7.7. Type families
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## 7.7. Type families

Indexed type families are a new GHC extension to facilitate type-level programming. Type families are a generalisation of associated data types (“Associated Types with Class”, M. Chakravarty, G. Keller, S. Peyton Jones, and S. Marlow. In Proceedings of “The 32nd Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (POPL'05)”, pages 1-13, ACM Press, 2005) and associated type synonyms (“Type Associated Type Synonyms”. M. Chakravarty, G. Keller, and S. Peyton Jones. In Proceedings of “The Tenth ACM SIGPLAN International Conference on Functional Programming”, ACM Press, pages 241-253, 2005). Type families themselves are described in the paper “Type Checking with Open Type Functions”, T. Schrijvers, S. Peyton-Jones, M. Chakravarty, and M. Sulzmann, in Proceedings of “ICFP 2008: The 13th ACM SIGPLAN International Conference on Functional Programming”, ACM Press, pages 51-62, 2008. Type families essentially provide type-indexed data types and named functions on types, which are useful for generic programming and highly parameterised library interfaces as well as interfaces with enhanced static information, much like dependent types. They might also be regarded as an alternative to functional dependencies, but provide a more functional style of type-level programming than the relational style of functional dependencies.

Indexed type families, or type families for short, are type constructors that represent sets of types. Set members are denoted by supplying the type family constructor with type parameters, which are called type indices. The difference between vanilla parametrised type constructors and family constructors is much like between parametrically polymorphic functions and (ad-hoc polymorphic) methods of type classes. Parametric polymorphic functions behave the same at all type instances, whereas class methods can change their behaviour in dependence on the class type parameters. Similarly, vanilla type constructors imply the same data representation for all type instances, but family constructors can have varying representation types for varying type indices.

Indexed type families come in two flavours: data families and type synonym families. They are the indexed family variants of algebraic data types and type synonyms, respectively. The instances of data families can be data types and newtypes.

Type families are enabled by the flag `-XTypeFamilies`. Additional information on the use of type families in GHC is available on the Haskell wiki page on type families.

### 7.7.1. Data families

Data families appear in two flavours: (1) they can be defined on the toplevel or (2) they can appear inside type classes (in which case they are known as associated types). The former is the more general variant, as it lacks the requirement for the type-indexes to coincide with the class parameters. However, the latter can lead to more clearly structured code and compiler warnings if some type instances were - possibly accidentally - omitted. In the following, we always discuss the general toplevel form first and then cover the additional constraints placed on associated types.

#### 7.7.1.1. Data family declarations

Indexed data families are introduced by a signature, such as

    data family GMap k :: * -> *

The special `family` distinguishes family from standard data declarations. The result kind annotation is optional and, as usual, defaults to `*` if omitted. An example is

    data family Array e

Named arguments can also be given explicit kind signatures if needed. Just as with [http://www.haskell.org/ghc/docs/latest/html/users_guide/gadt.html GADT declarations] named arguments are entirely optional, so that we can declare `Array` alternatively with

    data family Array :: * -> *

##### 7.7.1.1.1. Associated data family declarations

When a data family is declared as part of a type class, we drop the `family` special. The `GMap` declaration takes the following form

    class GMapKey k where
      data GMap k :: * -> *
      ...

In contrast to toplevel declarations, named arguments must be used for all type parameters that are to be used as type-indexes. Moreover, the argument names must be class parameters. Each class parameter may only be used at most once per associated type, but some may be omitted and they may be in an order other than in the class head. Hence, the following contrived example is admissible:

      class C a b c where
      data T c a :: *

#### 7.7.1.2. Data instance declarations

Instance declarations of data and newtype families are very similar to standard data and newtype declarations. The only two differences are that the keyword `data` or `newtype` is followed by `instance` and that some or all of the type arguments can be non-variable types, but may not contain forall types or type synonym families. However, data families are generally allowed in type parameters, and type synonyms are allowed as long as they are fully applied and expand to a type that is itself admissible - exactly as this is required for occurrences of type synonyms in class instance parameters. For example, the `Either` instance for `GMap` is

    data instance GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)

In this example, the declaration has only one variant. In general, it can be any number.

Data and newtype instance declarations are only legit when an appropriate family declaration is in scope - just like class instances require the class declaration to be visible. Moreover, each instance declaration has to conform to the kind determined by its family declaration. This implies that the number of parameters of an instance declaration matches the arity determined by the kind of the family. Although, all data families are declared with the `data` keyword, instances can be either `data` or `newtype`s, or a mix of both.

Even if type families are defined as toplevel declarations, functions that perform different computations for different family instances still need to be defined as methods of type classes. In particular, the following is not possible:

    data family T a
    data instance T Int  = A
    data instance T Char = B
    nonsence :: T a -> Int
    nonsence A = 1             -- WRONG: These two equations together...
    nonsence B = 2             -- ...will produce a type error.

Given the functionality provided by GADTs (Generalised Algebraic Data Types), it might seem as if a definition, such as the above, should be feasible. However, type families are - in contrast to GADTs - are open; i.e., new instances can always be added, possibly in other modules. Supporting pattern matching across different data instances would require a form of extensible case construct.

##### 7.7.1.2.1. Associated data instances

When an associated data family instance is declared within a type class instance, we drop the `instance` keyword in the family instance. So, the `Either` instance for `GMap` becomes:

    instance (GMapKey a, GMapKey b) => GMapKey (Either a b) where
      data GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
      ...

The most important point about associated family instances is that the type indexes corresponding to class parameters must be identical to the type given in the instance head; here this is the first argument of `GMap`, namely `Either a b`, which coincides with the only class parameter. Any parameters to the family constructor that do not correspond to class parameters, need to be variables in every instance; here this is the variable `v`.

Instances for an associated family can only appear as part of instances declarations of the class in which the family was declared - just as with the equations of the methods of a class. Also in correspondence to how methods are handled, declarations of associated types can be omitted in class instances. If an associated family instance is omitted, the corresponding instance type is not inhabited; i.e., only diverging expressions, such as `undefined`, can assume the type.

##### 7.7.1.2.2. Scoping of class parameters

In the case of multi-parameter type classes, the visibility of class parameters in the right-hand side of associated family instances depends solely on the parameters of the data family. As an example, consider the simple class declaration

    class C a b where
      data T a

Only one of the two class parameters is a parameter to the data family. Hence, the following instance declaration is invalid:

    instance C [c] d where
      data T [c] = MkT (c, d)    -- WRONG!!  'd' is not in scope

Here, the right-hand side of the data instance mentions the type variable `d` that does not occur in its left-hand side. We cannot admit such data instances as they would compromise type safety.

##### 7.7.1.2.3. Type class instances of family instances

Type class instances of instances of data families can be defined as usual, and in particular data instance declarations can have `deriving` clauses. For example, we can write

    data GMap () v = GMapUnit (Maybe v)
                   deriving Show

which implicitly defines an instance of the form

    instance Show v => Show (GMap () v) where ...

Note that class instances are always for particular instances of a data family and never for an entire family as a whole. This is for essentially the same reasons that we cannot define a toplevel function that performs pattern matching on the data constructors of different instances of a single type family. It would require a form of extensible case construct.

##### 7.7.1.2.4. Overlap of data instances

The instance declarations of a data family used in a single program may not overlap at all, independent of whether they are associated or not. In contrast to type class instances, this is not only a matter of consistency, but one of type safety.

#### 7.7.1.3. Import and export

The association of data constructors with type families is more dynamic than that is the case with standard data and newtype declarations. In the standard case, the notation `T(..)` in an import or export list denotes the type constructor and all the data constructors introduced in its declaration. However, a family declaration never introduces any data constructors; instead, data constructors are introduced by family instances. As a result, which data constructors are associated with a type family depends on the currently visible instance declarations for that family. Consequently, an import or export item of the form `T(..)` denotes the family constructor and all currently visible data constructors - in the case of an export item, these may be either imported or defined in the current module. The treatment of import and export items that explicitly list data constructors, such as `GMap(GMapEither)`, is analogous.

##### 7.7.1.3.1. Associated families

As expected, an import or export item of the form `C(..)` denotes all of the class' methods and associated types. However, when associated types are explicitly listed as subitems of a class, we need some new syntax, as uppercase identifiers as subitems are usually data constructors, not type constructors. To clarify that we denote types here, each associated type name needs to be prefixed by the keyword `type`. So for example, when explicitly listing the components of the `GMapKey` class, we write `GMapKey(type GMap, empty, lookup, insert)`.

##### 7.7.1.3.2. Examples

Assuming our running `GMapKey` class example, let us look at some export lists and their meaning:

- `module GMap (GMapKey) where...`: Exports just the class name.

- `module GMap (GMapKey(..)) where...`: Exports the class, the associated type `GMap` and the member functions `empty`, `lookup`, and `insert`. None of the data constructors is exported.

- `module GMap (GMapKey(..), GMap(..)) where...`: As before, but also exports all the data constructors `GMapInt`, `GMapChar`, `GMapUnit`, `GMapPair`, and `GMapUnit`.

- `module GMap (GMapKey(empty, lookup, insert), GMap(..)) where...`: As before.

- `module GMap (GMapKey, empty, lookup, insert, GMap(..)) where...`: As before.

Finally, you can write `GMapKey(type GMap)` to denote both the class `GMapKey` as well as its associated type `GMap`. However, you cannot write `GMapKey(type GMap(..))` — i.e., sub-component specifications cannot be nested. To specify `GMap`'s data constructors, you have to list it separately.

##### 7.7.1.3.3. Instances

Family instances are implicitly exported, just like class instances. However, this applies only to the heads of instances, not to the data constructors an instance defines.

### 7.7.2. Synonym families

Type families appear in two flavours: (1) they can be defined on the toplevel or (2) they can appear inside type classes (in which case they are known as associated type synonyms). The former is the more general variant, as it lacks the requirement for the type-indexes to coincide with the class parameters. However, the latter can lead to more clearly structured code and compiler warnings if some type instances were - possibly accidentally - omitted. In the following, we always discuss the general toplevel form first and then cover the additional constraints placed on associated types.

#### 7.7.2.1. Type family declarations

Indexed type families are introduced by a signature, such as

    type family Elem c :: *

The special `family` distinguishes family from standard type declarations. The result kind annotation is optional and, as usual, defaults to `*` if omitted. An example is

    type family Elem c

Parameters can also be given explicit kind signatures if needed. We call the number of parameters in a type family declaration, the family's arity, and all applications of a type family must be fully saturated w.r.t. to that arity. This requirement is unlike ordinary type synonyms and it implies that the kind of a type family is not sufficient to determine a family's arity, and hence in general, also insufficient to determine whether a type family application is well formed. As an example, consider the following declaration:

    type family F a b :: * -> *   -- F's arity is 2,
                                  -- although it's overall kind is * -> * -> * -> *

Given this declaration the following are examples of well-formed and malformed types:

    F Char [Int]       -- OK!  Kind: * -> *
    F Char [Int] Bool  -- OK!  Kind: *
    F IO Bool          -- WRONG: kind mismatch in the first argument
    F Bool             -- WRONG: unsaturated application

##### 7.7.2.1.1. Associated type family declarations

When a type family is declared as part of a type class, we drop the `family` special. The `Elem` declaration takes the following form

    class Collects ce where
      type Elem ce :: *
      ...

The argument names of the type family must be class parameters. Each class parameter may only be used at most once per associated type, but some may be omitted and they may be in an order other than in the class head. Hence, the following contrived example is admissible:

    class C a b c where
      type T c a :: *

These rules are exactly as for associated data families.

#### 7.7.2.2. Type instance declarations

Instance declarations of type families are very similar to standard type synonym declarations. The only two differences are that the keyword `type` is followed by `instance` and that some or all of the type arguments can be non-variable types, but may not contain forall types or type synonym families. However, data families are generally allowed, and type synonyms are allowed as long as they are fully applied and expand to a type that is admissible - these are the exact same requirements as for data instances. For example, the `[e]` instance for `Elem` is

    type instance Elem [e] = e

Type family instance declarations are only legitimate when an appropriate family declaration is in scope - just like class instances require the class declaration to be visible. Moreover, each instance declaration has to conform to the kind determined by its family declaration, and the number of type parameters in an instance declaration must match the number of type parameters in the family declaration. Finally, the right-hand side of a type instance must be a monotype (i.e., it may not include foralls) and after the expansion of all saturated vanilla type synonyms, no synonyms, except family synonyms may remain. Here are some examples of admissible and illegal type instances:

    type family F a :: *
    type instance F [Int]              = Int         -- OK!
    type instance F String             = Char        -- OK!
    type instance F (F a)              = a           -- WRONG: type parameter mentions a type family
    type instance F (forall a. (a, b)) = b           -- WRONG: a forall type appears in a type parameter
    type instance F Float              = forall a.a  -- WRONG: right-hand side may not be a forall type

    type family G a b :: * -> *
    type instance G Int            = (,)     -- WRONG: must be two type parameters
    type instance G Int Char Float = Double  -- WRONG: must be two type parameters

##### 7.7.2.2.1. Associated type instance declarations

When an associated family instance is declared within a type class instance, we drop the `instance` keyword in the family instance. So, the `[e]` instance for `Elem` becomes:

    instance (Eq (Elem [e])) => Collects ([e]) where
      type Elem [e] = e
      ...

The most important point about associated family instances is that the type indexes corresponding to class parameters must be identical to the type given in the instance head; here this is `[e]`, which coincides with the only class parameter.

Instances for an associated family can only appear as part of instances declarations of the class in which the family was declared - just as with the equations of the methods of a class. Also in correspondence to how methods are handled, declarations of associated types can be omitted in class instances. If an associated family instance is omitted, the corresponding instance type is not inhabited; i.e., only diverging expressions, such as `undefined`, can assume the type.

##### 7.7.2.2.2. Overlap of type synonym instances

The instance declarations of a type family used in a single program may only overlap if the right-hand sides of the overlapping instances coincide for the overlapping types. More formally, two instance declarations overlap if there is a substitution that makes the left-hand sides of the instances syntactically the same. Whenever that is the case, the right-hand sides of the instances must also be syntactically equal under the same substitution. This condition is independent of whether the type family is associated or not, and it is not only a matter of consistency, but one of type safety.

Here are two example to illustrate the condition under which overlap is permitted.

    type instance F (a, Int) = [a]
    type instance F (Int, b) = [b]   -- overlap permitted

    type instance G (a, Int)  = [a]
    type instance G (Char, a) = [a]  -- ILLEGAL overlap, as [Char] /= [Int]

##### 7.7.2.2.3. Decidability of type synonym instances

In order to guarantee that type inference in the presence of type families decidable, we need to place a number of additional restrictions on the formation of type instance declarations (c.f., Definition 5 (Relaxed Conditions) of “Type Checking with Open Type Functions”). Instance declarations have the general form

    type instance F t1 .. tn = t

where we require that for every type family application `(G s1 .. sm)` in `t`,

1. `s1 .. sm` do not contain any type family constructors,

2. the total number of symbols (data type constructors and type variables) in `s1 .. sm` is strictly smaller than in `t1 .. tn`, and

3. for every type variable `a`, `a` occurs in `s1 .. sm` at most as often as in `t1 .. tn`.

These restrictions are easily verified and ensure termination of type inference. However, they are not sufficient to guarantee completeness of type inference in the presence of, so called, ''loopy equalities'', such as `a ~ [F a]`, where a recursive occurrence of a type variable is underneath a family application and data constructor application - see the above mentioned paper for details.

If the option `-XUndecidableInstances` is passed to the compiler, the above restrictions are not enforced and it is on the programmer to ensure termination of the normalisation of type families during type inference.

#### 7.7.2.3. Equality constraints

Type context can include equality constraints of the form `t1 ~ t2`, which denote that the types `t1` and `t2` need to be the same. In the presence of type families, whether two types are equal cannot generally be decided locally. Hence, the contexts of function signatures may include equality constraints, as in the following example:

    sumCollects :: (Collects c1, Collects c2, Elem c1 ~ Elem c2) => c1 -> c2 -> c2

where we require that the element type of `c1` and `c2` are the same. In general, the types `t1` and `t2` of an equality constraint may be arbitrary monotypes; i.e., they may not contain any quantifiers, independent of whether higher-rank types are otherwise enabled.

Equality constraints can also appear in class and instance contexts. The former enable a simple translation of programs using functional dependencies into programs using family synonyms instead. The general idea is to rewrite a class declaration of the form

    class C a b | a -> b

to

    class (F a ~ b) => C a b where
      type F a

That is, we represent every functional dependency (FD) `a1 .. an -> b` by an FD type family `F a1 .. an` and a superclass context equality `F a1 .. an ~ b`, essentially giving a name to the functional dependency. In class instances, we define the type instances of FD families in accordance with the class head. Method signatures are not affected by that process.

NB: Equalities in superclass contexts are not fully implemented in GHC 6.10.

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