+tcDefaults []
+ = getDefaultTys -- No default declaration, so get the
+ -- default types from the envt;
+ -- i.e. use the curent ones
+ -- (the caller will put them back there)
+ -- It's important not to return defaultDefaultTys here (which
+ -- we used to do) because in a TH program, tcDefaults [] is called
+ -- repeatedly, once for each group of declarations between top-level
+ -- splices. We don't want to carefully set the default types in
+ -- one group, only for the next group to ignore them and install
+ -- defaultDefaultTys
+
+tcDefaults [DefaultDecl [] locn]
+ = returnM [] -- Default declaration specifying no types
+
+tcDefaults [DefaultDecl mono_tys locn]
+ = tcLookupGlobal_maybe numClassName `thenM` \ maybe_num ->
+ case maybe_num of
+ Just (AClass num_class) -> common_case num_class
+ other -> returnM []
+ -- 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
+ = addSrcLoc locn $
+ addErrCtxt defaultDeclCtxt $
+ mappM tc_default_ty mono_tys `thenM` \ tau_tys ->
+
+ -- Check that all the types are instances of Num
+ -- We only care about whether it worked or not
+ tcSimplifyDefault [mkClassPred num_class [ty] | ty <- tau_tys] `thenM_`
+
+ returnM tau_tys
+
+tcDefaults decls@(DefaultDecl _ loc : _) =
+ addSrcLoc loc $
+ failWithTc (dupDefaultDeclErr decls)
+
+
+tc_default_ty hs_ty
+ = tcHsType hs_ty `thenM` \ ty ->
+ checkTc (isTauTy ty) (polyDefErr hs_ty) `thenM_`
+ returnM ty