankNTypes, RoleAnnotations, ScopedTypeVariables #-}

    {-# OPTIONS_GHC -fno-warn-inline-rule-shadowing #-}

    #if MIN_VERSION_base(4,8,0)
    #define HAS_NATURAL
    #endif

    #if MIN_VERSION_base(4,9,0)
    # define HAS_MONADFAIL 1
    #endif

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Language.Haskell.Syntax
    -- Copyright   :  (c) The University of Glasgow 2003
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Abstract syntax definitions for Template Haskell.
    --
    -----------------------------------------------------------------------------

    module Language.Haskell.TH.Syntax
        ( module Language.Haskell.TH.Syntax
          -- * Language extensions
        , module Language.Haskell.TH.LanguageExtensions
        ) where

    import Data.Data hiding (Fixity(..))
    #if __GLASGOW_HASKELL__ < 709
    import Control.Applicative( Applicative(..) )
    #endif
    import Data.IORef
    import System.IO.Unsafe ( unsafePerformIO )
    import Control.Monad (liftM)
    import System.IO        ( hPutStrLn, stderr )
    import Data.Char        ( isAlpha, isAlphaNum, isUpper )
    import Data.Int
    import Data.Word
    import Data.Ratio
    import GHC.Generics     ( Generic )
    import GHC.Lexeme       ( startsVarSym, startsVarId )
    import Language.Haskell.TH.LanguageExtensions

    #ifdef HAS_NATURAL
    import Numeric.Natural
    #endif

    #if HAS_MONADFAIL
    import qualified Control.Monad.Fail as Fail
    #endif

    -----------------------------------------------------
    --
    --              The Quasi class
    --
    -----------------------------------------------------

    #if HAS_MONADFAIL
    class Fail.MonadFail m => Quasi m where
    #else
    class (Applicative m, Monad m) => Quasi m where
    #endif
      qNewName :: String -> m Name
            -- ^ Fresh names

            -- Error reporting and recovery
      qReport  :: Bool -> String -> m ()    -- ^ Report an error (True) or warning (False)
                                            -- ...but carry on; use 'fail' to stop
      qRecover :: m a -- ^ the error handler
               -> m a -- ^ action which may fail
               -> m a               -- ^ Recover from the monadic 'fail'

            -- Inspect the type-checker's environment
      qLookupName :: Bool -> String -> m (Maybe Name)
           -- True <=> type namespace, False <=> value namespace
      qReify          :: Name -> m Info
      qReifyFixity    :: Name -> m (Maybe Fixity)
      qReifyInstances :: Name -> [Type] -> m [Dec]
           -- Is (n tys) an instance?
           -- Returns list of matching instance Decs
           --    (with empty sub-Decs)
           -- Works for classes and type functions
      qReifyRoles         :: Name -> m [Role]
      qReifyAnnotations   :: Data a => AnnLookup -> m [a]
      qReifyModule        :: Module -> m ModuleInfo
      qReifyConStrictness :: Name -> m [DecidedStrictness]

      qLocation :: m Loc

      qRunIO :: IO a -> m a
      -- ^ Input/output (dangerous)

      qAddDependentFile :: FilePath -> m ()

      qAddTopDecls :: [Dec] -> m ()

      qAddModFinalizer :: Q () -> m ()

      qGetQ :: Typeable a => m (Maybe a)

      qPutQ :: Typeable a => a -> m ()

      qIsExtEnabled :: Extension -> m Bool
      qExtsEnabled :: m [Extension]

    -----------------------------------------------------
    --      The IO instance of Quasi
    --
    --  This instance is used only when running a Q
    --  computation in the IO monad, usually just to
    --  print the result.  There is no interesting
    --  type environment, so reification isn't going to
    --  work.
    --
    -----------------------------------------------------

    instance Quasi IO where
      qNewName s = do { n <- readIORef counter
                     ; writeIORef counter (n+1)
                     ; return (mkNameU s n) }

      qReport True  msg = hPutStrLn stderr ("Template Haskell error: " ++ msg)
      qReport False msg = hPutStrLn stderr ("Template Haskell error: " ++ msg)

      qLookupName _ _       = badIO "lookupName"
      qReify _              = badIO "reify"
      qReifyFixity _        = badIO "reifyFixity"
      qReifyInstances _ _   = badIO "reifyInstances"
      qReifyRoles _         = badIO "reifyRoles"
      qReifyAnnotations _   = badIO "reifyAnnotations"
      qReifyModule _        = badIO "reifyModule"
      qReifyConStrictness _ = badIO "reifyConStrictness"
      qLocation             = badIO "currentLocation"
      qRecover _ _          = badIO "recover" -- Maybe we could fix this?
      qAddDependentFile _   = badIO "addDependentFile"
      qAddTopDecls _        = badIO "addTopDecls"
      qAddModFinalizer _    = badIO "addModFinalizer"
      qGetQ                 = badIO "getQ"
      qPutQ _               = badIO "putQ"
      qIsExtEnabled _       = badIO "isExtEnabled"
      qExtsEnabled          = badIO "extsEnabled"

      qRunIO m = m

    badIO :: String -> IO a
    badIO op = do   { qReport True ("Can't do `" ++ op ++ "' in the IO monad")
                    ; fail "Template Haskell failure" }

    -- Global variable to generate unique symbols
    counter :: IORef Int
    {-# NOINLINE counter #-}
    counter = unsafePerformIO (newIORef 0)

    -----------------------------------------------------
    --
    --              The Q monad
    --
    -----------------------------------------------------

    newtype Q a = Q { unQ :: forall m. Quasi m => m a }

    -- \"Runs\" the 'Q' monad. Normal users of Template Haskell
    -- should not need this function, as the splice brackets @$( ... )@
    -- are the usual way of running a 'Q' computation.
    --
    -- This function is primarily used in GHC internals, and for debugging
    -- splices by running them in 'IO'.
    --
    -- Note that many functions in 'Q', such as 'reify' and other compiler
    -- queries, are not supported when running 'Q' in 'IO'; these operations
    -- simply fail at runtime. Indeed, the only operations guaranteed to succeed
    -- are 'newName', 'runIO', 'reportError' and 'reportWarning'.
    runQ :: Quasi m => Q a -> m a
    runQ (Q m) = m

    instance Monad Q where
      Q m >>= k  = Q (m >>= \x -> unQ (k x))
      (>>) = (*>)

      return     = pure

    #if !HAS_MONADFAIL
      fail s     = report True s >> Q (fail "Q monad failure")
    #else
      fail       = Fail.fail

    instance Fail.MonadFail Q where
      fail s     = report True s >> Q (Fail.fail "Q monad failure")
    #endif

    instance Functor Q where
      fmap f (Q x) = Q (fmap f x)

    instance Applicative Q where
      pure x = Q (pure x)
      Q f <*> Q x = Q (f <*> x)
      Q m *> Q n = Q (m *> n)

    -----------------------------------------------------
    --
    --              The TExp type
    --
    -----------------------------------------------------

    type role TExp nominal   -- See Note [Role of TExp]
    newtype TExp a = TExp { unType :: Exp }

    unTypeQ :: Q (TExp a) -> Q Exp
    unTypeQ m = do { TExp e <- m
                   ; return e }

    unsafeTExpCoerce :: Q Exp -> Q (TExp a)
    unsafeTExpCoerce m = do { e <- m
                            ; return (TExp e) }

    {- Note [Role of TExp]
    ~~~~~~~~~~~~~~~~~~~~~~
    TExp's argument must have a nominal role, not phantom as would
    be inferred (Trac #8459).  Consider

      e :: TExp Age
      e = MkAge 3

      foo = $(coerce e) + 4::Int

    The splice will evaluate to (MkAge 3) and you can't add that to
    4::Int. So you can't coerce a (TExp Age) to a (TExp Int). -}

    ----------------------------------------------------
    -- Packaged versions for the programmer, hiding the Quasi-ness

    {- |
    Generate a fresh name, which cannot be captured.

    For example, this:

    @f = $(do
      nm1 <- newName \"x\"
      let nm2 = 'mkName' \"x\"
      return ('LamE' ['VarP' nm1] (LamE [VarP nm2] ('VarE' nm1)))
     )@

    will produce the splice

    >f = \x0 -> \x -> x0

    In particular, the occurrence @VarE nm1@ refers to the binding @VarP nm1@,
    and is not captured by the binding @VarP nm2@.

    Although names generated by @newName@ cannot /be captured/, they can
    /capture/ other names. For example, this:

    >g = $(do
    >  nm1 <- newName "x"
    >  let nm2 = mkName "x"
    >  return (LamE [VarP nm2] (LamE [VarP nm1] (VarE nm2)))
    > )

    will produce the splice

    >g = \x -> \x0 -> x0

    since the occurrence @VarE nm2@ is captured by the innermost binding
    of @x@, namely @VarP nm1@.
    -}
    newName :: String -> Q Name
    newName s = Q (qNewName s)

    -- | Report an error (True) or warning (False),
    -- but carry on; use 'fail' to stop.
    report  :: Bool -> String -> Q ()
    report b s = Q (qReport b s)
    {-# DEPRECATED report "Use reportError or reportWarning instead" #-} -- deprecated in 7.6

    -- | Report an error to the user, but allow the current splice's computation to carry on. To abort the computation, use 'fail'.
    reportError :: String -> Q ()
    reportError = report True

    -- | Report a warning to the user, and carry on.
    reportWarning :: String -> Q ()
    reportWarning = report False

    -- | Recover from errors raised by 'reportError' or 'fail'.
    recover :: Q a -- ^ handler to invoke on failure
            -> Q a -- ^ computation to run
            -> Q a
    recover (Q r) (Q m) = Q (qRecover r m)

    -- We don't export lookupName; the Bool isn't a great API
    -- Instead we export lookupTypeName, lookupValueName
    lookupName :: Bool -> String -> Q (Maybe Name)
    lookupName ns s = Q (qLookupName ns s)

    -- | Look up the given name in the (type namespace of the) current splice's scope. See "Language.Haskell.TH.Syntax#namelookup" for more details.
    lookupTypeName :: String -> Q (Maybe Name)
    lookupTypeName  s = Q (qLookupName True s)

    -- | Look up the given name in the (value namespace of the) current splice's scope. See "Language.Haskell.TH.Syntax#namelookup" for more details.
    lookupValueName :: String -> Q (Maybe Name)
    lookupValueName s = Q (qLookupName False s)

    {-
    Note [Name lookup]
    ~~~~~~~~~~~~~~~~~~
    -}
    {- $namelookup #namelookup#
    The functions 'lookupTypeName' and 'lookupValueName' provide
    a way to query the current splice's context for what names
    are in scope. The function 'lookupTypeName' queries the type
    namespace, whereas 'lookupValueName' queries the value namespace,
    but the functions are otherwise identical.

    A call @lookupValueName s@ will check if there is a value
    with name @s@ in scope at the current splice's location. If
    there is, the @Name@ of this value is returned;
    if not, then @Nothing@ is returned.

    The returned name cannot be \"captured\".
    For example:

    > f = "global"
    > g = $( do
    >          Just nm <- lookupValueName "f"
    >          [| let f = "local" in $( varE nm ) |]

    In this case, @g = \"global\"@; the call to @lookupValueName@
    returned the global @f@, and this name was /not/ captured by
    the local definition of @f@.

    The lookup is performed in the context of the /top-level/ splice
    being run. For example:

    > f = "global"
    > g = $( [| let f = "local" in
    >            $(do
    >                Just nm <- lookupValueName "f"
    >                varE nm
    >             ) |] )

    Again in this example, @g = \"global\"@, because the call to
    @lookupValueName@ queries the context of the outer-most @$(...)@.

    Operators should be queried without any surrounding parentheses, like so:

    > lookupValueName "+"

    Qualified names are also supported, like so:

    > lookupValueName "Prelude.+"
    > lookupValueName "Prelude.map"

    -}

    {- | 'reify' looks up information about the 'Name'.

    It is sometimes useful to construct the argument name using 'lookupTypeName' or 'lookupValueName'
    to ensure that we are reifying from the right namespace. For instance, in this context:

    > data D = D

    which @D@ does @reify (mkName \"D\")@ return information about? (Answer: @D@-the-type, but don't rely on it.)
    To ensure we get information about @D@-the-value, use 'lookupValueName':

    > do
    >   Just nm <- lookupValueName "D"
    >   reify nm

    and to get information about @D@-the-type, use 'lookupTypeName'.
    -}
    reify :: Name -> Q Info
    reify v = Q (qReify v)

    {- | @reifyFixity nm@ attempts to find a fixity declaration for @nm@. For
    example, if the function @foo@ has the fixity declaration @infixr 7 foo@, then
    @reifyFixity 'foo@ would return @'Just' ('Fixity' 7 'InfixR')@. If the function
    @bar@ does not have a fixity declaration, then @reifyFixity 'bar@ returns
    'Nothing', so you may assume @bar@ has 'defaultFixity'.
    -}
    reifyFixity :: Name -> Q (Maybe Fixity)
    reifyFixity nm = Q (qReifyFixity nm)

    {- | @reifyInstances nm tys@ returns a list of visible instances of @nm tys@. That is,
    if @nm@ is the name of a type class, then all instances of this class at the types @tys@
    are returned. Alternatively, if @nm@ is the name of a data family or type family,
    all instances of this family at the types @tys@ are returned.
    -}
    reifyInstances :: Name -> [Type] -> Q [InstanceDec]
    reifyInstances cls tys = Q (qReifyInstances cls tys)

    {- | @reifyRoles nm@ returns the list of roles associated with the parameters of
    the tycon @nm@. Fails if @nm@ cannot be found or is not a tycon.
    The returned list should never contain 'InferR'.
    -}
    reifyRoles :: Name -> Q [Role]
    reifyRoles nm = Q (qReifyRoles nm)

    -- | @reifyAnnotations target@ returns the list of annotations
    -- associated with @target@.  Only the annotations that are
    -- appropriately typed is returned.  So if you have @Int@ and @String@
    -- annotations for the same target, you have to call this function twice.
    reifyAnnotations :: Data a => AnnLookup -> Q [a]
    reifyAnnotations an = Q (qReifyAnnotations an)

    -- | @reifyModule mod@ looks up information about module @mod@.  To
    -- look up the current module, call this function with the return
    -- value of @thisModule@.
    reifyModule :: Module -> Q ModuleInfo
    reifyModule m = Q (qReifyModule m)

    -- | @reifyConStrictness nm@ looks up the strictness information for the fields
    -- of the constructor with the name @nm@. Note that the strictness information
    -- that 'reifyConStrictness' returns may not correspond to what is written in
    -- the source code. For example, in the following data declaration:
    --
    -- @
    -- data Pair a = Pair a a
    -- @
    --
    -- 'reifyConStrictness' would return @['DecidedLazy', DecidedLazy]@ under most
    -- circumstances, but it would return @['DecidedStrict', DecidedStrict]@ if the
    -- @-XStrictData@ language extension was enabled.
    reifyConStrictness :: Name -> Q [DecidedStrictness]
    reifyConStrictness n = Q (qReifyConStrictness n)

    -- | Is the list of instances returned by 'reifyInstances' nonempty?
    isInstance :: Name -> [Type] -> Q Bool
    isInstance nm tys = do { decs <- reifyInstances nm tys
                           ; return (not (null decs)) }

    -- | The location at which this computation is spliced.
    location :: Q Loc
    location = Q qLocation

    -- |The 'runIO' function lets you run an I\/O computation in the 'Q' monad.
    -- Take care: you are guaranteed the ordering of calls to 'runIO' within
    -- a single 'Q' computation, but not about the order in which splices are run.
    --
    -- Note: for various murky reasons, stdout and stderr handles are not
    -- necessarily flushed when the compiler finishes running, so you should
    -- flush them yourself.
    runIO :: IO a -> Q a
    runIO m = Q (qRunIO m)

    -- | Record external files that runIO is using (dependent upon).
    -- The compiler can then recognize that it should re-compile the Haskell file
    -- when an external file changes.
    --
    -- Expects an absolute file path.
    --
    -- Notes:
    --
    --   * ghc -M does not know about these dependencies - it does not execute TH.
    --
    --   * The dependency is based on file content, not a modification time
    addDependentFile :: FilePath -> Q ()
    addDependentFile fp = Q (qAddDependentFile fp)

    -- | Add additional top-level declarations. The added declarations will be type
    -- checked along with the current declaration group.
    addTopDecls :: [Dec] -> Q ()
    addTopDecls ds = Q (qAddTopDecls ds)

    -- | Add a finalizer that will run in the Q monad after the current module has
    -- been type checked. This only makes sense when run within a top-level splice.
    addModFinalizer :: Q () -> Q ()
    addModFinalizer act = Q (qAddModFinalizer (unQ act))

    -- | Get state from the 'Q' monad.
    getQ :: Typeable a => Q (Maybe a)
    getQ = Q qGetQ

    -- | Replace the state in the 'Q' monad.
    putQ :: Typeable a => a -> Q ()
    putQ x = Q (qPutQ x)

    -- | Determine whether the given language extension is enabled in the 'Q' monad.
    isExtEnabled :: Extension -> Q Bool
    isExtEnabled ext = Q (qIsExtEnabled ext)

    -- | List all enabled language extensions.
    extsEnabled :: Q [Extension]
    extsEnabled = Q qExtsEnabled

    instance Quasi Q where
      qNewName            = newName
      qReport             = report
      qRecover            = recover
      qReify              = reify
      qReifyFixity        = reifyFixity
      qReifyInstances     = reifyInstances
      qReifyRoles         = reifyRoles
      qReifyAnnotations   = reifyAnnotations
      qReifyModule        = reifyModule
      qReifyConStrictness = reifyConStrictness
      qLookupName         = lookupName
      qLocation           = location
      qRunIO              = runIO
      qAddDependentFile   = addDependentFile
      qAddTopDecls        = addTopDecls
      qAddModFinalizer    = addModFinalizer
      qGetQ               = getQ
      qPutQ               = putQ
      qIsExtEnabled       = isExtEnabled
      qExtsEnabled        = extsEnabled

    ----------------------------------------------------
    -- The following operations are used solely in DsMeta when desugaring brackets
    -- They are not necessary for the user, who can use ordinary return and (>>=) etc

    returnQ :: a -> Q a
    returnQ = return

    bindQ :: Q a -> (a -> Q b) -> Q b
    bindQ = (>>=)

    sequenceQ :: [Q a] -> Q [a]
    sequenceQ = sequence

    -----------------------------------------------------
    --
    --              The Lift class
    --
    -----------------------------------------------------

    -- | A 'Lift' instance can have any of its values turned into a Template
    -- Haskell expression. This is needed when a value used within a Template
    -- Haskell quotation is bound outside the Oxford brackets (@[| ... |]@) but not
    -- at the top level. As an example:
    --
    -- > add1 :: Int -> Q Exp
    -- > add1 x = [| x + 1 |]
    --
    -- Template Haskell has no way of knowing what value @x@ will take on at
    -- splice-time, so it requires the type of @x@ to be an instance of 'Lift'.
    --
    -- 'Lift' instances can be derived automatically by use of the @-XDeriveLift@
    -- GHC language extension:
    --
    -- > {-# LANGUAGE DeriveLift #-}
    -- > module Foo where
    -- >
    -- > import Language.Haskell.TH.Syntax
    -- >
    -- > data Bar a = Bar1 a (Bar a) | Bar2 String
    -- >   deriving Lift
    class Lift t where
      -- | Turn a value into a Template Haskell expression, suitable for use in
      -- a splice.
      lift :: t -> Q Exp
      default lift :: Data t => t -> Q Exp
      lift = liftData

    -- If you add any instances here, consider updating test th/TH_Lift
    instance Lift Integer where
      lift x = return (LitE (IntegerL x))

    instance Lift Int where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Int8 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Int16 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Int32 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Int64 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Word where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Word8 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Word16 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Word32 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    instance Lift Word64 where
      lift x = return (LitE (IntegerL (fromIntegral x)))

    #ifdef HAS_NATURAL
    instance Lift Natural where
      lift x = return (LitE (IntegerL (fromIntegral x)))
    #endif

    instance Integral a => Lift (Ratio a) where
      lift x = return (LitE (RationalL (toRational x)))

    instance Lift Float where
      lift x = return (LitE (RationalL (toRational x)))

    instance Lift Double where
      lift x = return (LitE (RationalL (toRational x)))

    instance Lift Char where
      lift x = return (LitE (CharL x))

    instance Lift Bool where
      lift True  = return (ConE trueName)
      lift False = return (ConE falseName)

    instance Lift a => Lift (Maybe a) where
      lift Nothing  = return (ConE nothingName)
      lift (Just x) = liftM (ConE justName `AppE`) (lift x)

    instance (Lift a, Lift b) => Lift (Either a b) where
      lift (Left x)  = liftM (ConE leftName  `AppE`) (lift x)
      lift (Right y) = liftM (ConE rightName `AppE`) (lift y)

    instance Lift a => Lift [a] where
      lift xs = do { xs' <- mapM lift xs; return (ListE xs') }

    liftString :: String -> Q Exp
    -- Used in TcExpr to short-circuit the lifting for strings
    liftString s = return (LitE (StringL s))

    instance Lift () where
      lift () = return (ConE (tupleDataName 0))

    instance (Lift a, Lift b) => Lift (a, b) where
      lift (a, b)
        = liftM TupE $ sequence [lift a, lift b]

    instance (Lift a, Lift b, Lift c) => Lift (a, b, c) where
      lift (a, b, c)
        = liftM TupE $ sequence [lift a, lift b, lift c]

    instance (Lift a, Lift b, Lift c, Lift d) => Lift (a, b, c, d) where
      lift (a, b, c, d)
        = liftM TupE $ sequence [lift a, lift b, lift c, lift d]

    instance (Lift a, Lift b, Lift c, Lift d, Lift e)
          => Lift (a, b, c, d, e) where
      lift (a, b, c, d, e)
        = liftM TupE $ sequence [lift a, lift b, lift c, lift d, lift e]

    instance (Lift a, Lift b, Lift c, Lift d, Lift e, Lift f)
          => Lift (a, b, c, d, e, f) where
      lift (a, b, c, d, e, f)
        = liftM TupE $ sequence [lift a, lift b, lift c, lift d, lift e, lift f]

    instance (Lift a, Lift b, Lift c, Lift d, Lift e, Lift f, Lift g)
          => Lift (a, b, c, d, e, f, g) where
      lift (a, b, c, d, e, f, g)
        = liftM TupE $ sequence [lift a, lift b, lift c, lift d, lift e, lift f, lift g]

    -- TH has a special form for literal strings,
    -- which we should take advantage of.
    -- NB: the lhs of the rule has no args, so that
    --     the rule will apply to a 'lift' all on its own
    --     which happens to be the way the type checker
    --     creates it.
    {-# RULES "TH:liftString" lift = \s -> return (LitE (StringL s)) #-}

    trueName, falseName :: Name
    trueName  = mkNameG DataName "ghc-prim" "GHC.Types" "True"
    falseName = mkNameG DataName "ghc-prim" "GHC.Types" "False"

    nothingName, justName :: Name
    nothingName = mkNameG DataName "base" "GHC.Base" "Nothing"
    justName    = mkNameG DataName "base" "GHC.Base" "Just"

    leftName, rightName :: Name
    leftName  = mkNameG DataName "base" "Data.Either" "Left"
    rightName = mkNameG DataName "base" "Data.Either" "Right"

    -----------------------------------------------------
    --
    --              Generic Lift implementations
    --
    -----------------------------------------------------

    -- | 'dataToQa' is an internal utility function for constructing generic
    -- conversion functions from types with 'Data' instances to various
    -- quasi-quoting representations.  See the source of 'dataToExpQ' and
    -- 'dataToPatQ' for two example usages: @mkCon@, @mkLit@
    -- and @appQ@ are overloadable to account for different syntax for
    -- expressions and patterns; @antiQ@ allows you to override type-specific
    -- cases, a common usage is just @const Nothing@, which results in
    -- no overloading.
    dataToQa  ::  forall a k q. Data a
              =>  (Name -> k)
              ->  (Lit -> Q q)
              ->  (k -> [Q q] -> Q q)
              ->  (forall b . Data b => b -> Maybe (Q q))
              ->  a
              ->  Q q
    dataToQa mkCon mkLit appCon antiQ t =
        case antiQ t of
          Nothing ->
              case constrRep constr of
                AlgConstr _ ->
                    appCon (mkCon funOrConName) conArgs
                  where
                    funOrConName :: Name
                    funOrConName =
                        case showConstr constr of
                          "(:)"       -> Name (mkOccName ":")
                                              (NameG DataName
                                                    (mkPkgName "ghc-prim")
                                                    (mkModName "GHC.Types"))
                          con@"[]"    -> Name (mkOccName con)
                                              (NameG DataName
                                                    (mkPkgName "ghc-prim")
                                                    (mkModName "GHC.Types"))
                          con@('(':_) -> Name (mkOccName con)
                                              (NameG DataName
                                                    (mkPkgName "ghc-prim")
                                                    (mkModName "GHC.Tuple"))
                          -- It is possible for a Data instance to be defined such
                          -- that toConstr uses a Constr defined using a function,
                          -- not a data constructor. In such a case, we must take
                          -- care to build the Name using mkNameG_v (for values),
                          -- not mkNameG_d (for data constructors).
                          -- See Trac #10796.
                          fun@(x:_)   | startsVarSym x || startsVarId x
                                      -> mkNameG_v tyconPkg tyconMod fun
                          con         -> mkNameG_d tyconPkg tyconMod con
                      where
                        tycon :: TyCon
                        tycon = (typeRepTyCon . typeOf) t

                        tyconPkg, tyconMod :: String
                        tyconPkg = tyConPackage tycon
                        tyconMod = tyConModule  tycon

                    conArgs :: [Q q]
                    conArgs = gmapQ (dataToQa mkCon mkLit appCon antiQ) t
                IntConstr n ->
                    mkLit $ IntegerL n
                FloatConstr n ->
                    mkLit $ RationalL n
                CharConstr c ->
                    mkLit $ CharL c
            where
              constr :: Constr
              constr = toConstr t

          Just y -> y

    -- | 'dataToExpQ' converts a value to a 'Q Exp' representation of the
    -- same value, in the SYB style. It is generalized to take a function
    -- override type-specific cases; see 'liftData' for a more commonly
    -- used variant.
    dataToExpQ  ::  Data a
                =>  (forall b . Data b => b -> Maybe (Q Exp))
                ->  a
                ->  Q Exp
    dataToExpQ = dataToQa varOrConE litE (foldl appE)
        where
              -- Make sure that VarE is used if the Constr value relies on a
              -- function underneath the surface (instead of a constructor).
              -- See Trac #10796.
              varOrConE s =
                case nameSpace s of
                     Just VarName  -> return (VarE s)
                     Just DataName -> return (ConE s)
                     _ -> fail $ "Can't construct an expression from name "
                              ++ showName s
              appE x y = do { a <- x; b <- y; return (AppE a b)}
              litE c = return (LitE c)

    -- | 'liftData' is a variant of 'lift' in the 'Lift' type class which
    -- works for any type with a 'Data' instance.
    liftData :: Data a => a -> Q Exp
    liftData = dataToExpQ (const Nothing)

    -- | 'dataToPatQ' converts a value to a 'Q Pat' representation of the same
    -- value, in the SYB style. It takes a function to handle type-specific cases,
    -- alternatively, pass @const Nothing@ to get default behavior.
    dataToPatQ  ::  Data a
                =>  (forall b . Data b => b -> Maybe (Q Pat))
                ->  a
                ->  Q Pat
    dataToPatQ = dataToQa id litP conP
        where litP l = return (LitP l)
              conP n ps =
                case nameSpace n of
                    Just DataName -> do
                        ps' <- sequence ps
                        return (ConP n ps')
                    _ -> fail $ "Can't construct a pattern from name "
                             ++ showName n

    -----------------------------------------------------
    --              Names and uniques
    -----------------------------------------------------

    newtype ModName = ModName String        -- Module name
     deriving (Show,Eq,Ord,Typeable,Data,Generic)

    newtype PkgName = PkgName String        -- package name
     deriving (Show,Eq,Ord,Typeable,Data,Generic)

    -- | Obtained from 'reifyModule' and 'thisModule'.
    data Module = Module PkgName ModName -- package qualified module name
     deriving (Show,Eq,Ord,Typeable,Data,Generic)

    newtype OccName = OccName String
     deriving (Show,Eq,Ord,Typeable,Data,Generic)

    mkModName :: String -> ModName
    mkModName s = ModName s

    modString :: ModName -> String
    modString (ModName m) = m

    mkPkgName :: String -> PkgName
    mkPkgName s = PkgName s

    pkgString :: PkgName -> String
    pkgString (PkgName m) = m

    -----------------------------------------------------
    --              OccName
    -----------------------------------------------------

    mkOccName :: String -> OccName
    mkOccName s = OccName s

    occString :: OccName -> String
    occString (OccName occ) = occ

    -----------------------------------------------------
    --               Names
    -----------------------------------------------------
    --
    -- For "global" names ('NameG') we need a totally unique name,
    -- so we must include the name-space of the thing
    --
    -- For unique-numbered things ('NameU'), we've got a unique reference
    -- anyway, so no need for name space
    --
    -- For dynamically bound thing ('NameS') we probably want them to
    -- in a context-dependent way, so again we don't want the name
    -- space.  For example:
    --
    -- > let v = mkName "T" in [| data $v = $v |]
    --
    -- Here we use the same Name for both type constructor and data constructor
    --
    --
    -- NameL and NameG are bound *outside* the TH syntax tree
    -- either globally (NameG) or locally (NameL). Ex:
    --
    -- > f x = $(h [| (map, x) |])
    --
    -- The 'map' will be a NameG, and 'x' wil be a NameL
    --
    -- These Names should never appear in a binding position in a TH syntax tree

    {- $namecapture #namecapture#
    Much of 'Name' API is concerned with the problem of /name capture/, which
    can be seen in the following example.

    > f expr = [| let x = 0 in $expr |]
    > ...
    > g x = $( f [| x |] )
    > h y = $( f [| y |] )

    A naive desugaring of this would yield:

    > g x = let x = 0 in x
    > h y = let x = 0 in y

    All of a sudden, @g@ and @h@ have different meanings! In this case,
    we say that the @x@ in the RHS of @g@ has been /captured/
    by the binding of @x@ in @f@.

    What we actually want is for the @x@ in @f@ to be distinct from the
    @x@ in @g@, so we get the following desugaring:

    > g x = let x' = 0 in x
    > h y = let x' = 0 in y

    which avoids name capture as desired.

    In the general case, we say that a @Name@ can be captured if
    the thing it refers to can be changed by adding new declarations.
    -}

    {- |
    An abstract type representing names in the syntax tree.

    'Name's can be constructed in several ways, which come with different
    name-capture guarantees (see "Language.Haskell.TH.Syntax#namecapture" for
    an explanation of name capture):

      * the built-in syntax @'f@ and @''T@ can be used to construct names,
        The expression @'f@ gives a @Name@ which refers to the value @f@
        currently in scope, and @''T@ gives a @Name@ which refers to the
        type @T@ currently in scope. These names can never be captured.

      * 'lookupValueName' and 'lookupTypeName' are similar to @'f@ and
         @''T@ respectively, but the @Name@s are looked up at the point
         where the current splice is being run. These names can never be
         captured.

      * 'newName' monadically generates a new name, which can never
         be captured.

      * 'mkName' generates a capturable name.

    Names constructed using @newName@ and @mkName@ may be used in bindings
    (such as @let x = ...@ or @\x -> ...@), but names constructed using
    @lookupValueName@, @lookupTypeName@, @'f@, @''T@ may not.
    -}
    data Name = Name OccName NameFlavour deriving (Typeable, Data, Eq, Generic)

    instance Ord Name where
        -- check if unique is different before looking at strings
      (Name o1 f1) `compare` (Name o2 f2) = (f1 `compare` f2)   `thenCmp`
                                            (o1 `compare` o2)

    data NameFlavour
      = NameS           -- ^ An unqualified name; dynamically bound
      | NameQ ModName   -- ^ A qualified name; dynamically bound
      | NameU !Int      -- ^ A unique local name
      | NameL !Int      -- ^ Local name bound outside of the TH AST
      | NameG NameSpace PkgName ModName -- ^ Global name bound outside of the TH AST:
                    -- An original name (occurrences only, not binders)
                    -- Need the namespace too to be sure which
                    -- thing we are naming
      deriving ( Typeable, Data, Eq, Ord, Show, Generic )

    data NameSpace = VarName        -- ^ Variables
                   | DataName       -- ^ Data constructors
                   | TcClsName      -- ^ Type constructors and classes; Haskell has them
                                    -- in the same name space for now.
                   deriving( Eq, Ord, Show, Data, Typeable, Generic )

    type Uniq = Int

    -- | The name without its module prefix.
    --
    -- ==== __Examples__
    --
    -- >>> nameBase ''Data.Either.Either
    -- "Either"
    -- >>> nameBase (mkName "foo")
    -- "foo"
    -- >>> nameBase (mkName "Module.foo")
    -- "foo"
    nameBase :: Name -> String
    nameBase (Name occ _) = occString occ

    -- | Module prefix of a name, if it exists.
    --
    -- ==== __Examples__
    --
    -- >>> nameModule ''Data.Either.Either
    -- Just "Data.Either"
    -- >>> nameModule (mkName "foo")
    -- Nothing
    -- >>> nameModule (mkName "Module.foo")
    -- Just "Module"
    nameModule :: Name -> Maybe String
    nameModule (Name _ (NameQ m))     = Just (modString m)
    nameModule (Name _ (NameG _ _ m)) = Just (modString m)
    nameModule _                      = Nothing

    -- | A name's package, if it exists.
    --
    -- ==== __Examples__
    --
    -- >>> namePackage ''Data.Either.Either
    -- Just "base"
    -- >>> namePackage (mkName "foo")
    -- Nothing
    -- >>> namePackage (mkName "Module.foo")
    -- Nothing
    namePackage :: Name -> Maybe String
    namePackage (Name _ (NameG _ p _)) = Just (pkgString p)
    namePackage _                      = Nothing

    -- | Returns whether a name represents an occurrence of a top-level variable
    -- ('VarName'), data constructor ('DataName'), type constructor, or type class
    -- ('TcClsName'). If we can't be sure, it returns 'Nothing'.
    --
    -- ==== __Examples__
    --
    -- >>> nameSpace 'Prelude.id
    -- Just VarName
    -- >>> nameSpace (mkName "id")
    -- Nothing -- only works for top-level variable names
    -- >>> nameSpace 'Data.Maybe.Just
    -- Just DataName
    -- >>> nameSpace ''Data.Maybe.Maybe
    -- Just TcClsName
    -- >>> nameSpace ''Data.Ord.Ord
    -- Just TcClsName
    nameSpace :: Name -> Maybe NameSpace
    nameSpace (Name _ (NameG ns _ _)) = Just ns
    nameSpace _                       = Nothing

    {- |
    Generate a capturable name. Occurrences of such names will be
    resolved according to the Haskell scoping rules at the occurrence
    site.

    For example:

    > f = [| pi + $(varE (mkName "pi")) |]
    > ...
    > g = let pi = 3 in $f

    In this case, @g@ is desugared to

    > g = Prelude.pi + 3

    Note that @mkName@ may be used with qualified names:

    > mkName "Prelude.pi"

    See also 'Language.Haskell.TH.Lib.dyn' for a useful combinator. The above example could
    be rewritten using 'dyn' as

    > f = [| pi + $(dyn "pi") |]
    -}
    mkName :: String -> Name
    -- The string can have a '.', thus "Foo.baz",
    -- giving a dynamically-bound qualified name,
    -- in which case we want to generate a NameQ
    --
    -- Parse the string to see if it has a "." in it
    -- so we know whether to generate a qualified or unqualified name
    -- It's a bit tricky because we need to parse
    --
    -- > Foo.Baz.x   as    Qual Foo.Baz x
    --
    -- So we parse it from back to front
    mkName str
      = split [] (reverse str)
      where
        split occ []        = Name (mkOccName occ) NameS
        split occ ('.':rev) | not (null occ)
                            , is_rev_mod_name rev
                            = Name (mkOccName occ) (NameQ (mkModName (reverse rev)))
            -- The 'not (null occ)' guard ensures that
            --      mkName "&." = Name "&." NameS
            -- The 'is_rev_mod' guards ensure that
            --      mkName ".&" = Name ".&" NameS
            --      mkName "^.." = Name "^.." NameS      -- Trac #8633
            --      mkName "Data.Bits..&" = Name ".&" (NameQ "Data.Bits")
            -- This rather bizarre case actually happened; (.&.) is in Data.Bits
        split occ (c:rev)   = split (c:occ) rev

        -- Recognises a reversed module name xA.yB.C,
        -- with at least one component,
        -- and each component looks like a module name
        --   (i.e. non-empty, starts with capital, all alpha)
        is_rev_mod_name rev_mod_str
          | (compt, rest) <- break (== '.') rev_mod_str
          , not (null compt), isUpper (last compt), all is_mod_char compt
          = case rest of
              []             -> True
              (_dot : rest') -> is_rev_mod_name rest'
          | otherwise
          = False

        is_mod_char c = isAlphaNum c || c == '_' || c == '\''

    -- | Only used internally
    mkNameU :: String -> Uniq -> Name
    mkNameU s u = Name (mkOccName s) (NameU u)

    -- | Only used internally
    mkNameL :: String -> Uniq -> Name
    mkNameL s u = Name (mkOccName s) (NameL u)

    -- | Used for 'x etc, but not available to the programmer
    mkNameG :: NameSpace -> String -> String -> String -> Name
    mkNameG ns pkg modu occ
      = Name (mkOccName occ) (NameG ns (mkPkgName pkg) (mkModName modu))

    mkNameS :: String -> Name
    mkNameS n = Name (mkOccName n) NameS

    mkNameG_v, mkNameG_tc, mkNameG_d :: String -> String -> String -> Name
    mkNameG_v  = mkNameG VarName
    mkNameG_tc = mkNameG TcClsName
    mkNameG_d  = mkNameG DataName

    data NameIs = Alone | Applied | Infix

    showName :: Name -> String
    showName = showName' Alone

    showName' :: NameIs -> Name -> String
    showName' ni nm
     = case ni of
           Alone        -> nms
           Applied
            | pnam      -> nms
            | otherwise -> "(" ++ nms ++ ")"
           Infix
            | pnam      -> "`" ++ nms ++ "`"
            | otherwise -> nms
        where
            -- For now, we make the NameQ and NameG print the same, even though
            -- NameQ is a qualified name (so what it means depends on what the
            -- current scope is), and NameG is an original name (so its meaning
            -- should be independent of what's in scope.
            -- We may well want to distinguish them in the end.
            -- Ditto NameU and NameL
            nms = case nm of
                        Name occ NameS         -> occString occ
                        Name occ (NameQ m)     -> modString m ++ "." ++ occString occ
                        Name occ (NameG _ _ m) -> modString m ++ "." ++ occString occ
                        Name occ (NameU u)     -> occString occ ++ "_" ++ show u
                        Name occ (NameL u)     -> occString occ ++ "_" ++ show u

            pnam = classify nms

            -- True if we are function style, e.g. f, [], (,)
            -- False if we are operator style, e.g. +, :+
            classify "" = False -- shouldn't happen; . operator is handled below
            classify (x:xs) | isAlpha x || (x `elem` "_[]()") =
                                case dropWhile (/='.') xs of
                                      (_:xs') -> classify xs'
                                      []      -> True
                            | otherwise = False

    instance Show Name where
      show = showName

    -- Tuple data and type constructors
    -- | Tuple data constructor
    tupleDataName :: Int -> Name
    -- | Tuple type constructor
    tupleTypeName :: Int -> Name

    tupleDataName 0 = mk_tup_name 0 DataName
    tupleDataName 1 = error "tupleDataName 1"
    tupleDataName n = mk_tup_name (n-1) DataName

    tupleTypeName 0 = mk_tup_name 0 TcClsName
    tupleTypeName 1 = error "tupleTypeName 1"
    tupleTypeName n = mk_tup_name (n-1) TcClsName

    mk_tup_name :: Int -> NameSpace -> Name
    mk_tup_name n_commas space
      = Name occ (NameG space (mkPkgName "ghc-prim") tup_mod)
      where
        occ = mkOccName ('(' : replicate n_commas ',' ++ ")")
        tup_mod = mkModName "GHC.Tuple"

    -- Unboxed tuple data and type constructors
    -- | Unboxed tuple data constructor
    unboxedTupleDataName :: Int -> Name
    -- | Unboxed tuple type constructor
    unboxedTupleTypeName :: Int -> Name

    unboxedTupleDataName 0 = error "unboxedTupleDataName 0"
    unboxedTupleDataName 1 = error "unboxedTupleDataName 1"
    unboxedTupleDataName n = mk_unboxed_tup_name (n-1) DataName

    unboxedTupleTypeName 0 = error "unboxedTupleTypeName 0"
    unboxedTupleTypeName 1 = error "unboxedTupleTypeName 1"
    unboxedTupleTypeName n = mk_unboxed_tup_name (n-1) TcClsName

    mk_unboxed_tup_name :: Int -> NameSpace -> Name
    mk_unboxed_tup_name n_commas space
      = Name occ (NameG space (mkPkgName "ghc-prim") tup_mod)
      where
        occ = mkOccName ("(#" ++ replicate n_commas ',' ++ "#)")
        tup_mod = mkModName "GHC.Tuple"

    -----------------------------------------------------
    --              Locations
    -----------------------------------------------------

    data Loc
      = Loc { loc_filename :: String
            , loc_package  :: String
            , loc_module   :: String
            , loc_start    :: CharPos
            , loc_end      :: CharPos }
       deriving( Show, Eq, Ord, Data, Typeable, Generic )

    type CharPos = (Int, Int)       -- ^ Line and character position

    -----------------------------------------------------
    --
    --      The Info returned by reification
    --
    -----------------------------------------------------

    -- | Obtained from 'reify' in the 'Q' Monad.
    data Info
      =
      -- | A class, with a list of its visible instances
      ClassI
          Dec
          [InstanceDec]

      -- | A class method
      | ClassOpI
           Name
           Type
           ParentName

      -- | A \"plain\" type constructor. \"Fancier\" type constructors are returned using 'PrimTyConI' or 'FamilyI' as appropriate
      | TyConI
            Dec

      -- | A type or data family, with a list of its visible instances. A closed
      -- type family is returned with 0 instances.
      | FamilyI
            Dec
            [InstanceDec]

      -- | A \"primitive\" type constructor, which can't be expressed with a 'Dec'. Examples: @(->)@, @Int#@.
      | PrimTyConI
           Name
           Arity
           Unlifted

      -- | A data constructor
      | DataConI
           Name
           Type
           ParentName

      {- |
      A \"value\" variable (as opposed to a type variable, see 'TyVarI').

      The @Maybe Dec@ field contains @Just@ the declaration which
      defined the variable -- including the RHS of the declaration --
      or else @Nothing@, in the case where the RHS is unavailable to
      the compiler. At present, this value is _always_ @Nothing@:
      returning the RHS has not yet been implemented because of
      lack of interest.
      -}
      | VarI
           Name
           Type
           (Maybe Dec)

      {- |
      A type variable.

      The @Type@ field contains the type which underlies the variable.
      At present, this is always @'VarT' theName@, but future changes
      may permit refinement of this.
      -}
      | TyVarI      -- Scoped type variable
            Name
            Type    -- What it is bound to
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Obtained from 'reifyModule' in the 'Q' Monad.
    data ModuleInfo =
      -- | Contains the import list of the module.
      ModuleInfo [Module]
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    {- |
    In 'ClassOpI' and 'DataConI', name of the parent class or type
    -}
    type ParentName = Name

    -- | In 'PrimTyConI', arity of the type constructor
    type Arity = Int

    -- | In 'PrimTyConI', is the type constructor unlifted?
    type Unlifted = Bool

    -- | 'InstanceDec' desribes a single instance of a class or type function.
    -- It is just a 'Dec', but guaranteed to be one of the following:
    --
    --   * 'InstanceD' (with empty @['Dec']@)
    --
    --   * 'DataInstD' or 'NewtypeInstD' (with empty derived @['Name']@)
    --
    --   * 'TySynInstD'
    type InstanceDec = Dec

    data Fixity          = Fixity Int FixityDirection
        deriving( Eq, Ord, Show, Data, Typeable, Generic )
    data FixityDirection = InfixL | InfixR | InfixN
        deriving( Eq, Ord, Show, Data, Typeable, Generic )

    -- | Highest allowed operator precedence for 'Fixity' constructor (answer: 9)
    maxPrecedence :: Int
    maxPrecedence = (9::Int)

    -- | Default fixity: @infixl 9@
    defaultFixity :: Fixity
    defaultFixity = Fixity maxPrecedence InfixL

    {-
    Note [Unresolved infix]
    ~~~~~~~~~~~~~~~~~~~~~~~
    -}
    {- $infix #infix#
    When implementing antiquotation for quasiquoters, one often wants
    to parse strings into expressions:

    > parse :: String -> Maybe Exp

    But how should we parse @a + b * c@? If we don't know the fixities of
    @+@ and @*@, we don't know whether to parse it as @a + (b * c)@ or @(a
    + b) * c@.

    In cases like this, use 'UInfixE', 'UInfixP', or 'UInfixT', which stand for
    \"unresolved infix expression/pattern/type\", respectively. When the compiler
    is given a splice containing a tree of @UInfixE@ applications such as

    > UInfixE
    >   (UInfixE e1 op1 e2)
    >   op2
    >   (UInfixE e3 op3 e4)

    it will look up and the fixities of the relevant operators and
    reassociate the tree as necessary.

      * trees will not be reassociated across 'ParensE', 'ParensP', or 'ParensT',
        which are of use for parsing expressions like

        > (a + b * c) + d * e

      * 'InfixE', 'InfixP', and 'InfixT' expressions are never reassociated.

      * The 'UInfixE' constructor doesn't support sections. Sections
        such as @(a *)@ have no ambiguity, so 'InfixE' suffices. For longer
        sections such as @(a + b * c -)@, use an 'InfixE' constructor for the
        outer-most section, and use 'UInfixE' constructors for all
        other operators:

        > InfixE
        >   Just (UInfixE ...a + b * c...)
        >   op
        >   Nothing

        Sections such as @(a + b +)@ and @((a + b) +)@ should be rendered
        into 'Exp's differently:

        > (+ a + b)   ---> InfixE Nothing + (Just $ UInfixE a + b)
        >                    -- will result in a fixity error if (+) is left-infix
        > (+ (a + b)) ---> InfixE Nothing + (Just $ ParensE $ UInfixE a + b)
        >                    -- no fixity errors

      * Quoted expressions such as

        > [| a * b + c |] :: Q Exp
        > [p| a : b : c |] :: Q Pat
        > [t| T + T |] :: Q Type

        will never contain 'UInfixE', 'UInfixP', 'UInfixT', 'InfixT', 'ParensE',
        'ParensP', or 'ParensT' constructors.

    -}

    -----------------------------------------------------
    --
    --      The main syntax data types
    --
    -----------------------------------------------------

    data Lit = CharL Char
             | StringL String
             | IntegerL Integer     -- ^ Used for overloaded and non-overloaded
                                    -- literals. We don't have a good way to
                                    -- represent non-overloaded literals at
                                    -- the moment. Maybe that doesn't matter?
             | RationalL Rational   -- Ditto
             | IntPrimL Integer
             | WordPrimL Integer
             | FloatPrimL Rational
             | DoublePrimL Rational
             | StringPrimL [Word8]  -- ^ A primitive C-style string, type Addr#
             | CharPrimL Char
        deriving( Show, Eq, Ord, Data, Typeable, Generic )

        -- We could add Int, Float, Double etc, as we do in HsLit,
        -- but that could complicate the
        -- supposedly-simple TH.Syntax literal type

    -- | Pattern in Haskell given in @{}@
    data Pat
      = LitP Lit                      -- ^ @{ 5 or \'c\' }@
      | VarP Name                     -- ^ @{ x }@
      | TupP [Pat]                    -- ^ @{ (p1,p2) }@
      | UnboxedTupP [Pat]             -- ^ @{ (\# p1,p2 \#) }@
      | ConP Name [Pat]               -- ^ @data T1 = C1 t1 t2; {C1 p1 p1} = e@
      | InfixP Pat Name Pat           -- ^ @foo ({x :+ y}) = e@
      | UInfixP Pat Name Pat          -- ^ @foo ({x :+ y}) = e@
                                      --
                                      -- See "Language.Haskell.TH.Syntax#infix"
      | ParensP Pat                   -- ^ @{(p)}@
                                      --
                                      -- See "Language.Haskell.TH.Syntax#infix"
      | TildeP Pat                    -- ^ @{ ~p }@
      | BangP Pat                     -- ^ @{ !p }@
      | AsP Name Pat                  -- ^ @{ x \@ p }@
      | WildP                         -- ^ @{ _ }@
      | RecP Name [FieldPat]          -- ^ @f (Pt { pointx = x }) = g x@
      | ListP [ Pat ]                 -- ^ @{ [1,2,3] }@
      | SigP Pat Type                 -- ^ @{ p :: t }@
      | ViewP Exp Pat                 -- ^ @{ e -> p }@
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    type FieldPat = (Name,Pat)

    data Match = Match Pat Body [Dec] -- ^ @case e of { pat -> body where decs }@
        deriving( Show, Eq, Ord, Data, Typeable, Generic )
    data Clause = Clause [Pat] Body [Dec]
                                      -- ^ @f { p1 p2 = body where decs }@
        deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Exp
      = VarE Name                          -- ^ @{ x }@
      | ConE Name                          -- ^ @data T1 = C1 t1 t2; p = {C1} e1 e2  @
      | LitE Lit                           -- ^ @{ 5 or \'c\'}@
      | AppE Exp Exp                       -- ^ @{ f x }@

      | InfixE (Maybe Exp) Exp (Maybe Exp) -- ^ @{x + y} or {(x+)} or {(+ x)} or {(+)}@

        -- It's a bit gruesome to use an Exp as the
        -- operator, but how else can we distinguish
        -- constructors from non-constructors?
        -- Maybe there should be a var-or-con type?
        -- Or maybe we should leave it to the String itself?

      | UInfixE Exp Exp Exp                -- ^ @{x + y}@
                                           --
                                           -- See "Language.Haskell.TH.Syntax#infix"
      | ParensE Exp                        -- ^ @{ (e) }@
                                           --
                                           -- See "Language.Haskell.TH.Syntax#infix"
      | LamE [Pat] Exp                     -- ^ @{ \\ p1 p2 -> e }@
      | LamCaseE [Match]                   -- ^ @{ \\case m1; m2 }@
      | TupE [Exp]                         -- ^ @{ (e1,e2) }  @
      | UnboxedTupE [Exp]                  -- ^ @{ (\# e1,e2 \#) }  @
      | CondE Exp Exp Exp                  -- ^ @{ if e1 then e2 else e3 }@
      | MultiIfE [(Guard, Exp)]            -- ^ @{ if | g1 -> e1 | g2 -> e2 }@
      | LetE [Dec] Exp                     -- ^ @{ let x=e1;   y=e2 in e3 }@
      | CaseE Exp [Match]                  -- ^ @{ case e of m1; m2 }@
      | DoE [Stmt]                         -- ^ @{ do { p <- e1; e2 }  }@
      | CompE [Stmt]                       -- ^ @{ [ (x,y) | x <- xs, y <- ys ] }@
          --
          -- The result expression of the comprehension is
          -- the /last/ of the @'Stmt'@s, and should be a 'NoBindS'.
          --
          -- E.g. translation:
          --
          -- > [ f x | x <- xs ]
          --
          -- > CompE [BindS (VarP x) (VarE xs), NoBindS (AppE (VarE f) (VarE x))]

      | ArithSeqE Range                    -- ^ @{ [ 1 ,2 .. 10 ] }@
      | ListE [ Exp ]                      -- ^ @{ [1,2,3] }@
      | SigE Exp Type                      -- ^ @{ e :: t }@
      | RecConE Name [FieldExp]            -- ^ @{ T { x = y, z = w } }@
      | RecUpdE Exp [FieldExp]             -- ^ @{ (f x) { z = w } }@
      | StaticE Exp                        -- ^ @{ static e }@
      | UnboundVarE Name                   -- ^ @{ _x }@ (hole)
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    type FieldExp = (Name,Exp)

    -- Omitted: implicit parameters

    data Body
      = GuardedB [(Guard,Exp)]   -- ^ @f p { | e1 = e2
                                     --      | e3 = e4 }
                                     -- where ds@
      | NormalB Exp              -- ^ @f p { = e } where ds@
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Guard
      = NormalG Exp -- ^ @f x { | odd x } = x@
      | PatG [Stmt] -- ^ @f x { | Just y <- x, Just z <- y } = z@
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Stmt
      = BindS Pat Exp
      | LetS [ Dec ]
      | NoBindS Exp
      | ParS [[Stmt]]
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Range = FromR Exp | FromThenR Exp Exp
               | FromToR Exp Exp | FromThenToR Exp Exp Exp
              deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Dec
      = FunD Name [Clause]            -- ^ @{ f p1 p2 = b where decs }@
      | ValD Pat Body [Dec]           -- ^ @{ p = b where decs }@
      | DataD Cxt Name [TyVarBndr]
              (Maybe Kind)            -- Kind signature (allowed only for GADTs)
              [Con] Cxt
                                      -- ^ @{ data Cxt x => T x = A x | B (T x)
                                      --       deriving (Z,W)}@
      | NewtypeD Cxt Name [TyVarBndr]
                 (Maybe Kind)         -- Kind signature
                 Con Cxt              -- ^ @{ newtype Cxt x => T x = A (B x)
                                      --       deriving (Z,W Q)}@
      | TySynD Name [TyVarBndr] Type  -- ^ @{ type T x = (x,x) }@
      | ClassD Cxt Name [TyVarBndr]
             [FunDep] [Dec]           -- ^ @{ class Eq a => Ord a where ds }@
      | InstanceD (Maybe Overlap) Cxt Type [Dec]
                                      -- ^ @{ instance {\-\# OVERLAPS \#-\}
                                      --        Show w => Show [w] where ds }@
      | SigD Name Type                -- ^ @{ length :: [a] -> Int }@
      | ForeignD Foreign              -- ^ @{ foreign import ... }
                                      --{ foreign export ... }@

      | InfixD Fixity Name            -- ^ @{ infix 3 foo }@

      -- | pragmas
      | PragmaD Pragma                -- ^ @{ {\-\# INLINE [1] foo \#-\} }@

      -- | data families (may also appear in [Dec] of 'ClassD' and 'InstanceD')
      | DataFamilyD Name [TyVarBndr]
                   (Maybe Kind)
             -- ^ @{ data family T a b c :: * }@

      | DataInstD Cxt Name [Type]
                 (Maybe Kind)         -- Kind signature
                 [Con] Cxt            -- ^ @{ data instance Cxt x => T [x]
                                      --       = A x | B (T x) deriving (Z,W)}@

      | NewtypeInstD Cxt Name [Type]
                     (Maybe Kind)     -- Kind signature
                     Con Cxt          -- ^ @{ newtype instance Cxt x => T [x]
                                      --        = A (B x) deriving (Z,W)}@
      | TySynInstD Name TySynEqn      -- ^ @{ type instance ... }@

      -- | open type families (may also appear in [Dec] of 'ClassD' and 'InstanceD')
      | OpenTypeFamilyD TypeFamilyHead
             -- ^ @{ type family T a b c = (r :: *) | r -> a b }@

      | ClosedTypeFamilyD TypeFamilyHead [TySynEqn]
           -- ^ @{ type family F a b = (r :: *) | r -> a where ... }@

      | RoleAnnotD Name [Role]        -- ^ @{ type role T nominal representational }@
      | StandaloneDerivD Cxt Type     -- ^ @{ deriving instance Ord a => Ord (Foo a) }@
      | DefaultSigD Name Type         -- ^ @{ default size :: Data a => a -> Int }@
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Varieties of allowed instance overlap.
    data Overlap = Overlappable   -- ^ May be overlapped by more specific instances
                 | Overlapping    -- ^ May overlap a more general instance
                 | Overlaps       -- ^ Both 'Overlapping' and 'Overlappable'
                 | Incoherent     -- ^ Both 'Overlappable' and 'Overlappable', and
                                  -- pick an arbitrary one if multiple choices are
                                  -- available.
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Common elements of 'OpenTypeFamilyD' and 'ClosedTypeFamilyD'.
    -- By analogy with with "head" for type classes and type class instances as
    -- defined in /Type classes: an exploration of the design space/, the
    -- @TypeFamilyHead@ is defined to be the elements of the declaration between
    -- @type family@ and @where@.
    data TypeFamilyHead =
      TypeFamilyHead Name [TyVarBndr] FamilyResultSig (Maybe InjectivityAnn)
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | One equation of a type family instance or closed type family. The
    -- arguments are the left-hand-side type patterns and the right-hand-side
    -- result.
    data TySynEqn = TySynEqn [Type] Type
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data FunDep = FunDep [Name] [Name]
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data FamFlavour = TypeFam | DataFam
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Foreign = ImportF Callconv Safety String Name Type
                 | ExportF Callconv        String Name Type
             deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- keep Callconv in sync with module ForeignCall in ghc/compiler/prelude/ForeignCall.hs
    data Callconv = CCall | StdCall | CApi | Prim | JavaScript
              deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Safety = Unsafe | Safe | Interruptible
            deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Pragma = InlineP         Name Inline RuleMatch Phases
                | SpecialiseP     Name Type (Maybe Inline) Phases
                | SpecialiseInstP Type
                | RuleP           String [RuleBndr] Exp Exp Phases
                | AnnP            AnnTarget Exp
                | LineP           Int String
            deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data Inline = NoInline
                | Inline
                | Inlinable
                deriving (Show, Eq, Ord, Data, Typeable, Generic)

    data RuleMatch = ConLike
                   | FunLike
                   deriving (Show, Eq, Ord, Data, Typeable, Generic)

    data Phases = AllPhases
                | FromPhase Int
                | BeforePhase Int
                deriving (Show, Eq, Ord, Data, Typeable, Generic)

    data RuleBndr = RuleVar Name
                  | TypedRuleVar Name Type
                  deriving (Show, Eq, Ord, Data, Typeable, Generic)

    data AnnTarget = ModuleAnnotation
                   | TypeAnnotation Name
                   | ValueAnnotation Name
                  deriving (Show, Eq, Ord, Data, Typeable, Generic)

    type Cxt = [Pred]                 -- ^ @(Eq a, Ord b)@

    -- | Since the advent of @ConstraintKinds@, constraints are really just types.
    -- Equality constraints use the 'EqualityT' constructor. Constraints may also
    -- be tuples of other constraints.
    type Pred = Type

    data SourceUnpackedness
      = NoSourceUnpackedness -- ^ @C a@
      | SourceNoUnpack       -- ^ @C { {\-\# NOUNPACK \#-\} } a@
      | SourceUnpack         -- ^ @C { {\-\# UNPACK \#-\} } a@
            deriving (Show, Eq, Ord, Data, Typeable, Generic)

    data SourceStrictness = NoSourceStrictness    -- ^ @C a@
                          | SourceLazy            -- ^ @C {~}a@
                          | SourceStrict          -- ^ @C {!}a@
            deriving (Show, Eq, Ord, Data, Typeable, Generic)

    -- | Unlike 'SourceStrictness' and 'SourceUnpackedness', 'DecidedStrictness'
    -- refers to the strictness that the compiler chooses for a data constructor
    -- field, which may be different from what is written in source code. See
    -- 'reifyConStrictness' for more information.
    data DecidedStrictness = DecidedLazy
                           | DecidedStrict
                           | DecidedUnpack
            deriving (Show, Eq, Ord, Data, Typeable, Generic)

    data Con = NormalC Name [BangType]       -- ^ @C Int a@
             | RecC Name [VarBangType]       -- ^ @C { v :: Int, w :: a }@
             | InfixC BangType Name BangType -- ^ @Int :+ a@
             | ForallC [TyVarBndr] Cxt Con   -- ^ @forall a. Eq a => C [a]@
             | GadtC [Name] [BangType]
                     Type                    -- See Note [GADT return type]
                                             -- ^ @C :: a -> b -> T b Int@
             | RecGadtC [Name] [VarBangType]
                        Type                 -- See Note [GADT return type]
                                             -- ^ @C :: { v :: Int } -> T b Int@
            deriving (Show, Eq, Ord, Data, Typeable, Generic)

    -- Note [GADT return type]
    -- ~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- The return type of a GADT constructor does not necessarily match the name of
    -- the data type:
    --
    -- type S = T
    --
    -- data T a where
    --     MkT :: S Int
    --
    --
    -- type S a = T
    --
    -- data T a where
    --     MkT :: S Char Int
    --
    --
    -- type Id a = a
    -- type S a = T
    --
    -- data T a where
    --     MkT :: Id (S Char Int)
    --
    --
    -- That is why we allow the return type stored by a constructor to be an
    -- arbitrary type. See also #11341

    data Bang = Bang SourceUnpackedness SourceStrictness
             -- ^ @C { {\-\# UNPACK \#-\} !}a@
            deriving (Show, Eq, Ord, Data, Typeable, Generic)

    type BangType    = (Bang, Type)
    type VarBangType = (Name, Bang, Type)

    -- | As of @template-haskell-2.11.0.0@, 'Strict' has been replaced by 'Bang'.
    type Strict      = Bang

    -- | As of @template-haskell-2.11.0.0@, 'StrictType' has been replaced by
    -- 'BangType'.
    type StrictType    = BangType

    -- | As of @template-haskell-2.11.0.0@, 'VarStrictType' has been replaced by
    -- 'VarBangType'.
    type VarStrictType = VarBangType

    data Type = ForallT [TyVarBndr] Cxt Type  -- ^ @forall \<vars\>. \<ctxt\> -> \<type\>@
              | AppT Type Type                -- ^ @T a b@
              | SigT Type Kind                -- ^ @t :: k@
              | VarT Name                     -- ^ @a@
              | ConT Name                     -- ^ @T@
              | PromotedT Name                -- ^ @'T@
              | InfixT Type Name Type         -- ^ @T + T@
              | UInfixT Type Name Type        -- ^ @T + T@
                                              --
                                              -- See "Language.Haskell.TH.Syntax#infix"
              | ParensT Type                  -- ^ @(T)@

              -- See Note [Representing concrete syntax in types]
              | TupleT Int                    -- ^ @(,), (,,), etc.@
              | UnboxedTupleT Int             -- ^ @(\#,\#), (\#,,\#), etc.@
              | ArrowT                        -- ^ @->@
              | EqualityT                     -- ^ @~@
              | ListT                         -- ^ @[]@
              | PromotedTupleT Int            -- ^ @'(), '(,), '(,,), etc.@
              | PromotedNilT                  -- ^ @'[]@
              | PromotedConsT                 -- ^ @(':)@
              | StarT                         -- ^ @*@
              | ConstraintT                   -- ^ @Constraint@
              | LitT TyLit                    -- ^ @0,1,2, etc.@
              | WildCardT                     -- ^ @_,
          deriving( Show, Eq, Ord, Data, Typeable, Generic )

    data TyVarBndr = PlainTV  Name            -- ^ @a@
                   | KindedTV Name Kind       -- ^ @(a :: k)@
          deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Type family result signature
    data FamilyResultSig = NoSig              -- ^ no signature
                         | KindSig  Kind      -- ^ @k@
                         | TyVarSig TyVarBndr -- ^ @= r, = (r :: k)@
          deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Injectivity annotation
    data InjectivityAnn = InjectivityAnn Name [Name]
      deriving ( Show, Eq, Ord, Data, Typeable, Generic )

    data TyLit = NumTyLit Integer             -- ^ @2@
               | StrTyLit String              -- ^ @"Hello"@
      deriving ( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Role annotations
    data Role = NominalR            -- ^ @nominal@
              | RepresentationalR   -- ^ @representational@
              | PhantomR            -- ^ @phantom@
              | InferR              -- ^ @_@
      deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | Annotation target for reifyAnnotations
    data AnnLookup = AnnLookupModule Module
                   | AnnLookupName Name
                   deriving( Show, Eq, Ord, Data, Typeable, Generic )

    -- | To avoid duplication between kinds and types, they
    -- are defined to be the same. Naturally, you would never
    -- have a type be 'StarT' and you would never have a kind
    -- be 'SigT', but many of the other constructors are shared.
    -- Note that the kind @Bool@ is denoted with 'ConT', not
    -- 'PromotedT'. Similarly, tuple kinds are made with 'TupleT',
    -- not 'PromotedTupleT'.

    type Kind = Type

    {- Note [Representing concrete syntax in types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Haskell has a rich concrete syntax for types, including
      t1 -> t2, (t1,t2), [t], and so on
    In TH we represent all of this using AppT, with a distinguished
    type constructor at the head.  So,
      Type              TH representation
      -----------------------------------------------
      t1 -> t2          ArrowT `AppT` t2 `AppT` t2
      [t]               ListT `AppT` t
      (t1,t2)           TupleT 2 `AppT` t1 `AppT` t2
      '(t1,t2)          PromotedTupleT 2 `AppT` t1 `AppT` t2

    But if the original HsSyn used prefix application, we won't use
    these special TH constructors.  For example
      [] t              ConT "[]" `AppT` t
      (->) t            ConT "->" `AppT` t
    In this way we can faithfully represent in TH whether the original
    HsType used concrete syntax or not.

    The one case that doesn't fit this pattern is that of promoted lists
      '[ Maybe, IO ]    PromotedListT 2 `AppT` t1 `AppT` t2
    but it's very smelly because there really is no type constructor
    corresponding to PromotedListT. So we encode HsExplicitListTy with
    PromotedConsT and PromotedNilT (which *do* have underlying type
    constructors):
      '[ Maybe, IO ]    PromotedConsT `AppT` Maybe `AppT`
                        (PromotedConsT  `AppT` IO `AppT` PromotedNilT)
    -}

    -----------------------------------------------------
    --              Internal helper functions
    -----------------------------------------------------

    cmpEq :: Ordering -> Bool
    cmpEq EQ = True
    cmpEq _  = False

    thenCmp :: Ordering -> Ordering -> Ordering
    thenCmp EQ o2 = o2
    thenCmp o1 _  = o1