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 Peyton-Jones, Proceedings of the 4th ACM Symposium on Haskell, pp. 118-129, 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 let-bound 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 let-bound
ref9 (y :: a) = let x = undefined :: a
                 in static x      -- x has a non-closed 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

See GHC.StaticPtr.IsStatic.

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))