[project @ 1999-12-20 10:34:27 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..a3c292b 100644 (file)
--- 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}
+