Comments only

2 files changed, 5 insertions, 1 deletions

diff --git a/compiler/deSugar/Desugar.lhs b/compiler/deSugar/Desugar.lhs
index 7e284ae949..3b30deaa2e 100644
--- a/compiler/deSugar/Desugar.lhs
+++ b/compiler/deSugar/Desugar.lhs
@@ -280,6 +280,6 @@ For the LHS of a RULE we do *not* want to desugar
     [x]   to    build (\cn. x `c` n)
 We want to leave explicit lists simply as chains
 of cons's. We can achieve that slightly indirectly by
-switching off EnableRewriteRules.
+switching off EnableRewriteRules.  See DsExpr.dsExplicitList.
 
 That keeps the desugaring of list comprehensions simple too.
diff --git a/compiler/deSugar/DsExpr.lhs b/compiler/deSugar/DsExpr.lhs
index d8bd7cdeea..a58e9b4b89 100644
--- a/compiler/deSugar/DsExpr.lhs
+++ b/compiler/deSugar/DsExpr.lhs
@@ -643,6 +643,9 @@ Example: the foldr/single rule in GHC.Base
    foldr k z [x] = ...
 We do not want to generate a build invocation on the LHS of this RULE!
 
+We fix this by disabling rules in rule LHSs, and testing that
+flag here; see Note [Desugaring RULE left hand sides] in Desugar
+
 To test this I've added a (static) flag -fsimple-list-literals, which
 makes all list literals be generated via the simple route.  
 
@@ -657,6 +660,7 @@ dsExplicitList elt_ty xs
        ; if opt_SimpleListLiterals 	       		-- -fsimple-list-literals
          || not (dopt Opt_EnableRewriteRules dflags)	-- Rewrite rules off
 	    	-- Don't generate a build if there are no rules to eliminate it!
+		-- See Note [Desugaring RULE left hand sides] in Desugar
          || null dynamic_prefix   -- Avoid build (\c n. foldr c n xs)!
          then return $ mkListExpr elt_ty xs'
          else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }