+mkMethodBind :: InstOrigin
+ -> Class -> [TcType] -- Class and instance types
+ -> LHsBinds Name -- Method binding (pick the right one from in here)
+ -> ClassOpItem
+ -> TcM (Maybe Inst, -- Method inst
+ MethodSpec)
+-- Find the binding for the specified method, or make
+-- up a suitable default method if it isn't there
+
+mkMethodBind origin clas inst_tys meth_binds (sel_id, dm_info)
+ = mkMethId origin clas sel_id inst_tys `thenM` \ (mb_inst, meth_id) ->
+ let
+ meth_name = idName meth_id
+ in
+ -- Figure out what method binding to use
+ -- If the user suppplied one, use it, else construct a default one
+ getSrcSpanM `thenM` \ loc ->
+ (case find_bind (idName sel_id) meth_name meth_binds of
+ Just user_bind -> returnM user_bind
+ Nothing ->
+ mkDefMethRhs origin clas inst_tys sel_id loc dm_info `thenM` \ rhs ->
+ -- Not infix decl
+ returnM (noLoc $ FunBind (noLoc meth_name) False
+ (mkMatchGroup [mkSimpleMatch [] rhs])
+ placeHolderNames)
+ ) `thenM` \ meth_bind ->
+
+ returnM (mb_inst, (sel_id, meth_id, meth_bind))
+
+mkMethId :: InstOrigin -> Class
+ -> Id -> [TcType] -- Selector, and instance types
+ -> TcM (Maybe Inst, Id)
+
+-- mkMethId instantiates the selector Id at the specified types
+mkMethId origin clas sel_id inst_tys
+ = let
+ (tyvars,rho) = tcSplitForAllTys (idType sel_id)
+ rho_ty = ASSERT( length tyvars == length inst_tys )
+ substTyWith tyvars inst_tys rho
+ (preds,tau) = tcSplitPhiTy rho_ty
+ first_pred = head preds
+ in
+ -- The first predicate should be of form (C a b)
+ -- where C is the class in question
+ ASSERT( not (null preds) &&
+ case getClassPredTys_maybe first_pred of
+ { Just (clas1,tys) -> clas == clas1 ; Nothing -> False }
+ )
+ if isSingleton preds then
+ -- If it's the only one, make a 'method'
+ getInstLoc origin `thenM` \ inst_loc ->
+ newMethod inst_loc sel_id inst_tys preds tau `thenM` \ meth_inst ->
+ returnM (Just meth_inst, instToId meth_inst)
+ else
+ -- If it's not the only one we need to be careful
+ -- For example, given 'op' defined thus:
+ -- class Foo a where
+ -- op :: (?x :: String) => a -> a
+ -- (mkMethId op T) should return an Inst with type
+ -- (?x :: String) => T -> T
+ -- That is, the class-op's context is still there.
+ -- BUT: it can't be a Method any more, because it breaks
+ -- INVARIANT 2 of methods. (See the data decl for Inst.)
+ newUnique `thenM` \ uniq ->
+ getSrcSpanM `thenM` \ loc ->
+ let
+ real_tau = mkPhiTy (tail preds) tau
+ meth_id = mkUserLocal (getOccName sel_id) uniq real_tau
+ (srcSpanStart loc) --TODO
+ in
+ returnM (Nothing, meth_id)
+
+ -- 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
+ = -- An polymorphic default method
+ lookupImportedName (mkDefMethRdrName sel_id) `thenM` \ dm_name ->
+ -- Might not be imported, but will be an OrigName
+ traceRn (text "mkDefMeth" <+> ppr dm_name) `thenM_`
+ returnM (nlHsVar 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
+ && reportIfUnused (getOccName sel_id))
+ (omittedMethodWarn sel_id) `thenM_`
+ returnM error_rhs