[project @ 2002-12-11 15:55:17 by simonmar]

Obj type must be forall a.a now, to avoid confusing GHC's new
tail-calling scheme.

diff --git a/Data/Dynamic.hs b/Data/Dynamic.hs
index c67ea20..6379d33 100644 (file)
--- a/Data/Dynamic.hs
+++ b/Data/Dynamic.hs
@@ -110,8 +110,16 @@ instance Show Dynamic where
          showsPrec 0 t   . 
          showString ">>"
 
-data Obj = Obj  
- -- dummy type to hold the dynamically typed value.
+type Obj = forall a . a
+ -- Dummy type to hold the dynamically typed value.
+ --
+ -- In GHC's new eval/apply execution model this type must
+ -- be polymorphic.  It can't be a constructor, because then
+ -- GHC will use the constructor convention when evaluating it,
+ -- and this will go wrong if the object is really a function.  On
+ -- the other hand, if we use a polymorphic type, GHC will use
+ -- a fallback convention for evaluating it that works for all types.
+ -- (using a function type here would also work).
 
 -- | A concrete representation of a (monomorphic) type.  'TypeRep'
 -- supports reasonably efficient equality.