From: simonmar <unknown>
Date: Wed, 11 Dec 2002 15:55:17 +0000 (+0000)
Subject: [project @ 2002-12-11 15:55:17 by simonmar]
X-Git-Tag: nhc98-1-18-release~788
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=3a3c5dde1a91357fa258682510f75e37e0340913;p=ghc-base.git

[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
--- 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.