6.4.9. Type families¶

TypeFamilies
¶ Implies: MonoLocalBinds
,KindSignatures
,ExplicitNamespaces
Since: 6.8.1 Allow use and definition of indexed type and data families.
Indexed type families form an extension to facilitate typelevel programming. Type families are a generalisation of associated data types [AssocDataTypes2005] and associated type synonyms [AssocTypeSyn2005] Type families themselves are described in Schrijvers 2008 [TypeFamilies2008]. Type families essentially provide typeindexed 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 typelevel 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 (adhoc 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 three flavours: data families, open type synonym families, and closed 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 language extension TypeFamilies
. Additional
information on the use of type families in GHC is available on the
Haskell wiki page on type
families.
[AssocDataTypes2005]  “Associated Types with Class”, M. Chakravarty, G. Keller, S. Peyton Jones, and S. Marlow. In Proceedings of “The 32nd Annual ACM SIGPLANSIGACT Symposium on Principles of Programming Languages (POPL‘05)”, pages 113, ACM Press, 2005. 
[AssocTypeSyn2005]  “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 241253, 2005. 
[TypeFamilies2008]  “Type Checking with Open Type Functions”, T. Schrijvers, S. PeytonJones, M. Chakravarty, and M. Sulzmann, in Proceedings of “ICFP 2008: The 13th ACM SIGPLAN International Conference on Functional Programming”, ACM Press, pages 5162, 2008. 
6.4.9.1. 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 typeindexes 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.
6.4.9.1.1. Data family declarations¶
Indexed data families are introduced by a signature, such as
data family GMap k :: Type > Type
The special family
distinguishes family from standard data
declarations. The result kind annotation is optional and, as usual,
defaults to Type
if omitted. An example is
data family Array e
Named arguments can also be given explicit kind signatures if needed.
Just as with GADT declarations named arguments are
entirely optional, so that we can declare Array
alternatively with
data family Array :: Type > Type
Unlike with ordinary data definitions, the result kind of a data family
does not need to be Type
. It can alternatively be:
Of the form
TYPE r
for somer
(see Levity polymorphism). For example:data family DF1 :: TYPE IntRep data family DF2 (r :: RuntimeRep) :: TYPE r data family DF3 :: Type > TYPE WordRep
A bare kind variable (with
PolyKinds
enabled). For example:data family DF4 :: k data family DF5 (a :: k) :: k data family DF6 :: (k > Type) > k
Data instances’ kinds must end in Type
, however. This restriction is
slightly relaxed when the UnliftedNewtypes
extension is enabled,
as it permits a newtype instance
‘s kind to end in TYPE r
for some
r
.
6.4.9.1.2. 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 nonvariable 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.
When ExplicitForAll
is enabled, type and kind variables used on
the left hand side can be explicitly bound. For example:
data instance forall a (b :: Proxy a). F (Proxy b) = FProxy Bool
When an explicit forall
is present, all type and kind variables mentioned
which are not already in scope must be bound by the forall
:
data instance forall (a :: k). F a = FOtherwise  rejected: k not in scope
data instance forall k (a :: k). F a = FOtherwise  accepted
When the flag Wunusedtypepatterns
is enabled, type
variables that are mentioned in the patterns on the left hand side, but not
used on the right hand side are reported. Variables that occur multiple times
on the left hand side are also considered used. To suppress the warnings,
unused variables should be either replaced or prefixed with underscores. Type
variables starting with an underscore (_x
) are otherwise treated as
ordinary type variables.
This resembles the wildcards that can be used in
Partial Type Signatures. However, there are some differences.
No error messages reporting the inferred types are generated, nor does
the extension PartialTypeSignatures
have any effect.
A type or kind variable explicitly bound using ExplicitForAll
but
not used on the left hand side will generate an error, not a warning.
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 eitherdata
ornewtype
. 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 adata instance
ornewtype instance
declaration.
Even if data 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
foo B = 2
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  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.
6.4.9.1.3. 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.
6.4.9.2. Synonym families¶
Type families appear in three flavours: (1) they can be defined as open families on the toplevel, (2) they can be defined as closed families on the toplevel, or (3) they can appear inside type classes (in which case they are known as associated type synonyms). Toplevel families are more general, as they lack the requirement for the typeindexes to coincide with the class parameters. However, associated type synonyms 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 forms first and then cover the additional constraints placed on associated types. Note that closed associated type synonyms do not exist.
6.4.9.2.1. Type family declarations¶
Open indexed type families are introduced by a signature, such as
type family Elem c :: Type
The special family
distinguishes family from standard type
declarations. The result kind annotation is optional and, as usual,
defaults to Type
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 with respect 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 :: Type > Type
 F's arity is 2,
 although its overall kind is Type > Type > Type > Type
Given this declaration the following are examples of wellformed and malformed types:
F Char [Int]  OK! Kind: Type > Type
F Char [Int] Bool  OK! Kind: Type
F IO Bool  WRONG: kind mismatch in the first argument
F Bool  WRONG: unsaturated application
The result kind annotation is optional and defaults to Type
(like
argument kinds) if omitted. Polykinded type families can be declared
using a parameter in the kind annotation:
type family F a :: k
In this case the kind parameter k
is actually an implicit parameter
of the type family.
At definition site, the arity determines what inputs can be matched on:
data PT (a :: Type)
type family F1 :: k > Type
type instance F1 = PT
 OK, 'k' can be matched on.
type family F0 :: forall k. k > Type
type instance F0 = PT
 Error:
 • Expected kind ‘forall k. k > Type’,
 but ‘PT’ has kind ‘Type > Type’
 • In the type ‘PT’
 In the type instance declaration for ‘F0’
Both F1
and F0
have kind forall k. k > Type
, but their arity
differs.
At use sites, the arity determines if the definition can be used in a higherrank scenario:
type HRK (f :: forall k. k > Type) = (f Int, f Maybe, f True)
type H1 = HRK F0  OK
type H2 = HRK F1
 Error:
 • Expected kind ‘forall k. k > Type’,
 but ‘F1’ has kind ‘k0 > Type’
 • In the first argument of ‘HRK’, namely ‘F1’
 In the type ‘HRK F1’
 In the type declaration for ‘H2’
This is a consequence of the requirement that all applications of a type family must be fully saturated with respect to their arity.
6.4.9.2.2. 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 nonvariable 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 arguments can be replaced with underscores (_
) if the names of
the arguments don’t matter. This is the same as writing type variables
with unique names. Unused type arguments can be replaced or prefixed
with underscores to avoid warnings when the
Wunusedtypepatterns
flag is enabled. The same rules apply
as for Data instance declarations.
Also in the same way as Data instance declarations, when
ExplicitForAll
is enabled, type and kind variables can be
explicitly bound in a type instance declaration.
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 righthand 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.
6.4.9.2.3. Closed type families¶
A type family can also be declared with a where
clause, defining the
full set of equations for that family. For example:
type family F a where
F Int = Double
F Bool = Char
F a = String
A closed type family’s equations are tried in order, from top to bottom,
when simplifying a type family application. In this example, we declare
an instance for F
such that F Int
simplifies to Double
,
F Bool
simplifies to Char
, and for any other type a
that is
known not to be Int
or Bool
, F a
simplifies to String
.
Note that GHC must be sure that a
cannot unify with Int
or
Bool
in that last case; if a programmer specifies just F a
in
their code, GHC will not be able to simplify the type. After all, a
might later be instantiated with Int
.
A closed type family’s equations have the same restrictions and extensions as
the equations for open type family instances. For instance, when
ExplicitForAll
is enabled, type or kind variables used on the
left hand side of an equation can be explicitly bound, such as in:
type family R a where
forall t a. R (t a) = [a]
forall a. R a = a
A closed type family may be declared with no equations. Such closed type
families are opaque typelevel definitions that will never reduce, are
not necessarily injective (unlike empty data types), and cannot be given
any instances. This is different from omitting the equations of a closed
type family in a hsboot
file, which uses the syntax where ..
,
as in that case there may or may not be equations given in the hs
file.
6.4.9.2.4. Type family examples¶
Here are some examples of admissible and illegal type instances:
type family F a :: Type
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: righthand side may not be a forall type
type family H a where  OK!
H Int = Int
H Bool = Bool
H a = String
type instance H Char = Char  WRONG: cannot have instances of closed family
type family K a where  OK!
type family G a b :: Type > Type
type instance G Int = (,)  WRONG: must be two type parameters
type instance G Int Char Float = Double  WRONG: must be two type parameters
6.4.9.2.5. Compatibility and apartness of type family equations¶
There must be some restrictions on the equations of type families, lest we define an ambiguous rewrite system. So, equations of open type families are restricted to be compatible. Two type patterns are compatible if
 all corresponding types and implicit kinds in the patterns are apart, or
 the two patterns unify producing a substitution, and the righthand sides are equal under that substitution.
Two types are considered apart if, for all possible substitutions, the types cannot reduce to a common reduct.
The first clause of “compatible” is the more straightforward one. It says that the patterns of two distinct type family instances cannot overlap. For example, the following is disallowed:
type instance F Int = Bool
type instance F Int = Char
The second clause is a little more interesting. It says that two overlapping type family instances are allowed if the righthand sides coincide in the region of overlap. Some examples help here:
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]
Note that this compatibility 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.
For a polykinded type family, the kinds are checked for apartness just like types. For example, the following is accepted:
type family J a :: k
type instance J Int = Bool
type instance J Int = Maybe
These instances are compatible because they differ in their implicit kind
parameter; the first uses Type
while the second uses Type > Type
.
The definition for “compatible” uses a notion of “apart”, whose definition in turn relies on type family reduction. This condition of “apartness”, as stated, is impossible to check, so we use this conservative approximation: two types are considered to be apart when the two types cannot be unified, even by a potentially infinite unifier. Allowing the unifier to be infinite disallows the following pair of instances:
type instance H x x = Int
type instance H [x] x = Bool
The type patterns in this pair equal if x
is replaced by an infinite
nesting of lists. Rejecting instances such as these is necessary for
type soundness.
Compatibility also affects closed type families. When simplifying an application of a closed type family, GHC will select an equation only when it is sure that no incompatible previous equation will ever apply. Here are some examples:
type family F a where
F Int = Bool
F a = Char
type family G a where
G Int = Int
G a = a
In the definition for F
, the two equations are incompatible – their
patterns are not apart, and yet their righthand sides do not coincide.
Thus, before GHC selects the second equation, it must be sure that the
first can never apply. So, the type F a
does not simplify; only a
type such as F Double
will simplify to Char
. In G
, on the
other hand, the two equations are compatible. Thus, GHC can ignore the
first equation when looking at the second. So, G a
will simplify to
a
.
Incompatibilities between closed type family equations can be displayed
in :info
when fprintaxiomincomps
is enabled.
However see Type, class and other declarations for the overlap rules in GHCi.
6.4.9.2.6. Decidability of type synonym instances¶

UndecidableInstances
Relax restrictions on the decidability of type synonym family instances.
In order to guarantee that type inference in the presence of type families is 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 int1 .. tn
, and  for every type variable
a
,a
occurs ins1 .. sm
at most as often as int1 .. 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 UndecidableInstances
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.
6.4.9.3. Wildcards on the LHS of data and type family instances¶
When the name of a type argument of a data or type instance
declaration doesn’t matter, it can be replaced with an underscore
(_
). This is the same as writing a type variable with a unique name.
data family F a b :: Type
data instance F Int _ = Int
 Equivalent to data instance F Int b = Int
type family T a :: Type
type instance T (a,_) = a
 Equivalent to type instance T (a,b) = a
This use of underscore for wildcard in a type pattern is exactly like pattern matching in the term language, but is rather different to the use of a underscore in a partial type signature (see Type Wildcards).
A type variable beginning with an underscore is not treated specially in a type or data instance declaration. For example:
data instance F Bool _a = _a > Int
 Equivalent to data instance F Bool a = a > Int
Contrast this with the special treatment of named wildcards in type signatures (Named Wildcards).
6.4.9.4. Associated data and type families¶
A data or type synonym family can be declared as part of a type class, thus:
class GMapKey k where
data GMap k :: Type > Type
...
class Collects ce where
type Elem ce :: Type
...
When doing so, we (optionally) may 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 :: Type
Here c
and a
are class parameters, but the type is also indexed
on a third parameter x
.
6.4.9.4.1. Associated instances¶
When an associated data or type synonym family instance is declared
within a type class instance, we (optionally) may 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 data or type family instance for an associated type must follow the rule that the type indexes corresponding to class parameters must be precisely the same as types given in the instance head. For example:
class Collects ce where
type Elem ce :: Type
instance Eq (Elem [e]) => Collects [e] where
 Choose one of the following alternatives:
type Elem [e] = e  OK
type Elem [x] = x  BAD; '[x]' is different to '[e]' from head
type Elem x = x  BAD; 'x' is different to '[e]'
type Elem [Maybe x] = x  BAD: '[Maybe x]' is different to '[e]'
Note the following points:
An instance for an associated family can only appear as part of an instance declarations of the class in which the family was declared, just as with the equations of the methods of a class.
The type variables on the right hand side of the type family equation must, as usual, be explicitly bound by the left hand side. This restriction is relaxed for kind variables, however, as the right hand side is allowed to mention kind variables that are implicitly bound. For example, these are legitimate:
data family Nat :: k > k > Type  k is implicitly bound by an invisible kind pattern newtype instance Nat :: (k > Type) > (k > Type) > Type where Nat :: (forall xx. f xx > g xx) > Nat f g class Funct f where type Codomain f :: Type instance Funct ('KProxy :: KProxy o) where  o is implicitly bound by the kind signature  of the LHS type pattern ('KProxy) type Codomain 'KProxy = NatTr (Proxy :: o > Type)
The instance for an associated type can be omitted in class instances. In that case, unless there is a default instance (see Associated type synonym defaults), the corresponding instance type is not inhabited; i.e., only diverging expressions, such as
undefined
, can assume the type.Although it is unusual, there (currently) 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 isInt
. 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 (unlikeType
).When
ExplicitForAll
is enabled, type and kind variables can be explicitly bound in associated data or type family instances in the same way (and with the same restrictions) as Data instance declarations or Type instance declarations. For example, adapting the above, the following is accepted:instance Eq (Elem [e]) => Collects [e] where type forall e. Elem [e] = e
6.4.9.4.2. Associated type synonym defaults¶
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 instance Key v = Int
lookupKey :: Key v > v > Maybe Bool
instance IsBoolMap [(Int, Bool)] where
lookupKey = lookup
In an instance
declaration for the class, if no explicit
type instance
declaration is given for the associated type, the
default declaration is used instead, just as with default class methods.
Note the following points:
The
instance
keyword is optional.There can be at most one default declaration for an associated type synonym.
A default declaration is not permitted for an associated data type.
The default declaration must mention only type variables on the left hand side, and type variables may not be repeated on the lefthand side. The right hand side must mention only type variables that are explicitly bound on the left hand side. This restriction is relaxed for kind variables, however, as the right hand side is allowed to mention kind variables that are implicitly bound on the left hand side.
Like with Associated instances, it is possible to explicitly bind type and kind variables in default declarations with a
forall
by using theExplicitForAll
language extension.Unlike the associated type family declaration itself, the type variables of the default instance are independent of those of the parent class.
Here are some examples:
class C (a :: Type) where
type F1 a :: Type
type instance F1 a = [a]  OK
type instance F1 a = a>a  BAD; only one default instance is allowed
type F2 b a  OK; note the family has more type
 variables than the class
type instance F2 c d = c>d  OK; you don't have to use 'a' in the type instance
type F3 a
type F3 [b] = b  BAD; only type variables allowed on the
 LHS, and the argument to F3 is
 instantiated to [b], which is not
 a bare type variable
type F4 x y
type F4 x x = x  BAD; the type variable x is repeated on
 the LHS
type F5 a
type F5 b = a  BAD; 'a' is not in scope in the RHS
type F6 a :: [k]
type F6 a = ('[] :: [x])  OK; the kind variable x is implicitly
 bound by an invisible kind pattern
 on the LHS
type F7 a
type F7 a =
Proxy ('[] :: [x])  BAD; the kind variable x is not bound,
 even by an invisible kind pattern
type F8 (x :: a) :: [a]
type F8 x = ('[] :: [a])  OK; the kind variable a is implicitly
 bound by the kind signature of the
 LHS type pattern
type F9 (a :: k)
type F9 a = Maybe a  BAD; the kind variable k is
 instantiated to Type, which is not
 a bare kind variable
type F10 (a :: j) (b :: k)
type F10 (a :: z) (b :: z)
= Proxy a  BAD; the kind variable z is repeated,
 as both j and k are instantiated to z
type F11 a b
type forall a b. F11 a b = a  OK; LHS type variables can be
 explicitly bound with 'forall'
type F12 (a :: k)
type F12 @k a = Proxy a  OK; visible kind application syntax is
 permitted in default declarations
6.4.9.4.3. Scoping of class parameters¶
The visibility of class parameters in the righthand 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 righthand side of the data instance mentions the type
variable d
that does not occur in its lefthand side. We cannot
admit such data instances as they would compromise type safety.
Bear in mind that it is also possible for the righthand side of an
associated family instance to contain kind parameters (by using the
PolyKinds
extension). For instance, this class and instance are
perfectly admissible:
class C k where
type T :: k
instance C (Maybe a) where
type T = (Nothing :: Maybe a)
Here, although the righthand side (Nothing :: Maybe a)
mentions a kind
variable a
which does not occur on the lefthand side, this is acceptable,
because a
is implicitly bound by T
‘s kind pattern.
A kind variable can also be bound implicitly in a LHS type pattern, as in this example:
class C a where
type T (x :: a) :: [a]
instance C (Maybe a) where
type T x = ('[] :: [Maybe a])
In ('[] :: [Maybe a])
, the kind variable a
is implicitly bound by the
kind signature of the LHS type pattern x
.
6.4.9.4.4. Instance contexts and associated type and data instances¶
Associated type and data instance declarations do not inherit any
context specified on the enclosing instance. For type instance
declarations, it is unclear what the context would mean. For data
instance declarations, it is unlikely a user would want the context
repeated for every data constructor. The only place where the context
might likely be useful is in a deriving
clause of an associated data
instance. However, even here, the role of the outer instance context is
murky. So, for clarity, we just stick to the rule above: the enclosing
instance context is ignored. If you need to use a nontrivial context on
a derived instance, use a standalone deriving
clause (at the top level).
6.4.9.5. Import and export¶
The rules for export lists (Haskell Report Section 5.2) need adjustment for type families:
 The form
T(..)
, whereT
is a data family, names the familyT
and all the inscope constructors (whether in scope qualified or unqualified) that are data instances ofT
.  The form
T(.., ci, .., fj, ..)
, whereT
is a data family, namesT
and the specified constructorsci
and fieldsfj
as usual. The constructors and field names must belong to some data instance ofT
, but are not required to belong to the same instance.  The form
C(..)
, whereC
is a class, names the classC
and all its methods and associated types.  The form
C(.., mi, .., type Tj, ..)
, whereC
is a class, names the classC
, and the specified methodsmi
and associated typesTj
. The types need a keyword “type
” to distinguish them from data constructors.  Whenever there is no export list and a data instance is defined, the corresponding data family type constructor is exported along with the new data constructors, regardless of whether the data family is defined locally or in another module.
6.4.9.5.1. Examples¶
Recall our running GMapKey
class example:
class GMapKey k where
data GMap k :: Type > Type
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 functionsempty
,lookup
, andinsert
. The data constructors ofGMap
(in this caseGMapEither
) 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
, namelyGMapEither
.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., subcomponent specifications cannot be nested. To specifyGMap
‘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 inY
, namely the data constructorsD1
andD2
, and implicitly the data familyD
, even though it’s defined inX
. This means you can writeimport Y( D(D1,D2) )
without giving an explicit export list like this:module Y( D(..) ) where ... or module Y( module Y, D ) where ...
6.4.9.5.2. 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.
6.4.9.6. Type families and instance declarations¶
Type families require us to extend the rules for the form of instance heads, which are given in 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 ...
6.4.9.7. Injective type families¶

TypeFamilyDependencies
¶ Implies: TypeFamilies
Since: 8.0.1 Allow functional dependency annotations on type families. This allows one to define injective type families.
Starting with GHC 8.0 type families can be annotated with injectivity information. This information is then used by GHC during type checking to resolve type ambiguities in situations where a type variable appears only under type family applications. Consider this contrived example:
type family Id a
type instance Id Int = Int
type instance Id Bool = Bool
id :: Id t > Id t
id x = x
Here the definition of id
will be rejected because type variable t
appears only under type family applications and is thus ambiguous. But this
code will be accepted if we tell GHC that Id
is injective, which means it
will be possible to infer t
at call sites from the type of the argument:
type family Id a = r  r > a
Injective type families are enabled with XTypeFamilyDependencies
language
extension. This extension implies XTypeFamilies
.
For full details on injective type families refer to Haskell Symposium 2015 paper Injective type families for Haskell.
6.4.9.7.1. Syntax of injectivity annotation¶
The injectivity annotation is added after the type family head and consists of two parts:
 a type variable that names the result of a type family. Syntax:
= tyvar
or= (tyvar :: kind)
. The type variable must be fresh.  an injectivity annotation of the form
 A > B
, whereA
is the result type variable (see previous bullet) andB
is a list of argument type and kind variables in which type family is injective. It is possible to omit some variables if the type family is not injective in them.
Examples:
type family Id a = result  result > a where
type family F a b c = d  d > a c b
type family G (a :: k) b c = foo  foo > k b where
For open and closed type families it is OK to name the result but skip the injectivity annotation. This is not the case for associated type synonyms, where the named result without injectivity annotation will be interpreted as associated type synonym default.
6.4.9.7.2. Verifying the injectivity annotation against type family equations¶
Once the user declares type family to be injective GHC must verify that this declaration is correct, i.e., that type family equations don’t violate the injectivity annotation. A general idea is that if at least one equation (bullets (1), (2) and (3) below) or a pair of equations (bullets (4) and (5) below) violates the injectivity annotation then a type family is not injective in a way the user claims and an error is reported. In the bullets below RHS refers to the righthand side of the type family equation being checked for injectivity. LHS refers to the arguments of that type family equation. Below are the rules followed when checking injectivity of a type family:
If a RHS of a type family equation is a type family application GHC reports that the type family is not injective.
If a RHS of a type family equation is a bare type variable we require that all LHS variables (including implicit kind variables) are also bare. In other words, this has to be a sole equation of that type family and it has to cover all possible patterns. If the patterns are not covering GHC reports that the type family is not injective.
If a LHS type variable that is declared as injective is not mentioned on injective position in the RHS GHC reports that the type family is not injective. Injective position means either argument to a type constructor or injective argument to a type family. Type inference can potentially loop when looking under injective type families in the RHS, so this requires
UndecidableInstances
; GHC suggests enabling the flag when it is necessary.Open type families Open type families are typechecked incrementally. This means that when a module is imported type family instances contained in that module are checked against instances present in already imported modules.
A pair of an open type family equations is checked by attempting to unify their RHSs. If the RHSs don’t unify this pair does not violate injectivity annotation. If unification succeeds with a substitution then LHSs of unified equations must be identical under that substitution. If they are not identical then GHC reports that the type family is not injective.
In a closed type family all equations are ordered and in one place. Equations are also checked pairwise but this time an equation has to be paired with all the preceding equations. Of course a singleequation closed type family is trivially injective (unless (1), (2) or (3) above holds).
When checking a pair of closed type family equations GHC tried to unify their RHSs. If they don’t unify this pair of equations does not violate injectivity annotation. If the RHSs can be unified under some substitution (possibly empty) then either the LHSs unify under the same substitution or the LHS of the latter equation is subsumed by earlier equations. If neither condition is met GHC reports that a type family is not injective.
Note that for the purpose of injectivity check in bullets (4) and (5) GHC uses a special variant of unification algorithm that treats type family applications as possibly unifying with anything.