Fix dependency information for RULES

[ghc-hetmet.git] / compiler / basicTypes / IdInfo.lhs

diff --git a/compiler/basicTypes/IdInfo.lhs b/compiler/basicTypes/IdInfo.lhs
index b009794..a3124f3 100644 (file)
--- a/compiler/basicTypes/IdInfo.lhs
+++ b/compiler/basicTypes/IdInfo.lhs
@@ -464,9 +464,11 @@ type InlinePragInfo = Activation
 %************************************************************************
 
 \begin{code}
--- CoreRules is used only in an idSpecialisation (move to IdInfo?)
 data SpecInfo 
-  = SpecInfo [CoreRule] VarSet -- Locally-defined free vars of RHSs
+  = SpecInfo 
+       [CoreRule] 
+       VarSet          -- Locally-defined free vars of *both* LHS and RHS of rules
+                       -- Note [Rule dependency info]
 
 emptySpecInfo :: SpecInfo
 emptySpecInfo = SpecInfo [] emptyVarSet
@@ -483,6 +485,17 @@ specInfoRules (SpecInfo rules _) = rules
 seqSpecInfo (SpecInfo rules fvs) = seqRules rules `seq` seqVarSet fvs
 \end{code}
 
+Note [Rule dependency info]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+THe VarSet in a SpecInfo is used for dependency analysis in the 
+occurrence analyser.  We must track free vars in *both* lhs and rhs.  Why both?  
+Consider
+       x = y
+       RULE f x = 4
+Then if we substitute y for x, we'd better do so in the
+ rule's LHS too, so we'd better ensure the dependency is respsected
+
+
 
 %************************************************************************
 %*                                                                     *