6.4.16. Type abstractions¶

TypeAbstractions
¶ Since: 9.8.1 Status: Partially implemented Allow the use of type abstraction syntax.
The TypeAbstractions
extension provides a way to explicitly bind
scoped type or kind variables using the @a
syntax. The feature is only
partially implemented, and this text covers only the implemented parts, whereas
the full specification can be found in GHC Proposals #448
and #425.
6.4.16.1. Type Abstractions in Patterns¶
The type abstraction syntax can be used in patterns that match a data constructor. The syntax can’t be used with record patterns or infix patterns. This is useful in particular to bind existential type variables associated with a GADT data constructor as in the following example:
{# LANGUAGE AllowAmbiguousTypes #}
{# LANGUAGE GADTs #}
{# LANGUAGE RankNTypes #}
{# LANGUAGE TypeApplications #}
import Data.Proxy
data Foo where
Foo :: forall a. (Show a, Num a) => Foo
test :: Foo > String
test x = case x of
Foo @t > show @t 0
main :: IO ()
main = print $ test (Foo @Float)
In this example, the case in test
is binding an existential variable introduced
by Foo
that otherwise could not be named and used.
It’s possible to bind variables to any part of the type arguments to a constructor; there is no need for them to be existential. In addition, it’s possible to “match” against part of the type argument using type constructors.
For a somewhatcontrived example:
foo :: (Num a) => Maybe [a] > String
foo (Nothing @[t]) = show (0 :: t)
foo (Just @[t] xs) = show (sum xs :: t)
Here, we’re binding the type variable t to be the type of the elements of the list type which is itself the argument to Maybe.
The order of the type arguments specified by the type applications in a pattern is the same
as that for an expression: either the order as given by the user in an explicit forall
in the
definition of the data constructor, or if that is not present, the order in which the type
variables appear in its type signature from left to right.
For example if we have the following declaration in GADT syntax:
data Foo :: * > * where
A :: forall s t. [(t,s)] > Foo (t,s)
B :: (t,s) > Foo (t,s)
Then the type arguments to A
will match first s
and then t
, while the type arguments
to B
will match first t
and then s
.
Type arguments appearing in patterns can influence the inferred type of a definition:
foo (Nothing @Int) = 0
foo (Just x) = x
will have inferred type:
foo :: Maybe Int > Int
which is more restricted than what it would be without the application:
foo :: Num a => Maybe a > a
For more information and detail regarding type applications in patterns, see the paper
Type variables in patterns by Eisenberg, Breitner
and Peyton Jones. Relative to that paper, the implementation in GHC for now at least makes one
additional conservative restriction, that type variables occurring in patterns must not
already be in scope, and so are always new variables that only bind whatever type is
matched, rather than ever referring to a variable from an outer scope. Type wildcards
_
may be used in any place where no new variable needs binding.
6.4.16.2. Invisible Binders in Type Declarations¶
6.4.16.2.1. Syntax¶
The type abstraction syntax can be used in type declaration headers, including
type
, data
, newtype
, class
, type family
, and data family
declarations. Here are a few examples:
type C :: forall k. k > Constraint
class C @k a where ...
^^
type D :: forall k j. k > j > Type
data D @k @j (a :: k) (b :: j) = ...
^^ ^^
type F :: forall p q. p > q > (p, q)
type family F @p @q a b where ...
^^ ^^
Just as ordinary type parameters, invisible type variable binders may have kind annotations:
type F :: forall p q. p > q > (p, q)
type family F @(p :: Type) @(q :: Type) (a :: p) (b :: q) where ...
6.4.16.2.2. Scope¶
The @k
binders scope over the body of the declaration and can be used to bring
implicit type or kind variables into scope. Consider:
type C :: forall i. (i > i > i) > Constraint
class C @i a where
p :: P a i
Without the @i
binder in C @i a
, the i
in P a i
would no longer
refer to the class variable i
and would be implicitly quantified in the
method signature instead.
6.4.16.2.3. Type checking¶
Invisible type variable binders require either a standalone kind signature or a complete usersupplied kind.
If a standalone kind signature is given, GHC will match up @k
binders with
the corresponding forall k.
quantifiers in the signature:
type B :: forall k. k > forall j. j > Type
data B @k (a :: k) @j (b :: j)
Quantifierbinder pairs of B 


forall k. 
@k 
k > 
(a :: k) 
forall j. 
@j 
j > 
(b :: j) 
The matching is done lefttoright. Consider:
type S :: forall a b. a > b > Type
type S @k x y = ...
In this example, @k
is matched with forall a.
, not forall b.
:
Quantifierbinder pairs of S 


forall a. 
@k 
forall b. 

a > 
x 
b > 
y 
When a standalone kind signature is absent but the definition has a complete
usersupplied kind (and the CUSKs
extension is enabled),
a @k
binder gives rise to a forall k.
quantifier in the inferred kind
signature. The inferred forall k.
does not float to the left; the order of
quantifiers continues to match the order of binders in the header:
 Inferred kind: forall k. k > forall j. j > Type
data B @(k :: Type) (a :: k) @(j :: Type) (b :: j)