[project @ 2002-05-27 14:54:27 by simonmar]

Add documentation

diff --git a/System/Mem/StableName.hs b/System/Mem/StableName.hs
index 29918c5..659dacb 100644 (file)
--- a/System/Mem/StableName.hs
+++ b/System/Mem/StableName.hs
@@ -8,15 +8,26 @@
 -- Stability   :  experimental
 -- Portability :  non-portable
 --
--- Giving an object a stable (GC-invariant) name.
+-- 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
-       ( StableName {-a-}   -- abstract, instance of Eq
-       , makeStableName     -- :: a -> IO (StableName a)
-       , hashStableName     -- :: StableName a -> Int
-       ) where
+module System.Mem.StableName (
+  -- * Stable Names
+  StableName,
+  makeStableName,
+  hashStableName,
+  ) where
 
 import Prelude
 
@@ -30,8 +41,37 @@ import GHC.Base              ( Int(..), StableName#, makeStableName#
 -----------------------------------------------------------------------------
 -- 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.
+
+  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)
 #if defined(__PARALLEL_HASKELL__)
 makeStableName a = 
@@ -41,6 +81,10 @@ makeStableName a = IO $ \ s ->
     case makeStableName# a s of (# s', sn #) -> (# s', StableName sn #)
 #endif
 
+-- | 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
 #if defined(__PARALLEL_HASKELL__)
 hashStableName (StableName sn) =