{-# LANGUAGE Trustworthy #-}{-# LANGUAGE StandaloneDeriving #-}{-# LANGUAGE MagicHash #-}{-# LANGUAGE UnboxedTuples #-}------------------------------------------------------------------------------- |-- Module : System.Mem.StableName-- Copyright : (c) The University of Glasgow 2001-- License : BSD-style (see the file libraries/base/LICENSE)---- Maintainer : libraries@haskell.org-- Stability : experimental-- Portability : non-portable---- Stable names are a way of performing fast (O(1)), not-quite-exact-- comparison between objects.---- Stable names solve the following problem: suppose you want to build-- a hash table with Haskell objects as keys, but you want to use-- pointer equality for comparison; maybe because the keys are large-- and hashing would be slow, or perhaps because the keys are infinite-- in size. We can\'t build a hash table using the address of the-- object as the key, because objects get moved around by the garbage-- collector, meaning a re-hash would be necessary after every garbage-- collection.---------------------------------------------------------------------------------moduleGHC.StableName(-- * Stable NamesStableName (..),makeStableName ,hashStableName ,eqStableName )whereimportGHC.IO (IO(..))importGHC.Base (Int(..),StableName#,makeStableName#,eqStableName#,stableNameToInt#)------------------------------------------------------------------------------- Stable Names{-|
 An abstract name for an object, that supports equality and hashing.
 Stable names have the following property:
 * If @sn1 :: StableName@ and @sn2 :: StableName@ and @sn1 == sn2@
 then @sn1@ and @sn2@ were created by calls to @makeStableName@ on
 the same object.
 The reverse is not necessarily true: if two stable names are not
 equal, then the objects they name may still be equal. Note in particular
 that `makeStableName` may return a different `StableName` after an
 object is evaluated.
 Stable Names are similar to Stable Pointers ("Foreign.StablePtr"),
 but differ in the following ways:
 * There is no @freeStableName@ operation, unlike "Foreign.StablePtr"s.
 Stable names are reclaimed by the runtime system when they are no
 longer needed.
 * There is no @deRefStableName@ operation. You can\'t get back from
 a stable name to the original Haskell object. The reason for
 this is that the existence of a stable name for an object does not
 guarantee the existence of the object itself; it can still be garbage
 collected.
-}dataStableName a =StableName (StableName#a )-- | Makes a 'StableName' for an arbitrary object. The object passed as-- the first argument is not evaluated by 'makeStableName'.makeStableName::a ->IO(StableName a )makeStableName a =IO$ \s ->casemakeStableName#a s of(#s' ,sn #)->(#s' ,StableName sn #)-- | Convert a 'StableName' to an 'Int'. The 'Int' returned is not-- necessarily unique; several 'StableName's may map to the same 'Int'-- (in practice however, the chances of this are small, so the result-- of 'hashStableName' makes a good hash key).hashStableName::StableName a ->InthashStableName (StableName sn )=I#(stableNameToInt#sn )-- | @since 2.01instanceEq(StableName a )where(StableName sn1 )== (StableName sn2 )=caseeqStableName#sn1 sn2 of0#->False_->True-- | Equality on 'StableName' that does not require that the types of-- the arguments match.---- @since 4.7.0.0eqStableName::StableName a ->StableName b ->BooleqStableName (StableName sn1 )(StableName sn2 )=caseeqStableName#sn1 sn2 of0#->False_->True-- Requested by Emil Axelsson on glasgow-haskell-users, who wants to-- use it for implementing observable sharing.