+ -- The user didn't supply a method binding,
+ -- so we have to make up a default binding
+ -- The RHS of a default method depends on the default-method info
+mkDefMethRhs origin clas inst_tys sel_id loc (DefMeth dm_name)
+ = -- An polymorphic default method
+ traceRn (text "mkDefMeth" <+> ppr dm_name) `thenM_`
+ returnM (HsVar dm_name)
+
+mkDefMethRhs origin clas inst_tys sel_id loc NoDefMeth
+ = -- No default method
+ -- Warn only if -fwarn-missing-methods
+ doptM Opt_WarnMissingMethods `thenM` \ warn ->
+ warnTc (isInstDecl origin && warn)
+ (omittedMethodWarn sel_id) `thenM_`
+ returnM error_rhs
+ where
+ error_rhs = HsLam (mkSimpleMatch wild_pats simple_rhs placeHolderType loc)
+ simple_rhs = HsApp (HsVar (getName nO_METHOD_BINDING_ERROR_ID))
+ (HsLit (HsStringPrim (mkFastString (stringToUtf8 error_msg))))
+ error_msg = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
+
+ -- When the type is of form t1 -> t2 -> t3
+ -- make a default method like (\ _ _ -> noMethBind "blah")
+ -- rather than simply (noMethBind "blah")
+ -- Reason: if t1 or t2 are higher-ranked types we get n
+ -- silly ambiguity messages.
+ -- Example: f :: (forall a. Eq a => a -> a) -> Int
+ -- f = error "urk"
+ -- Here, tcSub tries to force (error "urk") to have the right type,
+ -- thus: f = \(x::forall a. Eq a => a->a) -> error "urk" (x t)
+ -- where 't' is fresh ty var. This leads directly to "ambiguous t".
+ --
+ -- NB: technically this changes the meaning of the default-default
+ -- method slightly, because `seq` can see the lambdas. Oh well.
+ (_,_,tau1) = tcSplitSigmaTy (idType sel_id)
+ (_,_,tau2) = tcSplitSigmaTy tau1
+ -- Need two splits because the selector can have a type like
+ -- forall a. Foo a => forall b. Eq b => ...
+ (arg_tys, _) = tcSplitFunTys tau2
+ wild_pats = [WildPat placeHolderType | ty <- arg_tys]
+
+mkDefMethRhs origin clas inst_tys sel_id loc GenDefMeth
+ = -- A generic default method
+ -- If the method is defined generically, we can only do the job if the
+ -- instance declaration is for a single-parameter type class with
+ -- a type constructor applied to type arguments in the instance decl
+ -- (checkTc, so False provokes the error)
+ ASSERT( isInstDecl origin ) -- We never get here from a class decl
+
+ checkTc (isJust maybe_tycon)
+ (badGenericInstance sel_id (notSimple inst_tys)) `thenM_`
+ checkTc (isJust (tyConGenInfo tycon))
+ (badGenericInstance sel_id (notGeneric tycon)) `thenM_`
+
+ ioToTcRn (dumpIfSet opt_PprStyle_Debug "Generic RHS" stuff) `thenM_`
+ returnM rhs
+ where
+ rhs = mkGenericRhs sel_id clas_tyvar tycon
+
+ stuff = vcat [ppr clas <+> ppr inst_tys,
+ nest 4 (ppr sel_id <+> equals <+> ppr rhs)]
+
+ -- The tycon is only used in the generic case, and in that
+ -- case we require that the instance decl is for a single-parameter
+ -- type class with type variable arguments:
+ -- instance (...) => C (T a b)
+ clas_tyvar = head (classTyVars clas)
+ Just tycon = maybe_tycon
+ maybe_tycon = case inst_tys of
+ [ty] -> case tcSplitTyConApp_maybe ty of
+ Just (tycon, arg_tys) | all tcIsTyVarTy arg_tys -> Just tycon
+ other -> Nothing
+ other -> Nothing
+
+isInstDecl InstanceDeclOrigin = True
+isInstDecl ClassDeclOrigin = False
+\end{code}