[project @ 2000-11-20 16:07:12 by simonpj]

[ghc-hetmet.git] / ghc / compiler / typecheck / TcDefaults.lhs

diff --git a/ghc/compiler/typecheck/TcDefaults.lhs b/ghc/compiler/typecheck/TcDefaults.lhs
index 6fe697b..b9b74c3 100644 (file)
--- a/ghc/compiler/typecheck/TcDefaults.lhs
+++ b/ghc/compiler/typecheck/TcDefaults.lhs
@@ -12,47 +12,55 @@ import HsSyn                ( HsDecl(..), DefaultDecl(..) )
 import RnHsSyn         ( RenamedHsDecl )
 
 import TcMonad
-import TcEnv           ( tcLookupClassByKey )
+import TcEnv           ( tcLookupGlobal_maybe )
 import TcMonoType      ( tcHsType )
 import TcSimplify      ( tcSimplifyCheckThetas )
 
 import TysWiredIn      ( integerTy, doubleTy )
 import Type             ( Type )
-import Unique          ( numClassKey )
-import ErrUtils                ( addShortErrLocLine )
+import PrelNames       ( numClassName )
 import Outputable
-import Util
+import HscTypes                ( TyThing(..) )
 \end{code}
 
 \begin{code}
-default_default = [integerTy, doubleTy ]
+default_default = [integerTy, doubleTy]
 
 tcDefaults :: [RenamedHsDecl]
-          -> TcM s [Type]          -- defaulting types to heave
+          -> TcM [Type]            -- defaulting types to heave
                                    -- into Tc monad for later use
                                    -- in Disambig.
 tcDefaults decls = tc_defaults [default_decl | DefD default_decl <- decls]
 
 tc_defaults [] = returnTc default_default
 
-tc_defaults [DefaultDecl mono_tys locn]
-  = 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
+tc_defaults [DefaultDecl [] locn]
+  = returnTc []                -- no defaults
 
-       tcAddErrCtxt defaultDeclCtxt            $
-       tcLookupClassByKey numClassKey          `thenNF_Tc` \ num ->
-       tcSimplifyCheckThetas
-               [{- Nothing given -}]
-               [ (num, [ty]) | ty <- tau_tys ] `thenTc_`
+tc_defaults [DefaultDecl mono_tys locn]
+  = tcLookupGlobal_maybe numClassName  `thenNF_Tc` \ maybe_num ->
+    case maybe_num of
+       Just (AClass num_class) -> common_case num_class
+       other                   -> returnTc []
+               -- In the Nothing case, 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
+  where
+    common_case num_class
+      = tcAddSrcLoc locn $
+       mapTc tcHsType mono_tys `thenTc` \ tau_tys ->
+    
+               -- Check that all the types are instances of Num
+               -- We only care about whether it worked or not
+       tcAddErrCtxt defaultDeclCtxt            $
+       tcSimplifyCheckThetas
+                   [{- Nothing given -}]
+                   [ (num_class, [ty]) | ty <- tau_tys ]       `thenTc_`
+    
+       returnTc tau_tys
 
-       returnTc tau_tys
 
 tc_defaults decls@(DefaultDecl _ loc : _) =
     tcAddSrcLoc loc $
@@ -69,3 +77,4 @@ dupDefaultDeclErr (DefaultDecl _ locn1 : dup_things)
   where
     pp (DefaultDecl _ locn) = ptext SLIT("here was another default declaration") <+> ppr locn
 \end{code}
+