Each reference is given a key which can be used to locate it at runtime with 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 deRefStaticPtr

The body e of a static e expression must be a closed expression. That is, there can be no free variables occurring in e, i.e. lambda- or let-bound variables bound locally in the context of the expression.

All of the following are permissible:

inc :: Int -> Int
inc x = x + 1

ref1 = static 1
ref2 = static inc
ref3 = static (inc 1)
ref4 = static ((\x -> x + 1) (1 :: Int))
ref5 y = static (let x = 1 in x)

While the following definitions are rejected:

ref6 = let x = 1 in static x
ref7 y = static (let x = 1 in y)

Informally, if we have a closed expression

e :: forall a_1 ... a_n . t

the static form is of type

static e :: (Typeable a_1, ... , Typeable a_n) => StaticPtr t

Furthermore, type t is constrained to have a Typeable instance. The following are therefore illegal:

static show                    -- No Typeable instance for (Show a => a -> String)
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 DeriveDataTypeable        #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE Rank2Types                #-}
{-# LANGUAGE StandaloneDeriving        #-}
{-# LANGUAGE StaticPointers            #-}

import Control.Monad.ST
import Data.Typeable
import GHC.StaticPtr

data Dict c = c => Dict
  deriving Typeable

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))