{-# LANGUAGE Trustworthy #-} {-# LANGUAGE CPP #-} {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE TypeSynonymInstances #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE DeriveGeneric #-} ----------------------------------------------------------------------------- -- | -- Module : GHC.Generics -- Copyright : (c) Universiteit Utrecht 2010-2011, University of Oxford 2012-2013 -- 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 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' D1Tree -- ('C1' C1_0Tree -- ('S1' 'NoSelector' ('Par0' a)) -- ':+:' -- 'C1' C1_1Tree -- ('S1' 'NoSelector' ('Rec0' (Tree a)) -- ':*:' -- 'S1' 'NoSelector' ('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. -- #if 0 -- /TODO:/ Newer GHC versions abandon the distinction between 'Par0' and 'Rec0' and will -- use 'Rec0' everywhere. -- #endif -- 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) = -- 'Par0' 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 @'Par0' 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' 'NoSelector'@ indicates that there is no record field selector associated with -- this field of the constructor. -- -- * The @'C1' C1_0Tree@ and @'C1' C1_1Tree@ invocations indicate that the enclosed part is -- the representation of the first and second constructor of datatype @Tree@, respectively. -- Here, @C1_0Tree@ and @C1_1Tree@ are datatypes generated by the compiler as part of -- @deriving 'Generic'@. These datatypes are proxy types with no values. They are useful -- because they are instances of the type class 'Constructor'. This type class can be used -- to obtain information about the constructor in question, such as its name -- or infix priority. -- -- * The @'D1' D1Tree@ tag indicates that the enclosed part is the representation of the -- datatype @Tree@. Again, @D1Tree@ is a datatype generated by the compiler. It is a -- proxy type, and is useful by being an instance of class 'Datatype', which -- can be used to obtain the name of a datatype, the module it has been defined in, and -- whether it has been defined using @data@ or @newtype@. -- ** 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 constructors 'Par0' and 'Rec0' are variants of 'K1': -- -- @ -- type 'Par0' = 'K1' 'P' -- type 'Rec0' = 'K1' 'R' -- @ -- -- Here, 'P' and 'R' are type-level proxies again that do not have any associated values. -- *** 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 'R' 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' D1Empty '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' D1Bool -- ('C1' C1_0Bool 'U1' ':+:' 'C1' C1_1Bool '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 -- -- | -- -- The instance for 'V1' is slightly awkward (but also rarely used): -- -- @ -- instance Encode' 'V1' where -- encode' x = undefined -- @ -- -- There are no values of type @V1 p@ to pass (except undefined), so this is -- actually impossible. 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 -- @ -- -- 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 'Par0' and 'Rec0' both being mapped to 'K1' allows us to define -- a uniform instance here. -- -- Similarly, 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) => 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) -- @ -- #if 0 -- /TODO:/ Add usage example? -- #endif -- 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). #if 0 -- *** Using meta-information -- -- | -- -- TODO #endif -- ** 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 'Functor', 'Traversable', or '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 @* -> *@. -- 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' D1Tree -- ('C1' C1_0Tree -- ('S1' 'NoSelector' 'Par1') -- ':+:' -- 'C1' C1_1Tree -- ('S1' 'NoSelector' ('Rec1' Tree) -- ':*:' -- 'S1' 'NoSelector' ('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 'Par0' and '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 -- -- @ -- class 'Rep1' WithInt where -- type 'Rep1' WithInt = -- 'D1' D1WithInt -- ('C1' C1_0WithInt -- ('S1' 'NoSelector' ('Rec0' Int) -- ':*:' -- 'S1' 'NoSelector' '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 -- -- @ -- class 'Rep1' Rose where -- type 'Rep1' Rose = -- 'D1' D1Rose -- ('C1' C1_0Rose -- ('S1' 'NoSelector' 'Par1' -- ':*:' -- 'S1' 'NoSelector' ([] ':.:' 'Rec1' Rose) -- @ -- -- where -- -- @ -- newtype (':.:') f g p = 'Comp1' { 'unComp1' :: f (g p) } -- @ #if 0 -- *** Limitations -- -- | -- -- /TODO/ -- -- /TODO:/ Also clear up confusion about 'Rec0' and 'Rec1' not really indicating recursion. -- #endif ----------------------------------------------------------------------------- -- * Generic representation types V1, U1(..), Par1(..), Rec1(..), K1(..), M1(..) , (:+:)(..), (:*:)(..), (:.:)(..) -- ** Synonyms for convenience , Rec0, Par0, R, P , D1, C1, S1, D, C, S -- * Meta-information , Datatype(..), Constructor(..), Selector(..), NoSelector , Fixity(..), Associativity(..), Arity(..), prec -- * Generic type classes , Generic(..), Generic1(..) ) where -- We use some base types import GHC.Types import Data.Maybe ( Maybe(..) ) import Data.Either ( Either(..) ) -- Needed for instances import GHC.Classes ( Eq, Ord ) import GHC.Read ( Read ) import GHC.Show ( Show ) import Data.Proxy -------------------------------------------------------------------------------- -- Representation types -------------------------------------------------------------------------------- -- | Void: used for datatypes without constructors data V1 p -- | Unit: used for constructors without arguments data U1 p = U1 deriving (Eq, Ord, Read, Show, Generic) -- | Used for marking occurrences of the parameter newtype Par1 p = Par1 { unPar1 :: p } deriving (Eq, Ord, Read, Show, Generic) -- | Recursive calls of kind * -> * newtype Rec1 f p = Rec1 { unRec1 :: f p } deriving (Eq, Ord, Read, Show, Generic) -- | Constants, additional parameters and recursion of kind * newtype K1 i c p = K1 { unK1 :: c } deriving (Eq, Ord, Read, Show, Generic) -- | Meta-information (constructor names, etc.) newtype M1 i c f p = M1 { unM1 :: f p } deriving (Eq, Ord, Read, Show, Generic) -- | Sums: encode choice between constructors infixr 5 :+: data (:+:) f g p = L1 (f p) | R1 (g p) deriving (Eq, Ord, Read, Show, Generic) -- | Products: encode multiple arguments to constructors infixr 6 :*: data (:*:) f g p = f p :*: g p deriving (Eq, Ord, Read, Show, Generic) -- | Composition of functors infixr 7 :.: newtype (:.:) f g p = Comp1 { unComp1 :: f (g p) } deriving (Eq, Ord, Read, Show, Generic) -- | Tag for K1: recursion (of kind *) data R -- | Tag for K1: parameters (other than the last) data P -- | Type synonym for encoding recursion (of kind *) type Rec0 = K1 R -- | Type synonym for encoding parameters (other than the last) type Par0 = K1 P {-# DEPRECATED Par0 "'Par0' is no longer used; use 'Rec0' instead" #-} -- deprecated in 7.6 {-# DEPRECATED P "'P' is no longer used; use 'R' instead" #-} -- deprecated in 7.6 -- | Tag for M1: datatype data D -- | Tag for M1: constructor data C -- | Tag for M1: record selector data S -- | Type synonym for encoding meta-information for datatypes type D1 = M1 D -- | Type synonym for encoding meta-information for constructors type C1 = M1 C -- | Type synonym for encoding meta-information for record selectors type S1 = M1 S -- | Class for datatypes that represent datatypes class Datatype d where -- | The name of the datatype (unqualified) datatypeName :: t d (f :: * -> *) a -> [Char] -- | The fully-qualified name of the module where the type is declared moduleName :: t d (f :: * -> *) a -> [Char] -- | Marks if the datatype is actually a newtype isNewtype :: t d (f :: * -> *) a -> Bool isNewtype _ = False -- | Class for datatypes that represent records class Selector s where -- | The name of the selector selName :: t s (f :: * -> *) a -> [Char] -- | Used for constructor fields without a name data NoSelector instance Selector NoSelector where selName _ = "" -- | Class for datatypes that represent data constructors class Constructor c where -- | The name of the constructor conName :: t c (f :: * -> *) a -> [Char] -- | The fixity of the constructor conFixity :: t c (f :: * -> *) a -> Fixity conFixity _ = Prefix -- | Marks if this constructor is a record conIsRecord :: t c (f :: * -> *) a -> Bool conIsRecord _ = False -- | Datatype to represent the arity of a tuple. data Arity = NoArity | Arity Int deriving (Eq, Show, Ord, Read, Generic) -- | Datatype to represent the fixity of a constructor. An infix -- | declaration directly corresponds to an application of 'Infix'. data Fixity = Prefix | Infix Associativity Int deriving (Eq, Show, Ord, Read, Generic) -- | Get the precedence of a fixity value. prec :: Fixity -> Int prec Prefix = 10 prec (Infix _ n) = n -- | Datatype to represent the associativity of a constructor data Associativity = LeftAssociative | RightAssociative | NotAssociative deriving (Eq, Show, Ord, Read, Generic) -- | Representable types of kind *. -- This class is derivable in GHC with the DeriveGeneric flag on. class Generic a where -- | Generic representation type type Rep a :: * -> * -- | Convert from the datatype to its representation from :: a -> (Rep a) x -- | Convert from the representation to the datatype to :: (Rep a) x -> a -- | Representable types of kind * -> *. -- This class is derivable in GHC with the DeriveGeneric flag on. class Generic1 f where -- | Generic representation type type Rep1 f :: * -> * -- | Convert from the datatype to its representation from1 :: f a -> (Rep1 f) a -- | Convert from the representation to the datatype to1 :: (Rep1 f) a -> f a -------------------------------------------------------------------------------- -- Derived instances -------------------------------------------------------------------------------- deriving instance Generic [a] deriving instance Generic (Maybe a) deriving instance Generic (Either a b) deriving instance Generic Bool deriving instance Generic Ordering deriving instance Generic () deriving instance Generic ((,) a b) deriving instance Generic ((,,) a b c) deriving instance Generic ((,,,) a b c d) deriving instance Generic ((,,,,) a b c d e) deriving instance Generic ((,,,,,) a b c d e f) deriving instance Generic ((,,,,,,) a b c d e f g) deriving instance Generic1 [] deriving instance Generic1 Maybe deriving instance Generic1 (Either a) deriving instance Generic1 ((,) a) deriving instance Generic1 ((,,) a b) deriving instance Generic1 ((,,,) a b c) deriving instance Generic1 ((,,,,) a b c d) deriving instance Generic1 ((,,,,,) a b c d e) deriving instance Generic1 ((,,,,,,) a b c d e f) -------------------------------------------------------------------------------- -- Primitive representations -------------------------------------------------------------------------------- -- Int data D_Int data C_Int instance Datatype D_Int where datatypeName _ = "Int" moduleName _ = "GHC.Int" instance Constructor C_Int where conName _ = "" -- JPM: I'm not sure this is the right implementation... instance Generic Int where type Rep Int = D1 D_Int (C1 C_Int (S1 NoSelector (Rec0 Int))) from x = M1 (M1 (M1 (K1 x))) to (M1 (M1 (M1 (K1 x)))) = x -- Float data D_Float data C_Float instance Datatype D_Float where datatypeName _ = "Float" moduleName _ = "GHC.Float" instance Constructor C_Float where conName _ = "" -- JPM: I'm not sure this is the right implementation... instance Generic Float where type Rep Float = D1 D_Float (C1 C_Float (S1 NoSelector (Rec0 Float))) from x = M1 (M1 (M1 (K1 x))) to (M1 (M1 (M1 (K1 x)))) = x -- Double data D_Double data C_Double instance Datatype D_Double where datatypeName _ = "Double" moduleName _ = "GHC.Float" instance Constructor C_Double where conName _ = "" -- JPM: I'm not sure this is the right implementation... instance Generic Double where type Rep Double = D1 D_Double (C1 C_Double (S1 NoSelector (Rec0 Double))) from x = M1 (M1 (M1 (K1 x))) to (M1 (M1 (M1 (K1 x)))) = x -- Char data D_Char data C_Char instance Datatype D_Char where datatypeName _ = "Char" moduleName _ = "GHC.Base" instance Constructor C_Char where conName _ = "" -- JPM: I'm not sure this is the right implementation... instance Generic Char where type Rep Char = D1 D_Char (C1 C_Char (S1 NoSelector (Rec0 Char))) from x = M1 (M1 (M1 (K1 x))) to (M1 (M1 (M1 (K1 x)))) = x deriving instance Generic (Proxy t)