*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.

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.

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 :: * -> *

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 permitted when an appropriate family declaration is in scope - just as a class instance declaration requires 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.

A data family instance declaration can use the full expressiveness of
ordinary `data`

or `newtype`

declarations:

Although, a data family is

*introduced*with the keyword "`data`

", a data family*instance*can use either`data`

or`newtype`

. For example:data family T a data instance T Int = T1 Int | T2 Bool newtype instance T Char = TC Bool

A

`data instance`

can use GADT syntax for the data constructors, and indeed can define a GADT. For example:data family G a b data instance G [a] b where G1 :: c -> G [Int] b G2 :: G [a] Bool

You can use a

`deriving`

clause on a`data instance`

or`newtype instance`

declaration.

Even if type families are defined as toplevel declarations, functions that perform different computations for different family instances may 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 foo :: T a -> Int foo A = 1 -- WRONG: These two equations together... foo B = 2 -- ...will produce a type error.

Instead, you would have to write `foo`

as a class operation, thus:

class Foo a where foo :: T a -> Int instance Foo Int where foo A = 1 instance Foo Char where foo B = 2

(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.)

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.

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 its 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

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

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]

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`

,

`s1 .. sm`

do not contain any type family constructors,the total number of symbols (data type constructors and type variables) in

`s1 .. sm`

is strictly smaller than in`t1 .. tn`

, andfor 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.

A data or type synonym family can be declared as part of a type class, thus:

class GMapKey k where data GMap k :: * -> * ... class Collects ce where type Elem ce :: * ...

When doing so, we drop the "`family`

" keyword.

The type parameters must all be type variables, of course, and some (but not necessarily all) of then can be the 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 x :: *

Here `c`

and `a`

are class parameters,
but the type is also indexed on a third parameter `x`

.

When an associated data or type synonym family instance is declared within a type
class instance, we drop the `instance`

keyword in the
family instance:

instance (GMapKey a, GMapKey b) => GMapKey (Either a b) where data GMap (Either a b) v = GMapEither (GMap a v) (GMap b v) ... 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 the first argument
of `GMap`

, namely `Either a b`

,
which coincides with the only class parameter.

Instances for an associated family can only appear as part of
instance 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.

Although it is unusual, there can be *multiple*
instances for an associated family in a single instance declaration.
For example, this is legitimate:

instance GMapKey Flob where data GMap Flob [v] = G1 v data GMap Flob Int = G2 Int ...

Here we give two data instance declarations, one in which the last
parameter is `[v]`

, and one for which it is `Int`

.
Since you cannot give any *subsequent* instances for
`(GMap Flob ...)`

, this facility is most useful when
the free indexed parameter is of a kind with a finite number of alternatives
(unlike `*`

).

It is possible for the class defining the associated type to specify a default for associated type instances. So for example, this is OK:

class IsBoolMap v where type Key v type Key v = Int lookupKey :: Key v -> v -> Maybe Bool instance IsBoolMap [(Int, Bool)] where lookupKey = lookup

There can also be multiple defaults for a single type, as long as they do not overlap:

class C a where type F a b type F a Int = Bool type F a Bool = Int

A default declaration is not permitted for an associated
*data* type.

The visibility of class
parameters in the right-hand side of associated family instances
depends *solely* on the parameters of the
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.

The rules for export lists (Haskell Report Section 5.2) needs adjustment for type families:

The form

`T(..)`

, where`T`

is a data family, names the family`T`

and all the in-scope constructors (whether in scope qualified or unqualified) that are data instances of`T`

.The form

`T(.., ci, .., fj, ..)`

, where`T`

is a data family, names`T`

and the specified constructors`ci`

and fields`fj`

as usual. The constructors and field names must belong to some data instance of`T`

, but are not required to belong to the*same*instance.The form

`C(..)`

, where`C`

is a class, names the class`C`

and all its methods*and associated types*.The form

`C(.., mi, .., type Tj, ..)`

, where`C`

is a class, names the class`C`

, and the specified methods`mi`

and associated types`Tj`

. The types need a keyword "`type`

" to distinguish them from data constructors.

Recall our running `GMapKey`

class example:

class GMapKey k where data GMap k :: * -> * insert :: GMap k v -> k -> v -> GMap k v lookup :: GMap k v -> k -> Maybe v empty :: GMap k v instance (GMapKey a, GMapKey b) => GMapKey (Either a b) where data GMap (Either a b) v = GMapEither (GMap a v) (GMap b v) ...method declarations...

Here are some export lists and their meaning:

`module GMap( GMapKey )`

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

: Exports the class, the associated type`GMap`

and the member functions`empty`

,`lookup`

, and`insert`

. The data constructors of`GMap`

(in this case`GMapEither`

) are not exported.`module GMap( GMapKey( type GMap, empty, lookup, insert ) )`

: Same as the previous item. Note the "`type`

" keyword.`module GMap( GMapKey(..), GMap(..) )`

: Same as previous item, but also exports all the data constructors for`GMap`

, namely`GMapEither`

.`module GMap ( GMapKey( empty, lookup, insert), GMap(..) )`

: Same as previous item.`module GMap ( GMapKey, empty, lookup, insert, GMap(..) )`

: Same as previous item.

Two things to watch out for:

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.Consider this example:

module X where data family D module Y where import X data instance D Int = D1 | D2

Module Y exports all the entities defined in Y, namely the data constructors

`D1`

and`D2`

,*but not the data family*. That (annoyingly) means that you cannot selectively import Y selectively, thus "`D`

`import Y( D(D1,D2) )`

", because Y does not export`D`

. Instead you should list the exports explicitly, thus:module Y( D(..) ) where ... or module Y( module Y, D ) where ...

Type families require us to extend the rules for the form of instance heads, which are given in Section 7.6.3.1, “Relaxed rules for the instance head”. Specifically:

Data type families may appear in an instance head

Type synonym families may not appear (at all) in an instance head

The reason for the latter restriction is that there is no way to check for instance matching. Consider

type family F a type instance F Bool = Int class C a instance C Int instance C (F a)

Now a constraint `(C (F Bool))`

would match both instances.
The situation is especially bad because the type instance for `F Bool`

might be in another module, or even in a module that is not yet written.

However, type class instances of instances of data families can be defined much like any other data type. For example, we can say

data instance T Int = T1 Int | T2 Bool instance Eq (T Int) where (T1 i) == (T1 j) = i==j (T2 i) == (T2 j) = i==j _ == _ = False

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.

Data instance declarations can also
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 ...