Safe Haskell | Safe-Inferred |
---|---|

Language | Haskell98 |

- 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

.
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 `Rep`

T`T`

and

. By using this technique, we merely need
a few generic instances in order to implement functionality that works for any
representable type.`Rep`

T

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:

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.

## 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:

*Generic instances*for the function, implementing it for most of the representation type constructors introduced above.

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)

### 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.

- If no

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 `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:

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`

.

### 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) }

module GHC.Generics