content: {-# 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. Alternatively the @DeriveAnyClass@ language extension can be
  -- used to derive Encode:
  --
  -- @
  -- {-# LANGUAGE DeriveAnyClass #-}
  -- data Tree a = Leaf a | Node (Tree a) (Tree a)
  --   deriving (Generic, Encode)
  -- @
  --
  -- 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
  