6.4.16. Arbitraryrank polymorphism¶

RankNTypes
¶ Implies: ExplicitForAll
Since: 6.8.1 Status: Included in GHC2021
Allow types of arbitrary rank.

Rank2Types
¶ Since: 6.8.1 Status: Deprecated A deprecated alias of
RankNTypes
.
GHC’s type system supports arbitraryrank explicit universal quantification in types. For example, all the following types are legal:
f1 :: forall a b. a > b > a
g1 :: forall a b. (Ord a, Eq b) => a > b > a
f2 :: (forall a. a>a) > Int > Int
g2 :: (forall a. Eq a => [a] > a > Bool) > Int > Int
f3 :: ((forall a. a>a) > Int) > Bool > Bool
Here, f1
and g1
are rank1 types, and can be written in standard
Haskell (e.g. f1 :: a>b>a
). The forall
makes explicit the
universal quantification that is implicitly added by Haskell.
The functions f2
and g2
have rank2 types; the forall
is on
the left of a function arrow. As g2
shows, the polymorphic type on
the left of the function arrow can be overloaded.
The function f3
has a rank3 type; it has rank2 types on the left
of a function arrow.
The language option RankNTypes
(which implies
ExplicitForAll
) enables higherrank
types. That is, you can nest forall
s arbitrarily deep in function
arrows. For example, a foralltype (also called a “type scheme”),
including a typeclass context, is legal:
 On the left or right of a function arrow.
 As the argument of a constructor, or type of a field, in a data type
declaration. For example, any of the
f1, f2, f3, g1, g2
above would be valid field type signatures.  As the type of an implicit parameter.
 In a pattern type signature (see Lexically scoped type variables).
In particular, in data
and newtype
declarations the constructor
arguments may be polymorphic types of any rank; see examples in
Examples. Note that the declared types are nevertheless always
monomorphic. This is important because by default GHC will not
instantiate type variables to a polymorphic type
(Impredicative polymorphism).
Note that the RankNTypes
option is also required for any type
with a forall
or context to the right of an arrow. For example:
h1 :: Int > (forall a. a > a)
h1' :: forall a. Int > (a > a)
k1 :: Int > Ord a => a > a
k1' :: Ord a => Int > a > a
The function h1
has a rank1 type; it has the same behaviour as h1'
,
except with a different order of arguments. This matters if one were to specify
the type explicitly using a visible type application (using TypeApplications
):
we would write h1 3 @Bool True
but h1' @Bool 3 True
.
Similarly, k1
has a rank1 type; it differs from k1'
only in the order of
arguments. As the types of h1
and k1
are not allowed in Haskell98, we also
require users to enable RankNTypes
to write them (which seems more
sensible than inventing a separate extension just for this case).
The obsolete language option Rank2Types
is a synonym for
RankNTypes
. They used to specify finer distinctions that GHC no
longer makes.
6.4.16.1. Examples¶
These are examples of data
and newtype
declarations whose data
constructors have polymorphic argument types:
data T a = T1 (forall b. b > b > b) a
data MonadT m = MkMonad { return :: forall a. a > m a,
bind :: forall a b. m a > (a > m b) > m b
}
newtype Swizzle = MkSwizzle (forall a. Ord a => [a] > [a])
The constructors have rank2 types:
T1 :: forall a. (forall b. b > b > b) > a > T a
MkMonad :: forall m. (forall a. a > m a)
> (forall a b. m a > (a > m b) > m b)
> MonadT m
MkSwizzle :: (forall a. Ord a => [a] > [a]) > Swizzle
In earlier versions of GHC, it was possible to omit the forall
in
the type of the constructor if there was an explicit context. For
example:
newtype Swizzle' = MkSwizzle' (Ord a => [a] > [a])
Since GHC 8.0 declarations such as MkSwizzle'
will cause an outofscope
error.
You construct values of types T1, MonadT, Swizzle
by applying the
constructor to suitable values, just as usual. For example,
a1 :: T Int
a1 = T1 (\x y>x) 3
a2, a3 :: Swizzle
a2 = MkSwizzle sort
a3 = MkSwizzle reverse
a4 :: MonadT Maybe
a4 = let r x = Just x
b m k = case m of
Just y > k y
Nothing > Nothing
in
MkMonad r b
mkTs :: (forall b. b > b > b) > a > a > [T a]
mkTs f x y = [T1 f x, T1 f y]
The type of the argument can, as usual, be more general than the type
required, as (MkSwizzle reverse)
shows. (reverse
does not need
the Ord
constraint.)
When you use pattern matching, the bound variables may now have polymorphic types. For example:
f :: T a > a > (a, Char)
f (T1 w k) x = (w k x, w 'c' 'd')
g :: (Ord a, Ord b) => Swizzle > [a] > (a > b) > [b]
g (MkSwizzle s) xs f = s (map f (s xs))
h :: MonadT m > [m a] > m [a]
h m [] = return m []
h m (x:xs) = bind m x $ \y >
bind m (h m xs) $ \ys >
return m (y:ys)
In the function h
we use the record selectors return
and
bind
to extract the polymorphic bind and return functions from the
MonadT
data structure, rather than using pattern matching.
6.4.16.2. Subsumption¶
Suppose:
f1 :: (forall a b. Int > a > b > b) > Bool
g1 :: forall x y. Int > y > x > x
f2 :: (forall a. (Eq a, Show a) => a > a) > Bool
g2 :: forall x. (Show x, Eq x) => x > x
then f1 g1
and f2 g2
are both well typed, despite the
different order of type variables and constraints. What happens is that the
argument is instantiated, and then regeneralised to match the type expected
by the function.
But this instantiation and regeneralisation happens only at the top level of a type. In particular, none of this happens if the foralls are underneath an arrow. For example:
f3 :: (Int > forall a b. a > b > b) > Bool
g3a :: Int > forall x y. x > y > y
g3b :: forall x. Int > forall y. x > y > y
g3c :: Int > forall x y. y > x > x
f4 :: (Int > forall a. (Eq a, Show a) => a > a) > Bool
g4 :: Int > forall x. (Show x, Eq x) => x > x) > Bool
Then the application f3 g3a
is welltyped, because g3a
has a type that matches the type
expected by f3
. But f3 g3b
is not well typed, because the foralls are in different places.
Nor is f3 g3c
, where the foralls are in the same place but the variables are in a different order.
Similarly f4 g4
is not well typed, because the constraints appear in a different order.
These examples can be made to typecheck by etaexpansion. For example f3 (\x > g3b x)
is well typed, and similarly f3 (\x > g3c x)
and f4 (\x > g4 x)
.
A similar phenomenon occurs for operator sections. For example,
(\`g3a\` "hello")
is not well typed, but it can be made to typecheck by eta
expanding it to \x > x \`g3a\` "hello"
.

DeepSubsumption
¶ Since: 9.2.4 Relax the simple subsumption rules, implicitly inserting etaexpansions when matching up function types with different quantification structures.
The DeepSubsumption
extension relaxes the aforementioned requirement that
foralls must appear in the same place. GHC will instead automatically rewrite expressions
like f x
of type ty1 > ty2
to become (\ (y :: ty1) > f x y)
; this is called etaexpansion.
See Section 4.6 of
Practical type inference for arbitraryrank types,
where this process is called “deep skolemisation”.
Note that these etaexpansions may silently change the semantics of the user’s program:
h1 :: Int > forall a. a > a
h1 = undefined
h2 :: forall b. Int > b > b
h2 = h1
With DeepSubsumption
, GHC will accept these definitions,
inserting an implicit etaexpansion:
h2 = \ i > h1 i
This means that h2 `seq` ()
will not crash, even though h1 `seq` ()
does crash.
Historical note: Deep skolemisation was initially removed from the language by
GHC Proposal #287,
but was reintroduced as part of the DeepSubsumption
extension following
GHC Proposal #511.
6.4.16.3. Type inference¶
In general, type inference for arbitraryrank types is undecidable. GHC uses an algorithm proposed by Odersky and Laufer (“Putting type annotations to work”, POPL‘96) to get a decidable algorithm by requiring some help from the programmer. We do not yet have a formal specification of “some help” but the rule is this:
For a lambdabound or casebound variable, x, either the programmer provides an explicit polymorphic type for x, or GHC’s type inference will assume that x’s type has no foralls in it.
What does it mean to “provide” an explicit type for x? You can do that by giving a type signature for x directly, using a pattern type signature (Lexically scoped type variables), thus:
\ f :: (forall a. a>a) > (f True, f 'c')
Alternatively, you can give a type signature to the enclosing context, which GHC can “push down” to find the type for the variable:
(\ f > (f True, f 'c')) :: (forall a. a>a) > (Bool,Char)
Here the type signature on the expression can be pushed inwards to give a type signature for f. Similarly, and more commonly, one can give a type signature for the function itself:
h :: (forall a. a>a) > (Bool,Char)
h f = (f True, f 'c')
You don’t need to give a type signature if the lambda bound variable is a constructor argument. Here is an example we saw earlier:
f :: T a > a > (a, Char)
f (T1 w k) x = (w k x, w 'c' 'd')
Here we do not need to give a type signature to w
, because it is an
argument of constructor T1
and that tells GHC all it needs to know.
6.4.16.4. Implicit quantification¶
GHC performs implicit quantification as follows. At the outermost level
(only) of userwritten types, if and only if there is no explicit
forall
, GHC finds all the type variables mentioned in the type that
are not already in scope, and universally quantifies them. For example,
the following pairs are equivalent:
f :: a > a
f :: forall a. a > a
g (x::a) = let
h :: a > b > b
h x y = y
in ...
g (x::a) = let
h :: forall b. a > b > b
h x y = y
in ...
Notice that GHC always adds implicit quantifiers at the outermost level of a userwritten type; it does not find the innermost possible quantification point. For example:
f :: (a > a) > Int
 MEANS
f :: forall a. (a > a) > Int
 NOT
f :: (forall a. a > a) > Int
g :: (Ord a => a > a) > Int
 MEANS
g :: forall a. (Ord a => a > a) > Int
 NOT
g :: (forall a. Ord a => a > a) > Int
If you want the latter type, you can write
your forall
s explicitly. Indeed, doing so is strongly advised for
rank2 types.
Sometimes there is no “outermost level”, in which case no implicit quantification happens:
data PackMap a b s t = PackMap (Monad f => (a > f b) > s > f t)
This is rejected because there is no “outermost level” for the types on the RHS
(it would obviously be terrible to add extra parameters to PackMap
),
so no implicit quantification happens, and the declaration is rejected
(with “f
is out of scope”). Solution: use an explicit forall
:
data PackMap a b s t = PackMap (forall f. Monad f => (a > f b) > s > f t)