[commit: ghc] ghc-7.2: remove out of date comment about instance versioning, replace it with (a1068ac)

[Ian Lynagh]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : ghc-7.2

http://hackage.haskell.org/trac/ghc/changeset/a1068ac1eb097ba857282ca47f9e1004dba30197

>---------------------------------------------------------------

commit a1068ac1eb097ba857282ca47f9e1004dba30197
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Wed Jun 1 11:25:12 2011 +0100

    remove out of date comment about instance versioning, replace it with
    a link to the Commentary where the current story is described.

>---------------------------------------------------------------

 compiler/iface/IfaceSyn.lhs |   38 +-------------------------------------
 1 files changed, 1 insertions(+), 37 deletions(-)

diff --git a/compiler/iface/IfaceSyn.lhs b/compiler/iface/IfaceSyn.lhs
index e03bc29..2204c9a 100644
--- a/compiler/iface/IfaceSyn.lhs
+++ b/compiler/iface/IfaceSyn.lhs
@@ -314,43 +314,7 @@ defined.)
 
 Note [Versioning of instances]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Now consider versioning.  If we *use* an instance decl in one compilation,
-we'll depend on the dfun id for that instance, so we'll recompile if it changes.
-But suppose we *don't* (currently) use an instance!  We must recompile if
-the instance is changed in such a way that it becomes important.  (This would
-only matter with overlapping instances, else the importing module wouldn't have
-compiled before and the recompilation check is irrelevant.)
-
-The is_orph field is set to (Just n) if the instance is not an orphan.
-The 'n' is *any* of the locally-defined names mentioned anywhere in the
-instance head.  This name is used for versioning; the instance decl is
-considered part of the defn of this 'n'.
-
-I'm worried about whether this works right if we pick a name from
-a functionally-dependent part of the instance decl.  E.g.
-
-  module M where { class C a b | a -> b }
-
-and suppose we are compiling module X:
-
-  module X where
-        import M
-        data S  = ...
-        data T = ...
-        instance C S T where ...
-
-If we base the instance version on T, I'm worried that changing S to S'
-would change T's version, but not S or S'.  But an importing module might
-not depend on T, and so might not be recompiled even though the new instance
-(C S' T) might be relevant.  I have not been able to make a concrete example,
-and it seems deeply obscure, so I'm going to leave it for now.
-
-
-Note [Versioning of rules]
-~~~~~~~~~~~~~~~~~~~~~~~~~~
-A rule that is not an orphan has an ifRuleOrph field of (Just n), where n
-appears on the LHS of the rule; any change in the rule changes the version of n.
-
+See [http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/RecompilationAvoidance#Instances]
 
 \begin{code}
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