6.11.4. Lexically scoped type variables¶

ScopedTypeVariables
¶ Implies: ExplicitForAll
Since: 6.8.1 Status: Included in GHC2021
Enable lexical scoping of type variables explicitly introduced with
forall
.
Tip
ScopedTypeVariables
breaks GHC’s usual rule that explicit forall
is optional and doesn’t affect semantics.
For the Declaration type signatures (or Expression type signatures) examples in this section,
the explicit forall
is required.
(If omitted, usually the program will not compile; in a few cases it will compile but the functions get a different signature.)
To trigger those forms of ScopedTypeVariables
, the forall
must appear against the toplevel signature (or outer expression)
but not against nested signatures referring to the same type variables.
Explicit forall
is not always required – see pattern signature equivalent for the example in this section, or Pattern type signatures.
GHC supports lexically scoped type variables, without which some type signatures are simply impossible to write. For example:
f :: forall a. [a] > [a]
f xs = ys ++ ys
where
ys :: [a]
ys = reverse xs
The type signature for f
brings the type variable a
into scope,
because of the explicit forall
(Declaration type signatures). The type
variables bound by a forall
scope over the entire definition of the
accompanying value declaration. In this example, the type variable a
scopes over the whole definition of f
, including over the type
signature for ys
. In Haskell 98 it is not possible to declare a type
for ys
; a major benefit of scoped type variables is that it becomes
possible to do so.
An equivalent form for that example, avoiding explicit forall
uses Pattern type signatures:
f :: [a] > [a]
f (xs :: [aa]) = xs ++ ys
where
ys :: [aa]
ys = reverse xs
Unlike the forall
form, type variable a
from f
’s signature is not scoped over f
’s equation(s).
Type variable aa
bound by the pattern signature is scoped over the righthand side of f
’s equation.
(Therefore there is no need to use a distinct type variable; using a
would be equivalent.)
6.11.4.1. Overview¶
The design follows the following principles
 A scoped type variable stands for a type variable, and not for a type. (This is a change from GHC’s earlier design.)
 Furthermore, distinct lexical type variables stand for distinct type variables. This means that every programmerwritten type signature (including one that contains free scoped type variables) denotes a rigid type; that is, the type is fully known to the type checker, and no inference is involved.
 Lexical type variables may be alpharenamed freely, without changing the program.
A lexically scoped type variable can be bound by:
 A declaration type signature (Declaration type signatures)
 An expression type signature (Expression type signatures)
 A pattern type signature (Pattern type signatures)
 Class and instance declarations (Class and instance declarations)
In Haskell, a programmerwritten type signature is implicitly quantified
over its free type variables (Section
4.1.2 of
the Haskell Report). Lexically scoped type variables affect this
implicit quantification rules as follows: any type variable that is in
scope is not universally quantified. For example, if type variable
a
is in scope, then
(e :: a > a) means (e :: a > a)
(e :: b > b) means (e :: forall b. b>b)
(e :: a > b) means (e :: forall b. a>b)
6.11.4.2. Declaration type signatures¶
A declaration type signature that has explicit quantification (using
forall
) brings into scope the explicitlyquantified type variables,
in the definition of the named function. For example:
f :: forall a. [a] > [a]
f (x:xs) = xs ++ [ x :: a ]
The “forall a
” brings “a
” into scope in the definition of
“f
”.
This only happens if:
The quantification in
f
’s type signature is explicit. For example:g :: [a] > [a] g (x:xs) = xs ++ [ x :: a ]
This program will be rejected, because “
a
” does not scope over the definition of “g
”, so “x::a
” means “x::forall a. a
” by Haskell’s usual implicit quantification rules.The type variable is quantified by the single, syntactically visible, outermost
forall
of the type signature. For example, GHC will reject all of the following examples:f1 :: forall a. forall b. a > [b] > [b] f1 _ (x:xs) = xs ++ [ x :: b ] f2 :: forall a. a > forall b. [b] > [b] f2 _ (x:xs) = xs ++ [ x :: b ] type Foo = forall b. [b] > [b] f3 :: Foo f3 (x:xs) = xs ++ [ x :: b ]
In
f1
andf2
, the type variableb
is not quantified by the outermostforall
, so it is not in scope over the bodies of the functions. Neither isb
in scope over the body off3
, as theforall
is tucked underneath theFoo
type synonym.The signature gives a type for a function binding or a bare variable binding, not a pattern binding. For example:
f1 :: forall a. [a] > [a] f1 (x:xs) = xs ++ [ x :: a ]  OK f2 :: forall a. [a] > [a] f2 = \(x:xs) > xs ++ [ x :: a ]  OK f3 :: forall a. [a] > [a] Just f3 = Just (\(x:xs) > xs ++ [ x :: a ])  Not OK!
f1
is a function binding, andf2
binds a bare variable; in both cases the type signature bringsa
into scope. However the binding forf3
is a pattern binding, and sof3
is a fresh variable brought into scope by the pattern, not connected with top levelf3
. Then type variablea
is not in scope of the righthand side ofJust f3 = ...
.
6.11.4.3. Expression type signatures¶
An expression type signature that has explicit quantification (using
forall
) brings into scope the explicitlyquantified type variables,
in the annotated expression. For example:
f = runST ( (op >>= \(x :: STRef s Int) > g x) :: forall s. ST s Bool )
Here, the type signature forall s. ST s Bool
brings the type
variable s
into scope, in the annotated expression
(op >>= \(x :: STRef s Int) > g x)
.
6.11.4.4. Pattern type signatures¶
A type signature may occur in any pattern; this is a pattern type signature. For example:
 f and g assume that 'a' is already in scope
f = \(x::Int, y::a) > x
g (x::a) = x
h ((x,y) :: (Int,Bool)) = (y,x)
In the case where all the type variables in the pattern type signature are already in scope (i.e. bound by the enclosing context), matters are simple: the signature simply constrains the type of the pattern in the obvious way.
Unlike expression and declaration type signatures, pattern type signatures are not implicitly generalised. The pattern in a pattern binding may only mention type variables that are already in scope. For example:
f :: forall a. [a] > (Int, [a])
f xs = (n, zs)
where
(ys::[a], n) = (reverse xs, length xs)  OK
(zs::[a]) = xs ++ ys  OK
Just (v::b) = ...  Not OK; b is not in scope
Here, the pattern signatures for ys
and zs
are fine, but the one
for v
is not because b
is not in scope.
However, in all patterns other than pattern bindings, a pattern type signature may mention a type variable that is not in scope; in this case, the signature brings that type variable into scope. For example:
 same f and g as above, now assuming that 'a' is not already in scope
f = \(x::Int, y::a) > x  'a' is in scope on RHS of >
g (x::a) = x :: a
hh (Just (v :: b)) = v :: b
The pattern type signature makes the type variable available on the righthand side of the equation.
Bringing type variables into scope is particularly important for existential data constructors. For example:
data T = forall a. MkT [a]
k :: T > T
k (MkT [t::a]) =
MkT t3
where
(t3::[a]) = [t,t,t]
Here, the pattern type signature [t::a]
mentions a lexical type
variable that is not already in scope. Indeed, it must not already be in
scope, because it is bound by the pattern match.
The effect is to bring it into scope,
standing for the existentiallybound type variable.
It does seem odd that the existentiallybound type variable must not be already in scope. Contrast that usually namebindings merely shadow (make a ‘hole’) in a samenamed outer variable’s scope. But we must have some way to bring such type variables into scope, else we could not name existentiallybound type variables in subsequent type signatures.
Compare the two (identical) definitions for examples f
, g
;
they are both legal whether or not a
is already in scope.
They differ in that if a
is already in scope, the signature constrains
the pattern, rather than the pattern binding the variable.
6.11.4.5. Class and instance declarations¶
ScopedTypeVariables
allow the type variables bound by the top of a
class
or instance
declaration to scope over the methods defined in the
where
part. Unlike Declaration type signatures, type variables from class and
instance declarations can be lexically scoped without an explicit forall
(although you are allowed an explicit forall
in an instance
declaration; see Explicit universal quantification (forall)). For example:
class C a where
op :: [a] > a
op xs = let ys::[a]
ys = reverse xs
in
head ys
instance C b => C [b] where
op xs = reverse (head (xs :: [[b]]))
 Alternatively, one could write the instance above as:
instance forall b. C b => C [b] where
op xs = reverse (head (xs :: [[b]]))
While ScopedTypeVariables
is required for type variables from the
top of a class or instance declaration to scope over the /bodies/ of the
methods, it is not required for the type variables to scope over the /type
signatures/ of the methods. For example, the following will be accepted without
explicitly enabling ScopedTypeVariables
:
class D a where
m :: [a] > a
instance D [a] where
m :: [a] > [a]
m = reverse
Note that writing m :: [a] > [a]
requires the use of the
InstanceSigs
extension.
Similarly, ScopedTypeVariables
is not required for type variables
from the top of the class or instance declaration to scope over associated type
families, which only requires the TypeFamilies
extension. For
instance, the following will be accepted without explicitly enabling
ScopedTypeVariables
:
class E a where
type T a
instance E [a] where
type T [a] = a
See Scoping of class parameters for further information.