    {-# LANGUAGE Safe #-}

    {-# LANGUAGE ExplicitNamespaces #-}

    -- |
    -- Module      :  GHC.Generics
    -- Copyright   :  (c) Universiteit Utrecht 2010-2011, University of Oxford 2012-2014
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable
    --
    -- @since 4.6.0.0
    --
    -- If you're using @GHC.Generics@, you should consider using the
    -- <http://hackage.haskell.org/package/generic-deriving> package, which
    -- contains many useful generic functions.

    module GHC.Generics  (
    -- * Introduction
    --
    -- |
    --
    -- Datatype-generic functions are based on the idea of converting values of
    -- a datatype @T@ into corresponding values of a (nearly) isomorphic type @'Rep' T@.
    -- The type @'Rep' T@ is
    -- built from a limited set of type constructors, all provided by this module. A
    -- datatype-generic function is then an overloaded function with instances
    -- for most of these type constructors, together with a wrapper that performs
    -- the mapping between @T@ and @'Rep' T@. By using this technique, we merely need
    -- a few generic instances in order to implement functionality that works for any
    -- representable type.
    --
    -- Representable types are collected in the 'Generic' class, which defines the
    -- associated type 'Rep' as well as conversion functions 'from' and 'to'.
    -- Typically, you will not define 'Generic' instances by hand, but have the compiler
    -- derive them for you.

    -- ** Representing datatypes
    --
    -- |
    --
    -- The key to defining your own datatype-generic functions is to understand how to
    -- represent datatypes using the given set of type constructors.
    --
    -- Let us look at an example first:
    --
    -- @
    -- data Tree a = Leaf a | Node (Tree a) (Tree a)
    --   deriving 'Generic'
    -- @
    --
    -- The above declaration (which requires the language pragma @DeriveGeneric@)
    -- causes the following representation to be generated:
    --
    -- @
    -- instance 'Generic' (Tree a) where
    --   type 'Rep' (Tree a) =
    --     'D1' ('MetaData \"Tree\" \"Main\" \"package-name\" 'False)
    --       ('C1' ('MetaCons \"Leaf\" 'PrefixI 'False)
    --          ('S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --                 ('Rec0' a))
    --        ':+:'
    --        'C1' ('MetaCons \"Node\" 'PrefixI 'False)
    --          ('S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --                ('Rec0' (Tree a))
    --           ':*:'
    --           'S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --                ('Rec0' (Tree a))))
    --   ...
    -- @
    --
    -- /Hint:/ You can obtain information about the code being generated from GHC by passing
    -- the @-ddump-deriv@ flag. In GHCi, you can expand a type family such as 'Rep' using
    -- the @:kind!@ command.
    --
    -- This is a lot of information! However, most of it is actually merely meta-information
    -- that makes names of datatypes and constructors and more available on the type level.
    --
    -- Here is a reduced representation for @Tree@ with nearly all meta-information removed,
    -- for now keeping only the most essential aspects:
    --
    -- @
    -- instance 'Generic' (Tree a) where
    --   type 'Rep' (Tree a) =
    --     'Rec0' a
    --     ':+:'
    --     ('Rec0' (Tree a) ':*:' 'Rec0' (Tree a))
    -- @
    --
    -- The @Tree@ datatype has two constructors. The representation of individual constructors
    -- is combined using the binary type constructor ':+:'.
    --
    -- The first constructor consists of a single field, which is the parameter @a@. This is
    -- represented as @'Rec0' a@.
    --
    -- The second constructor consists of two fields. Each is a recursive field of type @Tree a@,
    -- represented as @'Rec0' (Tree a)@. Representations of individual fields are combined using
    -- the binary type constructor ':*:'.
    --
    -- Now let us explain the additional tags being used in the complete representation:
    --
    --    * The @'S1' ('MetaSel 'Nothing 'NoSourceUnpackedness 'NoSourceStrictness
    --      'DecidedLazy)@ tag indicates several things. The @'Nothing@ indicates
    --      that there is no record field selector associated with this field of
    --      the constructor (if there were, it would have been marked @'Just
    --      \"recordName\"@ instead). The other types contain meta-information on
    --      the field's strictness:
    --
    --      * There is no @{\-\# UNPACK \#-\}@ or @{\-\# NOUNPACK \#-\}@ annotation
    --        in the source, so it is tagged with @'NoSourceUnpackedness@.
    --
    --      * There is no strictness (@!@) or laziness (@~@) annotation in the
    --        source, so it is tagged with @'NoSourceStrictness@.
    --
    --      * The compiler infers that the field is lazy, so it is tagged with
    --        @'DecidedLazy@. Bear in mind that what the compiler decides may be
    --        quite different from what is written in the source. See
    --        'DecidedStrictness' for a more detailed explanation.
    --
    --      The @'MetaSel@ type is also an instance of the type class 'Selector',
    --      which can be used to obtain information about the field at the value
    --      level.
    --
    --    * The @'C1' ('MetaCons \"Leaf\" 'PrefixI 'False)@ and
    --      @'C1' ('MetaCons \"Node\" 'PrefixI 'False)@ invocations indicate that the enclosed part is
    --      the representation of the first and second constructor of datatype @Tree@, respectively.
    --      Here, the meta-information regarding constructor names, fixity and whether
    --      it has named fields or not is encoded at the type level. The @'MetaCons@
    --      type is also an instance of the type class 'Constructor'. This type class can be used
    --      to obtain information about the constructor at the value level.
    --
    --    * The @'D1' ('MetaData \"Tree\" \"Main\" \"package-name\" 'False)@ tag
    --      indicates that the enclosed part is the representation of the
    --      datatype @Tree@. Again, the meta-information is encoded at the type level.
    --      The @'MetaData@ type is an instance of class 'Datatype', which
    --      can be used to obtain the name of a datatype, the module it has been
    --      defined in, the package it is located under, and whether it has been
    --      defined using @data@ or @newtype@ at the value level.

    -- ** Derived and fundamental representation types
    --
    -- |
    --
    -- There are many datatype-generic functions that do not distinguish between positions that
    -- are parameters or positions that are recursive calls. There are also many datatype-generic
    -- functions that do not care about the names of datatypes and constructors at all. To keep
    -- the number of cases to consider in generic functions in such a situation to a minimum,
    -- it turns out that many of the type constructors introduced above are actually synonyms,
    -- defining them to be variants of a smaller set of constructors.

    -- *** Individual fields of constructors: 'K1'
    --
    -- |
    --
    -- The type constructor 'Rec0' is a variant of 'K1':
    --
    -- @
    -- type 'Rec0' = 'K1' 'R'
    -- @
    --
    -- Here, 'R' is a type-level proxy that does not have any associated values.
    --
    -- There used to be another variant of 'K1' (namely @Par0@), but it has since
    -- been deprecated.

    -- *** Meta information: 'M1'
    --
    -- |
    --
    -- The type constructors 'S1', 'C1' and 'D1' are all variants of 'M1':
    --
    -- @
    -- type 'S1' = 'M1' 'S'
    -- type 'C1' = 'M1' 'C'
    -- type 'D1' = 'M1' 'D'
    -- @
    --
    -- The types 'S', 'C' and 'D' are once again type-level proxies, just used to create
    -- several variants of 'M1'.

    -- *** Additional generic representation type constructors
    --
    -- |
    --
    -- Next to 'K1', 'M1', ':+:' and ':*:' there are a few more type constructors that occur
    -- in the representations of other datatypes.

    -- **** Empty datatypes: 'V1'
    --
    -- |
    --
    -- For empty datatypes, 'V1' is used as a representation. For example,
    --
    -- @
    -- data Empty deriving 'Generic'
    -- @
    --
    -- yields
    --
    -- @
    -- instance 'Generic' Empty where
    --   type 'Rep' Empty =
    --     'D1' ('MetaData \"Empty\" \"Main\" \"package-name\" 'False) 'V1'
    -- @

    -- **** Constructors without fields: 'U1'
    --
    -- |
    --
    -- If a constructor has no arguments, then 'U1' is used as its representation. For example
    -- the representation of 'Bool' is
    --
    -- @
    -- instance 'Generic' Bool where
    --   type 'Rep' Bool =
    --     'D1' ('MetaData \"Bool\" \"Data.Bool\" \"package-name\" 'False)
    --       ('C1' ('MetaCons \"False\" 'PrefixI 'False) 'U1' ':+:' 'C1' ('MetaCons \"True\" 'PrefixI 'False) 'U1')
    -- @

    -- *** Representation of types with many constructors or many fields
    --
    -- |
    --
    -- As ':+:' and ':*:' are just binary operators, one might ask what happens if the
    -- datatype has more than two constructors, or a constructor with more than two
    -- fields. The answer is simple: the operators are used several times, to combine
    -- all the constructors and fields as needed. However, users /should not rely on
    -- a specific nesting strategy/ for ':+:' and ':*:' being used. The compiler is
    -- free to choose any nesting it prefers. (In practice, the current implementation
    -- tries to produce a more-or-less balanced nesting, so that the traversal of
    -- the structure of the datatype from the root to a particular component can be
    -- performed in logarithmic rather than linear time.)

    -- ** Defining datatype-generic functions
    --
    -- |
    --
    -- A datatype-generic function comprises two parts:
    --
    --    1. /Generic instances/ for the function, implementing it for most of the representation
    --       type constructors introduced above.
    --
    --    2. A /wrapper/ that for any datatype that is in `Generic`, performs the conversion
    --       between the original value and its `Rep`-based representation and then invokes the
    --       generic instances.
    --
    -- As an example, let us look at a function @encode@ that produces a naive, but lossless
    -- bit encoding of values of various datatypes. So we are aiming to define a function
    --
    -- @
    -- encode :: 'Generic' a => a -> [Bool]
    -- @
    --
    -- where we use 'Bool' as our datatype for bits.
    --
    -- For part 1, we define a class @Encode'@. Perhaps surprisingly, this class is parameterized
    -- over a type constructor @f@ of kind @* -> *@. This is a technicality: all the representation
    -- type constructors operate with kind @* -> *@ as base kind. But the type argument is never
    -- being used. This may be changed at some point in the future. The class has a single method,
    -- and we use the type we want our final function to have, but we replace the occurrences of
    -- the generic type argument @a@ with @f p@ (where the @p@ is any argument; it will not be used).
    --
    -- > class Encode' f where
    -- >   encode' :: f p -> [Bool]
    --
    -- With the goal in mind to make @encode@ work on @Tree@ and other datatypes, we now define
    -- instances for the representation type constructors 'V1', 'U1', ':+:', ':*:', 'K1', and 'M1'.

    -- *** Definition of the generic representation types
    --
    -- |
    --
    -- In order to be able to do this, we need to know the actual definitions of these types:
    --
    -- @
    -- data    'V1'        p                       -- lifted version of Empty
    -- data    'U1'        p = 'U1'                  -- lifted version of ()
    -- data    (':+:') f g p = 'L1' (f p) | 'R1' (g p) -- lifted version of 'Either'
    -- data    (':*:') f g p = (f p) ':*:' (g p)     -- lifted version of (,)
    -- newtype 'K1'    i c p = 'K1' { 'unK1' :: c }    -- a container for a c
    -- newtype 'M1'  i t f p = 'M1' { 'unM1' :: f p }  -- a wrapper
    -- @
    --
    -- So, 'U1' is just the unit type, ':+:' is just a binary choice like 'Either',
    -- ':*:' is a binary pair like the pair constructor @(,)@, and 'K1' is a value
    -- of a specific type @c@, and 'M1' wraps a value of the generic type argument,
    -- which in the lifted world is an @f p@ (where we do not care about @p@).

    -- *** Generic instances
    --
    -- |
    --
    -- To deal with the 'V1' case, we use the following code (which requires the pragma @EmptyCase@):
    --
    -- @
    -- instance Encode' 'V1' where
    --   encode' x = case x of { }
    -- @
    --
    -- There are no values of type @V1 p@ to pass, so it is impossible for this
    -- function to be invoked. One can ask why it is useful to define an instance
    -- for 'V1' at all in this case? Well, an empty type can be used as an argument
    -- to a non-empty type, and you might still want to encode the resulting type.
    -- As a somewhat contrived example, consider @[Empty]@, which is not an empty
    -- type, but contains just the empty list. The 'V1' instance ensures that we
    -- can call the generic function on such types.
    --
    -- There is exactly one value of type 'U1', so encoding it requires no
    -- knowledge, and we can use zero bits:
    --
    -- @
    -- instance Encode' 'U1' where
    --   encode' 'U1' = []
    -- @
    --
    -- In the case for ':+:', we produce 'False' or 'True' depending on whether
    -- the constructor of the value provided is located on the left or on the right:
    --
    -- @
    -- instance (Encode' f, Encode' g) => Encode' (f ':+:' g) where
    --   encode' ('L1' x) = False : encode' x
    --   encode' ('R1' x) = True  : encode' x
    -- @
    --
    -- (Note that this encoding strategy may not be reliable across different
    -- versions of GHC. Recall that the compiler is free to choose any nesting
    -- of ':+:' it chooses, so if GHC chooses @(a ':+:' b) ':+:' c@, then the
    -- encoding for @a@ would be @[False, False]@, @b@ would be @[False, True]@,
    -- and @c@ would be @[True]@. However, if GHC chooses @a ':+:' (b ':+:' c)@,
    -- then the encoding for @a@ would be @[False]@, @b@ would be @[True, False]@,
    -- and @c@ would be @[True, True]@.)
    --
    -- In the case for ':*:', we append the encodings of the two subcomponents:
    --
    -- @
    -- instance (Encode' f, Encode' g) => Encode' (f ':*:' g) where
    --   encode' (x ':*:' y) = encode' x ++ encode' y
    -- @
    --
    -- The case for 'K1' is rather interesting. Here, we call the final function
    -- @encode@ that we yet have to define, recursively. We will use another type
    -- class @Encode@ for that function:
    --
    -- @
    -- instance (Encode c) => Encode' ('K1' i c) where
    --   encode' ('K1' x) = encode x
    -- @
    --
    -- Note how we can define a uniform instance for 'M1', because we completely
    -- disregard all meta-information:
    --
    -- @
    -- instance (Encode' f) => Encode' ('M1' i t f) where
    --   encode' ('M1' x) = encode' x
    -- @
    --
    -- Unlike in 'K1', the instance for 'M1' refers to @encode'@, not @encode@.

    -- *** The wrapper and generic default
    --
    -- |
    --
    -- We now define class @Encode@ for the actual @encode@ function:
    --
    -- @
    -- class Encode a where
    --   encode :: a -> [Bool]
    --   default encode :: (Generic a, Encode' (Rep a)) => a -> [Bool]
    --   encode x = encode' ('from' x)
    -- @
    --
    -- The incoming @x@ is converted using 'from', then we dispatch to the
    -- generic instances using @encode'@. We use this as a default definition
    -- for @encode@. We need the @default encode@ signature because ordinary
    -- Haskell default methods must not introduce additional class constraints,
    -- but our generic default does.
    --
    -- Defining a particular instance is now as simple as saying
    --
    -- @
    -- instance (Encode a) => Encode (Tree a)
    -- @
    --
    -- The generic default is being used. In the future, it will hopefully be
    -- possible to use @deriving Encode@ as well, but GHC does not yet support
    -- that syntax for this situation.
    --
    -- Having @Encode@ as a class has the advantage that we can define
    -- non-generic special cases, which is particularly useful for abstract
    -- datatypes that have no structural representation. For example, given
    -- a suitable integer encoding function @encodeInt@, we can define
    --
    -- @
    -- instance Encode Int where
    --   encode = encodeInt
    -- @

    -- *** Omitting generic instances
    --
    -- |
    --
    -- It is not always required to provide instances for all the generic
    -- representation types, but omitting instances restricts the set of
    -- datatypes the functions will work for:
    --
    --    * If no ':+:' instance is given, the function may still work for
    --      empty datatypes or datatypes that have a single constructor,
    --      but will fail on datatypes with more than one constructor.
    --
    --    * If no ':*:' instance is given, the function may still work for
    --      datatypes where each constructor has just zero or one field,
    --      in particular for enumeration types.
    --
    --    * If no 'K1' instance is given, the function may still work for
    --      enumeration types, where no constructor has any fields.
    --
    --    * If no 'V1' instance is given, the function may still work for
    --      any datatype that is not empty.
    --
    --    * If no 'U1' instance is given, the function may still work for
    --      any datatype where each constructor has at least one field.
    --
    -- An 'M1' instance is always required (but it can just ignore the
    -- meta-information, as is the case for @encode@ above).
    --
    -- ** Generic constructor classes
    --
    -- |
    --
    -- Datatype-generic functions as defined above work for a large class
    -- of datatypes, including parameterized datatypes. (We have used @Tree@
    -- as our example above, which is of kind @* -> *@.) However, the
    -- 'Generic' class ranges over types of kind @*@, and therefore, the
    -- resulting generic functions (such as @encode@) must be parameterized
    -- by a generic type argument of kind @*@.
    --
    -- What if we want to define generic classes that range over type
    -- constructors (such as 'Data.Functor.Functor',
    -- 'Data.Traversable.Traversable', or 'Data.Foldable.Foldable')?

    -- *** The 'Generic1' class
    --
    -- |
    --
    -- Like 'Generic', there is a class 'Generic1' that defines a
    -- representation 'Rep1' and conversion functions 'from1' and 'to1',
    -- only that 'Generic1' ranges over types of kind @* -> *@. (More generally,
    -- it can range over types of kind @k -> *@, for any kind @k@, if the
    -- @PolyKinds@ extension is enabled. More on this later.)
    -- The 'Generic1' class is also derivable.
    --
    -- The representation 'Rep1' is ever so slightly different from 'Rep'.
    -- Let us look at @Tree@ as an example again:
    --
    -- @
    -- data Tree a = Leaf a | Node (Tree a) (Tree a)
    --   deriving 'Generic1'
    -- @
    --
    -- The above declaration causes the following representation to be generated:
    --
    -- @
    -- instance 'Generic1' Tree where
    --   type 'Rep1' Tree =
    --     'D1' ('MetaData \"Tree\" \"Main\" \"package-name\" 'False)
    --       ('C1' ('MetaCons \"Leaf\" 'PrefixI 'False)
    --          ('S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --                'Par1')
    --        ':+:'
    --        'C1' ('MetaCons \"Node\" 'PrefixI 'False)
    --          ('S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --                ('Rec1' Tree)
    --           ':*:'
    --           'S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --                ('Rec1' Tree)))
    --   ...
    -- @
    --
    -- The representation reuses 'D1', 'C1', 'S1' (and thereby 'M1') as well
    -- as ':+:' and ':*:' from 'Rep'. (This reusability is the reason that we
    -- carry around the dummy type argument for kind-@*@-types, but there are
    -- already enough different names involved without duplicating each of
    -- these.)
    --
    -- What's different is that we now use 'Par1' to refer to the parameter
    -- (and that parameter, which used to be @a@), is not mentioned explicitly
    -- by name anywhere; and we use 'Rec1' to refer to a recursive use of @Tree a@.

    -- *** Representation of @* -> *@ types
    --
    -- |
    --
    -- Unlike 'Rec0', the 'Par1' and 'Rec1' type constructors do not
    -- map to 'K1'. They are defined directly, as follows:
    --
    -- @
    -- newtype 'Par1'   p = 'Par1' { 'unPar1' ::   p } -- gives access to parameter p
    -- newtype 'Rec1' f p = 'Rec1' { 'unRec1' :: f p } -- a wrapper
    -- @
    --
    -- In 'Par1', the parameter @p@ is used for the first time, whereas 'Rec1' simply
    -- wraps an application of @f@ to @p@.
    --
    -- Note that 'K1' (in the guise of 'Rec0') can still occur in a 'Rep1' representation,
    -- namely when the datatype has a field that does not mention the parameter.
    --
    -- The declaration
    --
    -- @
    -- data WithInt a = WithInt Int a
    --   deriving 'Generic1'
    -- @
    --
    -- yields
    --
    -- @
    -- instance 'Generic1' WithInt where
    --   type 'Rep1' WithInt =
    --     'D1' ('MetaData \"WithInt\" \"Main\" \"package-name\" 'False)
    --       ('C1' ('MetaCons \"WithInt\" 'PrefixI 'False)
    --         ('S1' ('MetaSel 'Nothing
    --                         'NoSourceUnpackedness
    --                         'NoSourceStrictness
    --                         'DecidedLazy)
    --               ('Rec0' Int)
    --          ':*:'
    --          'S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --               'Par1'))
    -- @
    --
    -- If the parameter @a@ appears underneath a composition of other type constructors,
    -- then the representation involves composition, too:
    --
    -- @
    -- data Rose a = Fork a [Rose a]
    -- @
    --
    -- yields
    --
    -- @
    -- instance 'Generic1' Rose where
    --   type 'Rep1' Rose =
    --     'D1' ('MetaData \"Rose\" \"Main\" \"package-name\" 'False)
    --       ('C1' ('MetaCons \"Fork\" 'PrefixI 'False)
    --         ('S1' ('MetaSel 'Nothing
    --                         'NoSourceUnpackedness
    --                         'NoSourceStrictness
    --                         'DecidedLazy)
    --               'Par1'
    --          ':*:'
    --          'S1' ('MetaSel 'Nothing
    --                          'NoSourceUnpackedness
    --                          'NoSourceStrictness
    --                          'DecidedLazy)
    --               ([] ':.:' 'Rec1' Rose)))
    -- @
    --
    -- where
    --
    -- @
    -- newtype (':.:') f g p = 'Comp1' { 'unComp1' :: f (g p) }
    -- @

    -- *** Representation of @k -> *@ types
    --
    -- |
    --
    -- The 'Generic1' class can be generalized to range over types of kind
    -- @k -> *@, for any kind @k@. To do so, derive a 'Generic1' instance with the
    -- @PolyKinds@ extension enabled. For example, the declaration
    --
    -- @
    -- data Proxy (a :: k) = Proxy deriving 'Generic1'
    -- @
    --
    -- yields a slightly different instance depending on whether @PolyKinds@ is
    -- enabled. If compiled without @PolyKinds@, then @'Rep1' Proxy :: * -> *@, but
    -- if compiled with @PolyKinds@, then @'Rep1' Proxy :: k -> *@.

    -- *** Representation of unlifted types
    --
    -- |
    --
    -- If one were to attempt to derive a Generic instance for a datatype with an
    -- unlifted argument (for example, 'Int#'), one might expect the occurrence of
    -- the 'Int#' argument to be marked with @'Rec0' 'Int#'@. This won't work,
    -- though, since 'Int#' is of an unlifted kind, and 'Rec0' expects a type of
    -- kind @*@.
    --
    -- One solution would be to represent an occurrence of 'Int#' with 'Rec0 Int'
    -- instead. With this approach, however, the programmer has no way of knowing
    -- whether the 'Int' is actually an 'Int#' in disguise.
    --
    -- Instead of reusing 'Rec0', a separate data family 'URec' is used to mark
    -- occurrences of common unlifted types:
    --
    -- @
    -- data family URec a p
    --
    -- data instance 'URec' ('Ptr' ()) p = 'UAddr'   { 'uAddr#'   :: 'Addr#'   }
    -- data instance 'URec' 'Char'     p = 'UChar'   { 'uChar#'   :: 'Char#'   }
    -- data instance 'URec' 'Double'   p = 'UDouble' { 'uDouble#' :: 'Double#' }
    -- data instance 'URec' 'Int'      p = 'UFloat'  { 'uFloat#'  :: 'Float#'  }
    -- data instance 'URec' 'Float'    p = 'UInt'    { 'uInt#'    :: 'Int#'    }
    -- data instance 'URec' 'Word'     p = 'UWord'   { 'uWord#'   :: 'Word#'   }
    -- @
    --
    -- Several type synonyms are provided for convenience:
    --
    -- @
    -- type 'UAddr'   = 'URec' ('Ptr' ())
    -- type 'UChar'   = 'URec' 'Char'
    -- type 'UDouble' = 'URec' 'Double'
    -- type 'UFloat'  = 'URec' 'Float'
    -- type 'UInt'    = 'URec' 'Int'
    -- type 'UWord'   = 'URec' 'Word'
    -- @
    --
    -- The declaration
    --
    -- @
    -- data IntHash = IntHash Int#
    --   deriving 'Generic'
    -- @
    --
    -- yields
    --
    -- @
    -- instance 'Generic' IntHash where
    --   type 'Rep' IntHash =
    --     'D1' ('MetaData \"IntHash\" \"Main\" \"package-name\" 'False)
    --       ('C1' ('MetaCons \"IntHash\" 'PrefixI 'False)
    --         ('S1' ('MetaSel 'Nothing
    --                         'NoSourceUnpackedness
    --                         'NoSourceStrictness
    --                         'DecidedLazy)
    --               'UInt'))
    -- @
    --
    -- Currently, only the six unlifted types listed above are generated, but this
    -- may be extended to encompass more unlifted types in the future.

      -- * Generic representation types
        V1, U1(..), Par1(..), Rec1(..), K1(..), M1(..)
      , (:+:)(..), (:*:)(..), (:.:)(..)

      -- ** Unboxed representation types
      , URec(..)
      , type UAddr, type UChar, type UDouble
      , type UFloat, type UInt, type UWord

      -- ** Synonyms for convenience
      , Rec0, R
      , D1, C1, S1, D, C, S

      -- * Meta-information
      , Datatype(..), Constructor(..), Selector(..)
      , Fixity(..), FixityI(..), Associativity(..), prec
      , SourceUnpackedness(..), SourceStrictness(..), DecidedStrictness(..)
      , Meta(..)

      -- * Generic type classes
      , Generic(..)
      , Generic1(..)

      -- * Generic wrapper
      , Generically(..)
      , Generically1(..)
      ) where

    import GHC.Internal.Generics