[project @ 2004-12-14 12:44:52 by simonmar]

[ghc-base.git] / Data / Typeable.hs

diff --git a/Data/Typeable.hs b/Data/Typeable.hs
index 3e5b7c2..d7fd008 100644 (file)
--- a/Data/Typeable.hs
+++ b/Data/Typeable.hs
@@ -9,15 +9,24 @@
 -- Stability   :  experimental
 -- Portability :  portable
 --
--- The Typeable class reifies types to some extent by associating type
+-- The 'Typeable' class reifies types to some extent by associating type
 -- representations to types. These type representations can be compared,
 -- and one can in turn define a type-safe cast operation. To this end,
 -- an unsafe cast is guarded by a test for type (representation)
--- equivalence. The module Data.Dynamic uses Typeable for an
--- implementation of dynamics. The module Data.Generics uses Typeable
+-- equivalence. The module "Data.Dynamic" uses Typeable for an
+-- implementation of dynamics. The module "Data.Generics" uses Typeable
 -- and type-safe cast (but not dynamics) to support the \"Scrap your
 -- boilerplate\" style of generic programming.
 --
+-- Note, only relevant if you use dynamic linking. If you have a program
+-- that is statically linked with Data.Typeable, and then dynamically link
+-- a program that also uses Data.Typeable, you'll get two copies of the module.
+-- That's fine, but behind the scenes, the module uses a mutable variable to
+-- allocate unique Ids to type constructors.  So in the situation described,
+-- there'll be two separate Id allocators, which aren't comparable to each other.
+-- This can lead to chaos.  (It's a bug that we will fix.)  None of
+-- this matters if you aren't using dynamic linking.
+--
 -----------------------------------------------------------------------------
 
 module Data.Typeable
@@ -478,7 +487,7 @@ instance Typeable6 (,,,,,) where
   typeOf6 tu = mkTyConApp tup6Tc []
 
 tup7Tc :: TyCon
-tup7Tc = mkTyCon ",,,,,"
+tup7Tc = mkTyCon ",,,,,,"
 
 instance Typeable7 (,,,,,,) where
   typeOf7 tu = mkTyConApp tup7Tc []
@@ -518,6 +527,7 @@ INSTANCE_TYPEABLE0(TypeRep,typeRepTc,"TypeRep")
 
 #ifdef __GLASGOW_HASKELL__
 INSTANCE_TYPEABLE0(Word,wordTc,"Word" )
+INSTANCE_TYPEABLE1(MVar,mvarTc,"MVar" )
 #endif
 
 ---------------------------------------------