6.4.11. Kind polymorphism¶

TypeInType
¶ Implies: PolyKinds
,DataKinds
,KindSignatures
Since: 8.0.1 The extension
TypeInType
is now deprecated: its sole effect is to switch onPolyKinds
(and henceKindSignatures
) andDataKinds
.

PolyKinds
¶ Implies: KindSignatures
Since: 7.4.1 Allow kind polymorphic types.
This section describes GHC’s kind system, as it appears in version 8.0 and beyond. The kind system as described here is always in effect, with or without extensions, although it is a conservative extension beyond standard Haskell. The extensions above simply enable syntax and tweak the inference algorithm to allow users to take advantage of the extra expressiveness of GHC’s kind system.
6.4.11.1. Overview of kind polymorphism¶
Consider inferring the kind for
data App f a = MkApp (f a)
In Haskell 98, the inferred kind for App
is (Type > Type) > Type >
Type
. But this is overly specific, because another suitable Haskell 98 kind
for App
is ((Type > Type) > Type) > (Type > Type) > Type
, where the
kind assigned to a
is Type > Type
. Indeed, without kind signatures
(KindSignatures
), it is necessary to use a dummy constructor to get
a Haskell compiler to infer the second kind. With kind polymorphism
(PolyKinds
), GHC infers the kind forall k. (k > Type) > k >
Type
for App
, which is its most general kind.
Thus, the chief benefit of kind polymorphism is that we can now infer these
most general kinds and use App
at a variety of kinds:
App Maybe Int  `k` is instantiated to Type
data T a = MkT (a Int)  `a` is inferred to have kind (Type > Type)
App T Maybe  `k` is instantiated to (Type > Type)
6.4.11.2. Overview of TypeinType¶
GHC 8 extends the idea of kind polymorphism by declaring that types and kinds
are indeed one and the same. Nothing within GHC distinguishes between types
and kinds. Another way of thinking about this is that the type Bool
and
the “promoted kind” Bool
are actually identical. (Note that term
True
and the type 'True
are still distinct, because the former can
be used in expressions and the latter in types.) This lack of distinction
between types and kinds is a hallmark of dependently typed languages.
Full dependently typed languages also remove the difference between expressions
and types, but doing that in GHC is a story for another day.
One simplification allowed by combining types and kinds is that the type of
Type
is just Type
. It is true that the Type :: Type
axiom can lead
to nontermination, but this is not a problem in GHC, as we already have other
means of nonterminating programs in both types and expressions. This decision
(among many, many others) does mean that despite the expressiveness of GHC’s
type system, a “proof” you write in Haskell is not an irrefutable mathematical
proof. GHC promises only partial correctness, that if your programs compile and
run to completion, their results indeed have the types assigned. It makes no
claim about programs that do not finish in a finite amount of time.
To learn more about this decision and the design of GHC under the hood please see the paper introducing this kind system to GHC/Haskell.
6.4.11.3. Principles of kind inference¶
Generally speaking, when PolyKinds
is on, GHC tries to infer the
most general kind for a declaration.
In many cases (for example, in a datatype declaration)
the definition has a righthand side to inform kind
inference. But that is not always the case. Consider
type family F a
Type family declarations have no righthand side, but GHC must still
infer a kind for F
. Since there are no constraints, it could infer
F :: forall k1 k2. k1 > k2
, but that seems too polymorphic. So
GHC defaults those entirelyunconstrained kind variables to Type
and we
get F :: Type > Type
. You can still declare F
to be kindpolymorphic
using kind signatures:
type family F1 a  F1 :: Type > Type
type family F2 (a :: k)  F2 :: forall k. k > Type
type family F3 a :: k  F3 :: forall k. Type > k
type family F4 (a :: k1) :: k2  F4 :: forall k1 k2. k1 > k2
The general principle is this:
 When there is a righthand side, GHC infers the most polymorphic kind consistent with the righthand side. Examples: ordinary data type and GADT declarations, class declarations. In the case of a class declaration the role of “right hand side” is played by the class method signatures.
 When there is no right hand side, GHC defaults argument and result
kinds to
Type
, except when directed otherwise by a kind signature. Examples: data and open type family declarations.
This rule has occasionallysurprising consequences (see #10132).
class C a where  Class declarations are generalised
 so C :: forall k. k > Constraint
data D1 a  No right hand side for these two family
type F1 a  declarations, but the class forces (a :: k)
 so D1, F1 :: forall k. k > Type
data D2 a  No righthand side so D2 :: Type > Type
type F2 a  No righthand side so F2 :: Type > Type
The kindpolymorphism from the class declaration makes D1
kindpolymorphic, but not so D2
; and similarly F1
, F2
.
6.4.11.4. Kind inference in type signatures¶
When kindchecking a type, GHC considers only what is written in that type when figuring out how to generalise the type’s kind.
For example,
consider these definitions (with ScopedTypeVariables
):
data Proxy a  Proxy :: forall k. k > Type
p :: forall a. Proxy a
p = Proxy :: Proxy (a :: Type)
GHC reports an error, saying that the kind of a
should be a kind variable
k
, not Type
. This is because, by looking at the type signature
forall a. Proxy a
, GHC assumes a
‘s kind should be generalised, not
restricted to be Type
. The function definition is then rejected for being
more specific than its type signature.
6.4.11.5. Explicit kind quantification¶
Enabled by PolyKinds
, GHC supports explicit kind quantification,
as in these examples:
data Proxy :: forall k. k > Type
f :: (forall k (a :: k). Proxy a > ()) > Int
Note that the second example has a forall
that binds both a kind k
and
a type variable a
of kind k
. In general, there is no limit to how
deeply nested this sort of dependency can work. However, the dependency must
be wellscoped: forall (a :: k) k. ...
is an error.
6.4.11.6. Inferring the order of variables in a type/class declaration¶
It is possible to get intricate dependencies among the type variables introduced in a type or class declaration. Here is an example:
data T a (b :: k) c = MkT (a c)
After analysing this declaration, GHC will discover that a
and
c
can be kindpolymorphic, with a :: k2 > Type
and
c :: k2
. We thus infer the following kind:
T :: forall {k2 :: Type} (k :: Type). (k2 > Type) > k > k2 > Type
Note that k2
is placed before k
, and that k
is placed before
a
. Also, note that k2
is written here in braces. As explained with
TypeApplications
(Inferred vs. specified type variables),
type and kind variables that GHC generalises
over, but not written in the original program, are not available for visible
type application. (These are called inferred variables.)
Such variables are written in braces.
The general principle is this:
 Variables not available for type application come first.
 Then come variables the user has written, implicitly brought into scope in a type variable’s kind.
 Lastly come the normal type variables of a declaration.
 Variables not given an explicit ordering by the user are sorted according to ScopedSort (Ordering of specified variables).
With the T
example above, we could bind k
after a
; doing so
would not violate dependency concerns. However, it would violate our general
principle, and so k
comes first.
Sometimes, this ordering does not respect dependency. For example:
data T2 k (a :: k) (c :: Proxy '[a, b])
It must be that a
and b
have the same kind. Note also that b
is implicitly declared in c
‘s kind. Thus, according to our general
principle, b
must come before k
. However, b
depends on
k
. We thus reject T2
with a suitable error message.
In associated types, we order the type variables as if the type family was a toplevel declaration, ignoring the visibilities of the class’s type variable binders. Here is an example:
class C (a :: k) b where
type F (c :: j) (d :: Proxy m) a b
We infer these kinds:
C :: forall {k1 :: Type} (k :: Type). k > k1 > Constraint
F :: forall {k1 :: Type} {k2 :: Type} {k3 :: Type} j (m :: k1).
j > Proxy m > k2 > k3 > Type
Note that the kind of a
is specified in the kind of C
but inferred in
the kind of F
.
The “general principle” described here is meant to make all this more predictable for users. It would not be hard to extend GHC to relax this principle. If you should want a change here, consider writing a proposal to do so.
6.4.11.7. Complete usersupplied kind signatures and polymorphic recursion¶

CUSKs
¶ Since: 8.10.1
NB! This is a legacy feature, see StandaloneKindSignatures
for the
modern replacement.
Just as in type inference, kind inference for recursive types can only use monomorphic recursion. Consider this (contrived) example:
data T m a = MkT (m a) (T Maybe (m a))
 GHC infers kind T :: (Type > Type) > Type > Type
The recursive use of T
forced the second argument to have kind
Type
. However, just as in type inference, you can achieve polymorphic
recursion by giving a complete usersupplied kind signature (or CUSK)
for T
. A CUSK is present when all argument kinds and the result kind
are known, without any need for inference. For example:
data T (m :: k > Type) :: k > Type where
MkT :: m a > T Maybe (m a) > T m a
The complete usersupplied kind signature specifies the polymorphic kind
for T
, and this signature is used for all the calls to T
including the recursive ones. In particular, the recursive use of T
is at kind Type
.
What exactly is considered to be a “complete usersupplied kind signature” for a type constructor? These are the forms:
For a datatype, every type variable must be annotated with a kind. In a GADTstyle declaration, there may also be a kind signature (with a toplevel
::
in the header), but the presence or absence of this annotation does not affect whether or not the declaration has a complete signature.data T1 :: (k > Type) > k > Type where ...  Yes; T1 :: forall k. (k>Type) > k > Type data T2 (a :: k > Type) :: k > Type where ...  Yes; T2 :: forall k. (k>Type) > k > Type data T3 (a :: k > Type) (b :: k) :: Type where ...  Yes; T3 :: forall k. (k>Type) > k > Type data T4 (a :: k > Type) (b :: k) where ...  Yes; T4 :: forall k. (k>Type) > k > Type data T5 a (b :: k) :: Type where ...  No; kind is inferred data T6 a b where ...  No; kind is inferred
For a datatype with a toplevel
::
: all kind variables introduced after the::
must be explicitly quantified.data T1 :: k > Type  No CUSK: `k` is not explicitly quantified data T2 :: forall k. k > Type  CUSK: `k` is bound explicitly data T3 :: forall (k :: Type). k > Type  still a CUSK
For a newtype, the rules are the same as they are for a data type unless
UnliftedNewtypes
is enabled. WithUnliftedNewtypes
, the type constructor only has a CUSK if a kind signature is present. As with a datatype with a toplevel::
, all kind variables introduced after the::
must be explicitly quantified{# LANGUAGE UnliftedNewtypes #} newtype N1 where  No; missing kind signature newtype N2 :: TYPE 'IntRep where  Yes; kind signature present newtype N3 (a :: Type) where  No; missing kind signature newtype N4 :: k > Type where  No; `k` is not explicitly quantified newtype N5 :: forall (k :: Type). k > Type where  Yes; good signature
For a class, every type variable must be annotated with a kind.
For a type synonym, every type variable and the result type must all be annotated with kinds:
type S1 (a :: k) = (a :: k)  Yes S1 :: forall k. k > k type S2 (a :: k) = a  No kind is inferred type S3 (a :: k) = Proxy a  No kind is inferred
Note that in
S2
andS3
, the kind of the righthand side is rather apparent, but it is still not considered to have a complete signature – no inference can be done before detecting the signature.An unassociated open type or data family declaration always has a CUSK; unannotated type variables default to kind
Type
:data family D1 a  D1 :: Type > Type data family D2 (a :: k)  D2 :: forall k. k > Type data family D3 (a :: k) :: Type  D3 :: forall k. k > Type type family S1 a :: k > Type  S1 :: forall k. Type > k > Type
An associated type or data family declaration has a CUSK precisely if its enclosing class has a CUSK.
class C a where  no CUSK type AT a b  no CUSK, b is defaulted class D (a :: k) where  yes CUSK type AT2 a b  yes CUSK, b is defaulted
A closed type family has a complete signature when all of its type variables are annotated and a return kind (with a toplevel
::
) is supplied.
It is possible to write a datatype that syntactically has a CUSK (according to the rules above) but actually requires some inference. As a very contrived example, consider
data Proxy a  Proxy :: forall k. k > Type
data X (a :: Proxy k)
According to the rules above X
has a CUSK. Yet, the kind of k
is undetermined.
It is thus quantified over, giving X
the kind forall k1 (k :: k1). Proxy k > Type
.
The detection of CUSKs is enabled by the CUSKs
flag, which is
switched on by default. This extension is scheduled for deprecation to be
replaced with StandaloneKindSignatures
.
6.4.11.8. Standalone kind signatures and polymorphic recursion¶
Just as in type inference, kind inference for recursive types can only use monomorphic recursion. Consider this (contrived) example:
data T m a = MkT (m a) (T Maybe (m a))
 GHC infers kind T :: (Type > Type) > Type > Type
The recursive use of T
forced the second argument to have kind
Type
. However, just as in type inference, you can achieve polymorphic
recursion by giving a standalone kind signature for T
:
type T :: (k > Type) > k > Type
data T m a = MkT (m a) (T Maybe (m a))
The standalone kind signature specifies the polymorphic kind
for T
, and this signature is used for all the calls to T
including the recursive ones. In particular, the recursive use of T
is at kind Type
.
While a standalone kind signature determines the kind of a type constructor, it does not determine its arity. This is of particular importance for type families and type synonyms, as they cannot be partially applied. See Type family declarations for more information about arity.
The arity can be specified using explicit binders and inline kind annotations:
 arity F0 = 0
type F0 :: forall k. k > Type
type family F0 :: forall k. k > Type
 arity F1 = 1
type F1 :: forall k. k > Type
type family F1 :: k > Type
 arity F2 = 2
type F2 :: forall k. k > Type
type family F2 a :: Type
In absence of an inline kind annotation, the inferred arity includes all explicitly bound parameters and all immediately following invisible parameters:
 arity FD1 = 1
type FD1 :: forall k. k > Type
type FD1
 arity FD2 = 2
type FD2 :: forall k. k > Type
type FD2 a
Note that F0
, F1
, F2
, FD1
, and FD2
all have identical
standalone kind signatures. The arity is inferred from the type family header.
6.4.11.9. Standalone kind signatures and declaration headers¶
GHC requires that in the presence of a standalone kind signature, data declarations must bind all their inputs. For example:
type Prox1 :: k > Type
data Prox1 a = MkProx1
 OK.
type Prox2 :: k > Type
data Prox2 = MkProx2
 Error:
 • Expected a type, but found something with kind ‘k > Type’
 • In the data type declaration for ‘Prox2’
GADTstyle data declarations may either bind their inputs or use an inline signature in addition to the standalone kind signature:
type GProx1 :: k > Type
data GProx1 a where MkGProx1 :: GProx1 a
 OK.
type GProx2 :: k > Type
data GProx2 where MkGProx2 :: GProx2 a
 Error:
 • Expected a type, but found something with kind ‘k > Type’
 • In the data type declaration for ‘GProx2’
type GProx3 :: k > Type
data GProx3 :: k > Type where MkGProx3 :: GProx3 a
 OK.
type GProx4 :: k > Type
data GProx4 :: w where MkGProx4 :: GProx4 a
 OK, w ~ (k > Type)
Classes are subject to the same rules:
type C1 :: Type > Constraint
class C1 a
 OK.
type C2 :: Type > Constraint
class C2
 Error:
 • Couldn't match expected kind ‘Constraint’
 with actual kind ‘Type > Constraint’
 • In the class declaration for ‘C2’
On the other hand, type families are exempt from this rule:
type F :: Type > Type
type family F
 OK.
Data families are tricky territory. Their headers are exempt from this rule, but their instances are not:
type T :: k > Type
data family T
 OK.
data instance T Int = MkT1
 OK.
data instance T = MkT3
 Error:
 • Expecting one more argument to ‘T’
 Expected a type, but ‘T’ has kind ‘k0 > Type’
 • In the data instance declaration for ‘T’
This also applies to GADTstyle data instances:
data instance T (a :: Nat) where MkN4 :: T 4
MKN9 :: T 9
 OK.
data instance T :: Symbol > Type where MkSN :: T "Neptune"
MkSJ :: T "Jupiter"
 OK.
data instance T where MkT4 :: T x
 Error:
 • Expecting one more argument to ‘T’
 Expected a type, but ‘T’ has kind ‘k0 > Type’
 • In the data instance declaration for ‘T’
6.4.11.10. Kind inference in data type declarations¶
Consider the declaration
data T1 f a = MkT1 (f a)
data T2 f a where
MkT2 :: f a > T f a
In both cases GHC looks at the data constructor declarations to
give constraints on the kind of T
, yielding
T1, T2 :: forall k. (k > Type) > k > Type
Consider the type
type G :: forall k. k > Type
data G (a :: k) where
GInt :: G Int
GMaybe :: G Maybe
This datatype G
is GADTlike in both its kind and its type. Suppose you
have g :: G a
, where a :: k
. Then pattern matching to discover that
g
is in fact GMaybe
tells you both that k ~ (Type > Type)
and
a ~ Maybe
. The definition for G
requires that PolyKinds
be in effect, but patternmatching on G
requires no extension beyond
GADTs
. That this works is actually a straightforward extension
of regular GADTs and a consequence of the fact that kinds and types are the
same.
Note that the datatype G
is used at different kinds in its body, and
therefore that kindindexed GADTs use a form of polymorphic recursion.
It is thus only possible to use this feature if you have provided a
complete usersupplied kind signature (CUSK)
for the datatype (Complete usersupplied kind signatures and polymorphic recursion), or a standalone
kind signature (Standalone kind signatures and polymorphic recursion);
in the case of G
we both.
If you wish to see the kind indexing explicitly, you can do so by enabling fprintexplicitkinds
and querying G
with GHCi’s :info
command:
> :set fprintexplicitkinds
> :info G
type role G nominal nominal
type G :: forall k. k > Type
data G @k a where
GInt :: G @Type Int
GMaybe :: G @(Type > Type) Maybe
where you can see the GADTlike nature of the two constructors.
6.4.11.11. Kind inference for data/newtype instance declarations¶
Consider these declarations
data family T :: forall k. (k>Type) > k > Type
data instance T p q where
MkT :: forall r. r Int > T r Int
Here T
has an invisible kind argument; and perhaps it is instantiated
to Type
in the instance, thus:
data instance T @Type (p :: Type>Type) (q :: Type) where
MkT :: forall r. r Int > T r Int
Or perhaps we intended the specialisation to be in the GADT data constructor, thus:
data instance T @k (p :: k>Type) (q :: k) where
MkT :: forall r. r Int > T @Type r Int
It gets more complicated if there are multiple constructors. In general, there is no principled way to tell which type specialisation comes from the data instance, and which from the individual GADT data constructors.
So GHC implements this rule: in data/newtype instance declararations (unlike ordinary data/newtype declarations) we do not look at the constructor declarations when inferring the shape of the instance header. The principle is that the instantiation of the data instance should be apparent from the header alone. This principle makes the program easier to understand, and avoids the swamp of complexity indicated above.
6.4.11.12. Kind inference in class instance declarations¶
Consider the following example of a polykinded class and an instance for it:
class C a where
type F a
instance C b where
type F b = b > b
In the class declaration, nothing constrains the kind of the type a
,
so it becomes a polykinded type variable (a :: k)
. Yet, in the
instance declaration, the righthand side of the associated type
instance b > b
says that b
must be of kind Type
. GHC could
theoretically propagate this information back into the instance head,
and make that instance declaration apply only to type of kind Type
, as
opposed to types of any kind. However, GHC does not do this.
In short: GHC does not propagate kind information from the members of a class instance declaration into the instance declaration head.
This lack of kind inference is simply an engineering problem within GHC,
but getting it to work would make a substantial change to the inference
infrastructure, and it’s not clear the payoff is worth it. If you want
to restrict b
‘s kind in the instance above, just use a kind
signature in the instance head.
6.4.11.13. Kind inference in type synonyms and type family instances¶
Consider the scoping rules for type synonyms and type family instances, such as these:
type TS a (b :: k) = <rhs>
type instance TF a (b :: k) = <rhs>
The basic principle is that all variables mentioned on the right hand side
<rhs>
must be bound on the left hand side:
type TS a (b :: k) = (k, a, Proxy b)  accepted
type TS a (b :: k) = (k, a, Proxy b, z)  rejected: z not in scope
But there is one exception: free variables mentioned in the outermost kind
signature on the right hand side are quantified implicitly. Thus, in the
following example the variables a
, b
, and k
are all in scope on the
right hand side of S
:
type S a b = <rhs> :: k > k
The reason for this exception is that there may be no other way to bind k
.
For example, suppose we wanted S
to have the following kind with an
invisible parameter k
:
S :: forall k. Type > Type > k > k
In this case, we could not simply bind k
on the lefthand side, as k
would become a visible parameter:
type S k a b = <rhs> :: k > k
S :: forall k > Type > Type > k > k
Note that we only look at the outermost kind signature to decide which
variables to quantify implicitly. As a counterexample, consider M1
:
type M1 = 'Just ('Nothing :: Maybe k)  rejected: k not in scope
Here, the kind signature is hidden inside 'Just
, and there is no outermost
kind signature. We can fix this example by providing an outermost kind signature:
type M2 = 'Just ('Nothing :: Maybe k) :: Maybe (Maybe k)
Here, k
is brought into scope by :: Maybe (Maybe k)
.
A kind signature is considered to be outermost regardless of redundant parentheses:
type P = 'Nothing :: Maybe a  accepted
type P = ((('Nothing :: Maybe a)))  accepted
Closed type family instances are subject to the same rules:
type family F where
F = 'Nothing :: Maybe k  accepted
type family F where
F = 'Just ('Nothing :: Maybe k)  rejected: k not in scope
type family F where
F = 'Just ('Nothing :: Maybe k) :: Maybe (Maybe k)  accepted
type family F :: Maybe (Maybe k) where
F = 'Just ('Nothing :: Maybe k)  rejected: k not in scope
type family F :: Maybe (Maybe k) where
F @k = 'Just ('Nothing :: Maybe k)  accepted
Kind variables can also be quantified in visible positions. Consider the following two examples:
data ProxyKInvis (a :: k)
data ProxyKVis k (a :: k)
In the first example, the kind variable k
is an invisible argument to
ProxyKInvis
. In other words, a user does not need to instantiate k
explicitly, as kind inference automatically determines what k
should be.
For instance, in ProxyKInvis True
, k
is inferred to be Bool
.
This is reflected in the kind of ProxyKInvis
:
ProxyKInvis :: forall k. k > Type
In the second example, k
is a visible argument to ProxyKVis
. That is
to say, k
is an argument that users must provide explicitly when applying
ProxyKVis
. For example, ProxyKVis Bool True
is a well formed type.
What is the kind of ProxyKVis
? One might say
forall k. Type > k > Type
, but this isn’t quite right, since this would
allow incorrect things like ProxyKVis Bool Int
, which should be rejected
due to the fact that Int
is not of kind Bool
. The key observation is that
the kind of the second argument depends on the first argument. GHC indicates
this dependency in the syntax that it gives for the kind of ProxyKVis
:
ProxyKVis :: forall k > k > Type
This kind is similar to the kind of ProxyKInvis
, but with a key difference:
the type variables quantified by the forall
are followed by an arrow
(>
), not a dot (.
). This is a visible, dependent quantifier. It is
visible in that the user must pass in a type for k
explicitly, and it is
dependent in the sense that k
appears later in the kind of ProxyKVis
.
As a counterpart, the k
binder in forall k. k > Type
can be thought
of as an invisible, dependent quantifier.
GHC permits writing kinds with this syntax, provided that the
ExplicitForAll
and PolyKinds
language extensions are enabled. Just
like the invisible forall
, one can put explicit kind signatures on visibly
bound kind variables, so the following is syntactically valid:
ProxyKVis :: forall (k :: Type) > k > Type
Currently, the ability to write visible, dependent quantifiers is limited to kinds. Consequently, visible dependent quantifiers are rejected in any context that is unambiguously the type of a term. They are also rejected in the types of data constructors.
6.4.11.14. Kind inference in closed type families¶
Although all open type families are considered to have a complete usersupplied kind signature (Complete usersupplied kind signatures and polymorphic recursion), we can relax this condition for closed type families, where we have equations on which to perform kind inference. GHC will infer kinds for the arguments and result types of a closed type family.
GHC supports kindindexed type families, where the family matches both on the kind and type. GHC will not infer this behaviour without a complete usersupplied kind signature or standalone kind signature (see Standalone kind signatures and polymorphic recursion), because doing so would sometimes infer nonprincipal types. Indeed, we can see kindindexing as a form of polymorphic recursion, where a type is used at a kind other than its most general in its own definition.
For example:
type family F1 a where
F1 True = False
F1 False = True
F1 x = x
 F1 fails to compile: kindindexing is not inferred
type family F2 (a :: k) where
F2 True = False
F2 False = True
F2 x = x
 F2 fails to compile: no complete signature
type family F3 (a :: k) :: k where
F3 True = False
F3 False = True
F3 x = x
 OK
6.4.11.15. Higherrank kinds¶
In concert with RankNTypes
, GHC supports higherrank kinds.
Here is an example:
 Heterogeneous propositional equality
data (a :: k1) :~~: (b :: k2) where
HRefl :: a :~~: a
class HTestEquality (t :: forall k. k > Type) where
hTestEquality :: forall k1 k2 (a :: k1) (b :: k2). t a > t b > Maybe (a :~~: b)
Note that hTestEquality
takes two arguments where the type variable t
is applied
to types of different kinds. That type variable must then be polykinded. Accordingly,
the kind of HTestEquality
(the class) is (forall k. k > Type) > Constraint
,
a higherrank kind.
A big difference with higherrank kinds as compared with higherrank types is that
forall
s in kinds cannot be moved. This is best illustrated by example.
Suppose we want to have an instance of HTestEquality
for (:~~:)
.
instance HTestEquality ((:~~:) a) where
hTestEquality HRefl HRefl = Just HRefl
With the declaration of (:~~:)
above, it gets kind forall k1 k2. k1 > k2 > Type
.
Thus, the type (:~~:) a
has kind k2 > Type
for some k2
. GHC cannot
then regeneralize this kind to become forall k2. k2 > Type
as desired. Thus, the
instance is rejected as illkinded.
To allow for such an instance, we would have to define (:~~:)
as follows:
data (:~~:) :: forall k1. k1 > forall k2. k2 > Type where
HRefl :: a :~~: a
In this redefinition, we give an explicit kind for (:~~:)
, deferring the choice
of k2
until after the first argument (a
) has been given. With this declaration
for (:~~:)
, the instance for HTestEquality
is accepted.
6.4.11.16. The kind Type
¶

StarIsType
¶ Since: 8.6.1 Treat the unqualified uses of the
*
type operator as nullary and desugar toData.Kind.Type
.
The kind Type
(imported from Data.Kind
) classifies ordinary types. With
StarIsType
(currently enabled by default), *
is desugared to
Type
, but using this legacy syntax is not recommended due to conflicts with
TypeOperators
. This also applies to ★
, the Unicode variant of
*
.
6.4.11.17. Inferring dependency in datatype declarations¶
If a type variable a
in a datatype, class, or type family declaration
depends on another such variable k
in the same declaration, two properties
must hold:
a
must appear afterk
in the declaration, andk
must appear explicitly in the kind of some type variable in that declaration.
The first bullet simply means that the dependency must be wellscoped. The
second bullet concerns GHC’s ability to infer dependency. Inferring this
dependency is difficult, and GHC currently requires the dependency to be
made explicit, meaning that k
must appear in the kind of a type variable,
making it obvious to GHC that dependency is intended. For example:
data Proxy k (a :: k)  OK: dependency is "obvious"
data Proxy2 k a = P (Proxy k a)  ERROR: dependency is unclear
In the second declaration, GHC cannot immediately tell that k
should
be a dependent variable, and so the declaration is rejected.
It is conceivable that this restriction will be relaxed in the future, but it is (at the time of writing) unclear if the difficulties around this scenario are theoretical (inferring this dependency would mean our type system does not have principal types) or merely practical (inferring this dependency is hard, given GHC’s implementation). So, GHC takes the easy way out and requires a little help from the user.
6.4.11.18. Inferring dependency in userwritten forall
s¶
A programmer may use forall
in a type to introduce new quantified type
variables. These variables may depend on each other, even in the same
forall
. However, GHC requires that the dependency be inferrable from
the body of the forall
. Here are some examples:
data Proxy k (a :: k) = MkProxy  just to use below
f :: forall k a. Proxy k a  This is just fine. We see that (a :: k).
f = undefined
g :: Proxy k a > ()  This is to use below.
g = undefined
data Sing a
h :: forall k a. Sing k > Sing a > ()  No obvious relationship between k and a
h _ _ = g (MkProxy :: Proxy k a)  This fails. We didn't know that a should have kind k.
Note that in the last example, it’s impossible to learn that a
depends on k
in the
body of the forall
(that is, the Sing k > Sing a > ()
). And so GHC rejects
the program.
6.4.11.19. Kind defaulting without PolyKinds
¶
Without PolyKinds
, GHC refuses to generalise over kind variables.
It thus defaults kind variables to Type
when possible; when this is not
possible, an error is issued.
Here is an example of this in action:
{# LANGUAGE PolyKinds #}
import Data.Kind (Type)
data Proxy a = P  inferred kind: Proxy :: k > Type
data Compose f g x = MkCompose (f (g x))
 inferred kind: Compose :: (b > Type) > (a > b) > a > Type
 separate module having imported the first
{# LANGUAGE NoPolyKinds, DataKinds #}
z = Proxy :: Proxy 'MkCompose
In the last line, we use the promoted constructor 'MkCompose
, which has
kind
forall (a :: Type) (b :: Type) (f :: b > Type) (g :: a > b) (x :: a).
f (g x) > Compose f g x
Now we must infer a type for z
. To do so without generalising over kind
variables, we must default the kind variables of 'MkCompose
. We can easily
default a
and b
to Type
, but f
and g
would be illkinded if
defaulted. The definition for z
is thus an error.
6.4.11.20. Prettyprinting in the presence of kind polymorphism¶
With kind polymorphism, there is quite a bit going on behind the scenes that
may be invisible to a Haskell programmer. GHC supports several flags that
control how types are printed in error messages and at the GHCi prompt.
See the discussion of type prettyprinting options
for further details. If you are using kind polymorphism and are confused as to
why GHC is rejecting (or accepting) your program, we encourage you to turn on
these flags, especially fprintexplicitkinds
.
6.4.11.21. Datatype return kinds¶
With KindSignatures
, we can give the kind of a datatype written
in GADTsyntax (see GADTSyntax
). For example:
data T :: Type > Type where ...
There are a number of restrictions around these return kinds. The text below
considers UnliftedNewtypes
and data families (enabled by TypeFamilies
).
The discussion also assumes familiarity with levity polymorphism.
data
anddata instance
declarations must have return kinds that end inTYPE LiftedRep
. (Recall thatType
is just a synonym forTYPE LiftedRep
.) By “end in”, we refer to the kind left over after all arguments (introduced either byforall
or>
) are stripped off and type synonyms expanded. Note that no type family expansion is done when performing this check.If
UnliftedNewtypes
is enabled, thennewtype
andnewtype instance
declarations must have return kinds that end inTYPE rep
for somerep
. Therep
may mention type families, but theTYPE
must be apparent without type family expansion. (Type synonym expansion is acceptable.)If
UnliftedNewtypes
is not enabled, thennewtype
andnewtype instance
declarations have the same restrictions asdata
declarations.A
data
ornewtype
instance actually can have two return kinds. The first is the kind derived by applying the data family to the patterns provided in the instance declaration. The second is given by a kind annotation. Both return kinds must satisfy the restrictions above.
Examples:
data T1 :: Type  good: Type expands to TYPE LiftedRep
data T2 :: TYPE LiftedRep  good
data T3 :: forall k. k > Type > Type  good: arguments are dropped
type LR = LiftedRep
data T3 :: TYPE LR  good: we look through type synonyms
type family F a where
F Int = LiftedRep
data T4 :: TYPE (F Int)  bad: we do not look through type families
type family G a where
G Int = Type
data T5 :: G Int  bad: we do not look through type families
 assume XUnliftedNewtypes
newtype T6 :: Type where ...  good
newtype T7 :: TYPE (F Int) where ...  good
newtype T8 :: G Int where ...  bad
data family DF a :: Type
data instance DF Int :: Type  good
data instance DF Bool :: TYPE LiftedRep  good
data instance DF Char :: G Int  bad
data family DF2 k :: k  good
data family DF2 Type  good
data family DF2 Bool  bad
data family DF2 (G Int)  bad for 2 reasons:
 a type family can't be in a pattern, and
 the kind fails the restrictions here