X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcDefaults.lhs;h=a3c292b82368ab392ef43638917fcc3a37f0480c;hb=e921b2e307532e0f30eefa88b11a124be592bde4;hp=6fe697ba590b246f1c620306797232326e350e5b;hpb=f7989a6dea8c43352f363117d9bb07439953ccdc;p=ghc-hetmet.git

diff --git a/ghc/compiler/typecheck/TcDefaults.lhs b/ghc/compiler/typecheck/TcDefaults.lhs
index 6fe697b..a3c292b 100644
--- a/ghc/compiler/typecheck/TcDefaults.lhs
+++ b/ghc/compiler/typecheck/TcDefaults.lhs
@@ -12,7 +12,7 @@ import HsSyn		( HsDecl(..), DefaultDecl(..) )
 import RnHsSyn		( RenamedHsDecl )
 
 import TcMonad
-import TcEnv		( tcLookupClassByKey )
+import TcEnv		( tcLookupClassByKey_maybe )
 import TcMonoType	( tcHsType )
 import TcSimplify	( tcSimplifyCheckThetas )
 
@@ -25,7 +25,7 @@ import Util
 \end{code}
 
 \begin{code}
-default_default = [integerTy, doubleTy ]
+default_default = [integerTy, doubleTy]
 
 tcDefaults :: [RenamedHsDecl]
 	   -> TcM s [Type] 	    -- defaulting types to heave
@@ -35,24 +35,33 @@ tcDefaults decls = tc_defaults [default_decl | DefD default_decl <- decls]
 
 tc_defaults [] = returnTc default_default
 
+tc_defaults [DefaultDecl [] locn]
+  = returnTc []		-- no defaults
+
 tc_defaults [DefaultDecl mono_tys locn]
-  = tcAddSrcLoc locn $
+  = tcLookupClassByKey_maybe numClassKey	`thenNF_Tc` \ maybe_num ->
+    case maybe_num of {
+	Nothing -> 	-- Num has not been sucked in, so the defaults will
+			-- never be used; so simply discard the default decl.
+			-- This slightly benefits modules that don't use any
+			-- numeric stuff at all, by avoid the necessity of
+			-- always sucking in Num
+		returnTc [] ;
+
+	Just num ->	-- The common case
+
+    tcAddSrcLoc locn $
     mapTc tcHsType mono_tys	`thenTc` \ tau_tys ->
 
-    case tau_tys of
-      [] -> returnTc []		-- no defaults
-
-      _  ->
 	    -- Check that all the types are instances of Num
 	    -- We only care about whether it worked or not
-
-	tcAddErrCtxt defaultDeclCtxt		$
-	tcLookupClassByKey numClassKey		`thenNF_Tc` \ num ->
-	tcSimplifyCheckThetas
+    tcAddErrCtxt defaultDeclCtxt		$
+    tcSimplifyCheckThetas
 		[{- Nothing given -}]
 		[ (num, [ty]) | ty <- tau_tys ]	`thenTc_`
 
-	returnTc tau_tys
+    returnTc tau_tys
+    }
 
 tc_defaults decls@(DefaultDecl _ loc : _) =
     tcAddSrcLoc loc $
@@ -69,3 +78,4 @@ dupDefaultDeclErr (DefaultDecl _ locn1 : dup_things)
   where
     pp (DefaultDecl _ locn) = ptext SLIT("here was another default declaration") <+> ppr locn
 \end{code}
+