6.19.3. Generic programming¶
There are a few ways to do datatypegeneric programming using the
GHC.Generics framework. One is making use of the
Generically
and Generically1
wrappers from GHC.Generics
,
instances can be derived via them using DerivingVia
:
{# LANGUAGE DeriveGeneric #}
{# LANGUAGE DerivingStrategies #}
{# LANGUAGE DerivingVia #}
import GHC.Generics
data V4 a = V4 a a a a
deriving
stock (Generic, Generic1)
deriving (Semigroup, Monoid)
via Generically (V4 a)
deriving (Functor, Applicative)
via Generically1 V4
The older approach uses DeriveGeneric
,
DefaultSignatures
, and DeriveAnyClass
. It
derives instances by providing a distinguished generic implementation
as part of the type class declaration. This section gives a very brief
overview of how to do it.
Generic programming support in GHC allows defining classes with methods
that do not need a user specification when instantiating: the method
body is automatically derived by GHC. This is similar to what happens
for standard classes such as Read
and Show
, for instance, but
now for userdefined classes.
Note
GHC used to have an implementation of generic classes as defined in the paper “Derivable type classes”, Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp. 94105. These have been removed and replaced by the more general support for generic programming.
6.19.3.1. Deriving representations¶
The first thing we need is generic representations. The GHC.Generics
module defines a couple of primitive types that are used to represent
Haskell datatypes:
  Unit: used for constructors without arguments
data U1 p = U1
  Constants, additional parameters and recursion of kind Type
newtype K1 i c p = K1 { unK1 :: c }
  Metainformation (constructor names, etc.)
newtype M1 i c f p = M1 { unM1 :: f p }
  Sums: encode choice between constructors
infixr 5 :+:
data (:+:) f g p = L1 (f p)  R1 (g p)
  Products: encode multiple arguments to constructors
infixr 6 :*:
data (:*:) f g p = f p :*: g p
The Generic
and Generic1
classes mediate between userdefined
datatypes and their internal representation as a sumofproducts:
class Generic a where
 Encode the representation of a user datatype
type Rep a :: Type > Type
 Convert from the datatype to its representation
from :: a > (Rep a) x
 Convert from the representation to the datatype
to :: (Rep a) x > a
class Generic1 (f :: k > Type) where
type Rep1 f :: k > Type
from1 :: f a > Rep1 f a
to1 :: Rep1 f a > f a
Generic1
is used for functions that can only be defined over type
containers, such as map
. Note that Generic1
ranges over types of kind
Type > Type
by default, but if the PolyKinds
extension is
enabled, then it can range of types of kind k > Type
, for any kind k
.

DeriveGeneric
¶ Since: 7.2.1 Allow automatic deriving of instances for the
Generic
typeclass.
Instances of these classes can be derived by GHC with the
DeriveGeneric
extension, and are necessary to be able to define
generic instances automatically.
For example, a userdefined datatype of trees
data UserTree a = Node a (UserTree a) (UserTree a)  Leaf
in a Main
module in a package named foo
will get the following
representation:
instance Generic (UserTree a) where
 Representation type
type Rep (UserTree a) =
M1 D ('MetaData "UserTree" "Main" "packagename" 'False) (
M1 C ('MetaCons "Node" 'PrefixI 'False) (
M1 S ('MetaSel 'Nothing
'NoSourceUnpackedness
'NoSourceStrictness
'DecidedLazy)
(K1 R a)
:*: M1 S ('MetaSel 'Nothing
'NoSourceUnpackedness
'NoSourceStrictness
'DecidedLazy)
(K1 R (UserTree a))
:*: M1 S ('MetaSel 'Nothing
'NoSourceUnpackedness
'NoSourceStrictness
'DecidedLazy)
(K1 R (UserTree a)))
:+: M1 C ('MetaCons "Leaf" 'PrefixI 'False) U1)
 Conversion functions
from (Node x l r) = M1 (L1 (M1 (M1 (K1 x) :*: M1 (K1 l) :*: M1 (K1 r))))
from Leaf = M1 (R1 (M1 U1))
to (M1 (L1 (M1 (M1 (K1 x) :*: M1 (K1 l) :*: M1 (K1 r))))) = Node x l r
to (M1 (R1 (M1 U1))) = Leaf
This representation is generated automatically if a deriving Generic
clause is attached to the datatype. Standalone
deriving can also be used.
6.19.3.2. Writing generic functions¶
A generic function is defined by creating a class and giving instances
for each of the representation types of GHC.Generics
. As an example
we show generic serialization:
data Bin = O  I
class GSerialize f where
gput :: f a > [Bin]
instance GSerialize U1 where
gput U1 = []
instance (GSerialize a, GSerialize b) => GSerialize (a :*: b) where
gput (x :*: y) = gput x ++ gput y
instance (GSerialize a, GSerialize b) => GSerialize (a :+: b) where
gput (L1 x) = O : gput x
gput (R1 x) = I : gput x
instance (GSerialize a) => GSerialize (M1 i c a) where
gput (M1 x) = gput x
instance (Serialize a) => GSerialize (K1 i a) where
gput (K1 x) = put x
A caveat: this encoding strategy may not be reliable across different versions
of GHC. When deriving a Generic
instance is free to choose any nesting of
:+:
and :*:
it chooses, so if GHC chooses (a :+: b) :+: c
, then the
encoding for a
would be [O, O]
, b
would be [O, I]
, and c
would be [I]
. However, if GHC chooses a :+: (b :+: c)
, then the
encoding for a
would be [O]
, b
would be [I, O]
, and c
would
be [I, I]
. (In practice, the current implementation tries to produce a
moreorless balanced nesting of :+:
and :*:
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.)
Typically this GSerialize
class will not be exported, as it only makes
sense to have instances for the representation types.
6.19.3.3. Unlifted representation types¶
The data family URec
is provided to enable generic programming over
datatypes with certain unlifted arguments. There are six instances corresponding
to 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 = UInt { uInt# :: Int# }
data instance URec Float p = UFloat { uFloat# :: Float# }
data instance URec Word p = UWord { uWord# :: Word# }
Six 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
As an example, this data declaration:
data IntHash = IntHash Int#
deriving Generic
results in the following Generic
instance:
instance 'Generic' IntHash where
type 'Rep' IntHash =
'D1' ('MetaData "IntHash" "Main" "packagename" 'False)
('C1' ('MetaCons "IntHash" 'PrefixI 'False)
('S1' ('MetaSel 'Nothing
'NoSourceUnpackedness
'NoSourceStrictness
'DecidedLazy)
'UInt'))
A user could provide, for example, a GSerialize UInt
instance so that a
Serialize IntHash
instance could be easily defined in terms of
GSerialize
.
6.19.3.4. Generic defaults¶
The only thing left to do now is to define a “frontend” class, which is exposed to the user:
class Serialize a where
put :: a > [Bin]
default put :: (Generic a, GSerialize (Rep a)) => a > [Bin]
put = gput . from
Here we use a default signature to
specify that the user does not have to provide an implementation for
put
, as long as there is a Generic
instance for the type to
instantiate. For the UserTree
type, for instance, the user can just
write:
instance (Serialize a) => Serialize (UserTree a)
The default method for put
is then used, corresponding to the
generic implementation of serialization. If you are using
DeriveAnyClass
, the same instance is generated by simply attaching
a deriving Serialize
clause to the UserTree
datatype
declaration. For more examples of generic functions please refer to the
genericderiving
package on Hackage.
6.19.3.5. More information¶
For more details please refer to the Haskell Wiki page or the original paper [Generics2010].
[Generics2010]  Jose Pedro Magalhaes, Atze Dijkstra, Johan Jeuring, and Andres Loeh. A generic deriving mechanism for Haskell. Proceedings of the third ACM Haskell symposium on Haskell (Haskell‘2010), pp. 3748, ACM, 2010. 