System/Mem/StableName.hs

    {-# 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.
    --
    -------------------------------------------------------------------------------

    module System.Mem.StableName (
      -- * Stable Names
      StableName,
      makeStableName,
      hashStableName,
      eqStableName
      ) where

    import GHC.IO           ( IO(..) )
    import GHC.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.
    -}

    data StableName 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 ->
        case makeStableName# 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 -> Int
    hashStableName (StableName sn) = I# (stableNameToInt# sn)

    -- | @since 2.01
    instance Eq (StableName a) where
        (StableName sn1) == (StableName sn2) =
           case eqStableName# sn1 sn2 of
             0# -> False
             _  -> True

    -- | Equality on 'StableName' that does not require that the types of
    -- the arguments match.
    --
    -- @since 4.7.0.0
    eqStableName :: StableName a -> StableName b -> Bool
    eqStableName (StableName sn1) (StableName sn2) =
           case eqStableName# sn1 sn2 of
             0# -> False
             _  -> True
      -- Requested by Emil Axelsson on glasgow-haskell-users, who wants to
      -- use it for implementing observable sharing.