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.

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:

 class 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.

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.

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.

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.

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

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

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)

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.)

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.

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).

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.

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)

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).

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)?

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:

class 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.

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) }