6.15.3. Static pointers¶

StaticPointers
¶ Since: 7.10.1 Allow use of static pointer syntax.
The language extension StaticPointers
adds a new syntactic form
static e
, which stands for a reference to the closed expression ⟨e⟩.
This reference is stable and portable, in the sense that it remains
valid across different processes on possibly different machines. Thus, a
process can create a reference and send it to another process that can
resolve it to ⟨e⟩.
With this extension turned on, static
is no longer a valid
identifier.
Static pointers were first proposed in the paper Towards Haskell in the cloud, Jeff Epstein, Andrew P. Black and Simon PeytonJones, Proceedings of the 4th ACM Symposium on Haskell, pp. 118129, ACM, 2011.
6.15.3.1. Using static pointers¶
Each reference is given a key which can be used to locate it at runtime with GHC.StaticPtr.unsafeLookupStaticPtr which uses a global and immutable table called the Static Pointer Table. The compiler includes entries in this table for all static forms found in the linked modules. The value can be obtained from the reference via GHC.StaticPtr.deRefStaticPtr.
The body e
of a static e
expression must be a closed expression. Where
we say an expression is closed when all of its free (type) variables are
closed. And a variable is closed if it is letbound to a closed expression
and its type is closed as well. And a type is closed if it has no free
variables.
All of the following are permissible:
inc :: Int > Int
inc x = x + 1
ref1 = static 'a'
ref2 = static inc
ref3 = static (inc 1)
ref4 = static ((\x > x + 1) (1 :: Int))
ref5 = static (let x = 'a' in x)
ref6 = let x = 'a' in static x
While the following definitions are rejected:
ref7 y = let x = y in static x  x is not closed
ref8 y = static (let x = 1 in y)  y is not letbound
ref9 (y :: a) = let x = undefined :: a
in static x  x has a nonclosed type
Note
While modules loaded in GHCi with the :load
command may use
StaticPointers
and static
expressions, statements
entered on the REPL may not. This is a limitation of GHCi; see
#12356 for details.
Note
The set of keys used for locating static pointers in the Static Pointer Table is not guaranteed to remain stable for different program binaries. Or in other words, only processes launched from the same program binary are guaranteed to use the same set of keys.
6.15.3.2. Static semantics of static pointers¶
Informally, if we have a closed expression
e :: forall a_1 ... a_n . t
the static form is of type
static e :: (IsStatic p, Typeable a_1, ... , Typeable a_n) => p t
A static form determines a value of type StaticPtr t
, but just
like OverloadedLists
and OverloadedStrings
, this literal
expression is overloaded to allow lifting a StaticPtr
into another
type implicitly, via the IsStatic
class:
class IsStatic p where
fromStaticPtr :: Typeable a => StaticPtr a > p a
The only predefined instance is the obvious one that does nothing:
instance IsStatic StaticPtr where
fromStaticPtr sptr = sptr
Furthermore, type t
is constrained to have a Typeable
instance.
The following are therefore illegal:
static id  No Typeable instance for (a > a)
static Control.Monad.ST.runST  No Typeable instance for ((forall s. ST s a) > a)
That being said, with the appropriate use of wrapper datatypes, the above limitations induce no loss of generality:
{# LANGUAGE ConstraintKinds #}
{# LANGUAGE ExistentialQuantification #}
{# LANGUAGE Rank2Types #}
{# LANGUAGE StandaloneDeriving #}
{# LANGUAGE StaticPointers #}
import Control.Monad.ST
import Data.Typeable
import GHC.StaticPtr
data Dict c = c => Dict
g1 :: Typeable a => StaticPtr (Dict (Show a) > a > String)
g1 = static (\Dict > show)
data Rank2Wrapper f = R2W (forall s. f s)
deriving Typeable
newtype Flip f a s = Flip { unFlip :: f s a }
deriving Typeable
g2 :: Typeable a => StaticPtr (Rank2Wrapper (Flip ST a) > a)
g2 = static (\(R2W f) > runST (unFlip f))