6.11.5. Lexically scoped type variables¶
Enable lexical scoping of type variables explicitly introduced with
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,
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
forall must appear against the top-level signature (or outer expression)
but not against nested signatures referring to the same type variables.
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
scopes over the whole definition of
f, including over the type
ys. In Haskell 98 it is not possible to declare a type
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
forall form, type variable
f‘s signature is not scoped over
aa bound by the pattern signature is scoped over the right-hand side of
(Therefore there is no need to use a distinct type variable; using
a would be equivalent.)
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 programmer-written 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 alpha-renamed 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 programmer-written type signature is implicitly quantified
over its free type variables (Section
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)
220.127.116.11. Declaration type signatures¶
A declaration type signature that has explicit quantification (using
forall) brings into scope the explicitly-quantified type variables,
in the definition of the named function. For example:
f :: forall a. [a] -> [a] f (x:xs) = xs ++ [ x :: a ]
forall a” brings “
a” into scope in the definition of
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
forallof 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 ]
f2, the type variable
bis not quantified by the outermost
forall, so it is not in scope over the bodies of the functions. Neither is
bin scope over the body of
f3, as the
forallis tucked underneath the
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!
f1is a function binding, and
f2binds a bare variable; in both cases the type signature brings
ainto scope. However the binding for
f3is a pattern binding, and so
f3is a fresh variable brought into scope by the pattern, not connected with top level
f3. Then type variable
ais not in scope of the right-hand side of
Just f3 = ....
18.104.22.168. Expression type signatures¶
An expression type signature that has explicit quantification (using
forall) brings into scope the explicitly-quantified 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
s into scope, in the annotated expression
(op >>= \(x :: STRef s Int) -> g x).
22.214.171.124. 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
zs are fine, but the one
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 right-hand 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 existentially-bound type variable.
It does seem odd that the existentially-bound type variable must not be already in scope. Contrast that usually name-bindings merely shadow (make a ‘hole’) in a same-named outer variable’s scope. But we must have some way to bring such type variables into scope, else we could not name existentially-bound type variables in subsequent type signatures.
Compare the two (identical) definitions for examples
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.
126.96.36.199. Class and instance declarations¶
ScopedTypeVariables allow the type variables bound by the top of a
instance declaration to scope over the methods defined in the
where part. Unlike :ref`decl-type-sigs`, type variables from class and
instance declarations can be lexically scoped without an explicit
(although you are allowed an explicit
forall in an
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]]))
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
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
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
class E a where type T a instance E [a] where type T [a] = a
See Scoping of class parameters for further information.