X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FcoreSyn%2FCoreArity.lhs;h=f39b6b92352ebe07a18c5851a536dc73423e9b1b;hb=ac38ece1e717cb412e89354aa95fd11d44c1cefb;hp=2dfed4916de299924e9c77416e2e55d46c510311;hpb=a602a61e9b361eea186d6979f1b4520ea67726e1;p=ghc-hetmet.git

diff --git a/compiler/coreSyn/CoreArity.lhs b/compiler/coreSyn/CoreArity.lhs
index 2dfed49..f39b6b9 100644
--- a/compiler/coreSyn/CoreArity.lhs
+++ b/compiler/coreSyn/CoreArity.lhs
@@ -24,6 +24,7 @@ import Var
 import VarEnv
 import Id
 import Type
+import TcType	( isDictLikeTy )
 import Coercion
 import BasicTypes
 import Unique
@@ -293,7 +294,7 @@ arityType dflags (Let b e)
   where
     cheap_bind (NonRec b e) = is_cheap (b,e)
     cheap_bind (Rec prs)    = all is_cheap prs
-    is_cheap (b,e) = (dopt Opt_DictsCheap dflags && isDictId b)
+    is_cheap (b,e) = (dopt Opt_DictsCheap dflags && isDictLikeTy (idType b))
 		   || exprIsCheap e
 	-- If the experimental -fdicts-cheap flag is on, we eta-expand through
 	-- dictionary bindings.  This improves arities. Thereby, it also
@@ -311,6 +312,9 @@ arityType dflags (Let b e)
 	--
 	-- One could go further and make exprIsCheap reply True to any
 	-- dictionary-typed expression, but that's more work.
+	-- 
+	-- See Note [Dictionary-like types] in TcType.lhs for why we use
+	-- isDictLikeTy here rather than isDictTy
 
 arityType _ _ = ATop
 \end{code}