diff --git a/_state.go b/_state.go
index 6e9febfb..b14202f6 100644
--- a/_state.go
+++ b/_state.go
@@ -615,7 +615,7 @@ func (ls *LState) printCallStack() {
 		if frame == nil {
 			break
 		}
-		if frame.Fn.IsG {
+		if frame.Fn.IsG() {
 			println("IsG:", true, "Frame:", frame, "Fn:", frame.Fn)
 		} else {
 			println("IsG:", false, "Frame:", frame, "Fn:", frame.Fn, "pc:", frame.Pc)
@@ -626,7 +626,7 @@ func (ls *LState) printCallStack() {
 
 func (ls *LState) closeAllUpvalues() { // +inline-start
 	for cf := ls.currentFrame; cf != nil; cf = cf.Parent {
-		if !cf.Fn.IsG {
+		if !cf.Fn.IsG() {
 			ls.closeUpvalues(cf.LocalBase)
 		}
 	}
@@ -653,7 +653,7 @@ func (ls *LState) raiseError(level int, format string, args ...interface{}) {
 
 func (ls *LState) findLocal(frame *callFrame, no int) string {
 	fn := frame.Fn
-	if !fn.IsG {
+	if !fn.IsG() {
 		if name, ok := fn.LocalName(no, frame.Pc-1); ok {
 			return name
 		}
@@ -700,7 +700,7 @@ func (ls *LState) stackTrace(level int) string {
 		for dbg, ok := ls.GetStack(i); ok; dbg, ok = ls.GetStack(i) {
 			cf := dbg.frame
 			buf = append(buf, fmt.Sprintf("\t%v in %v", ls.Where(i), ls.formattedFrameFuncName(cf)))
-			if !cf.Fn.IsG && cf.TailCall > 0 {
+			if !cf.Fn.IsG() && cf.TailCall > 0 {
 				for tc := cf.TailCall; tc > 0; tc-- {
 					buf = append(buf, "\t(tailcall): ?")
 					i++
@@ -746,19 +746,19 @@ func (ls *LState) frameFuncName(fr *callFrame) (string, bool) {
 			return "corountine", true
 		}
 	}
-	if !frame.Fn.IsG {
+	if !frame.Fn.IsG() {
 		pc := frame.Pc - 1
 		for _, call := range frame.Fn.Proto.DbgCalls {
 			if call.Pc == pc {
 				name := call.Name
-				if (name == "?" || fr.TailCall > 0) && !fr.Fn.IsG {
+				if (name == "?" || fr.TailCall > 0) && !fr.Fn.IsG() {
 					name = fmt.Sprintf("<%v:%v>", fr.Fn.Proto.SourceName, fr.Fn.Proto.LineDefined)
 				}
 				return name, false
 			}
 		}
 	}
-	if !fr.Fn.IsG {
+	if !fr.Fn.IsG() {
 		return fmt.Sprintf("<%v:%v>", fr.Fn.Proto.SourceName, fr.Fn.Proto.LineDefined), false
 	}
 	return "(anonymous)", false
@@ -934,7 +934,7 @@ func (ls *LState) metaCall(lvalue LValue) (*LFunction, bool) {
 }
 
 func (ls *LState) initCallFrame(cf *callFrame) { // +inline-start
-	if cf.Fn.IsG {
+	if cf.Fn.IsG() {
 		ls.reg.SetTop(cf.LocalBase + cf.NArgs)
 	} else {
 		proto := cf.Fn.Proto
@@ -1549,20 +1549,20 @@ func (ls *LState) GetInfo(what string, dbg *Debug, fn LValue) (LValue, error) {
 		case 'S':
 			if dbg.frame != nil && dbg.frame.Parent == nil {
 				dbg.What = "main"
-			} else if f.IsG {
+			} else if f.IsG() {
 				dbg.What = "G"
 			} else if dbg.frame != nil && dbg.frame.TailCall > 0 {
 				dbg.What = "tail"
 			} else {
 				dbg.What = "Lua"
 			}
-			if !f.IsG {
+			if !f.IsG() {
 				dbg.Source = f.Proto.SourceName
 				dbg.LineDefined = f.Proto.LineDefined
 				dbg.LastLineDefined = f.Proto.LastLineDefined
 			}
 		case 'l':
-			if !f.IsG && dbg.frame != nil {
+			if !f.IsG() && dbg.frame != nil {
 				if dbg.frame.Pc > 0 {
 					dbg.CurrentLine = f.Proto.DbgSourcePositions[dbg.frame.Pc-1]
 				}
@@ -1591,7 +1591,7 @@ func (ls *LState) GetStack(level int) (*Debug, bool) {
 	frame := ls.currentFrame
 	for ; level > 0 && frame != nil; frame = frame.Parent {
 		level--
-		if !frame.Fn.IsG {
+		if !frame.Fn.IsG() {
 			level -= frame.TailCall
 		}
 	}
@@ -1622,7 +1622,7 @@ func (ls *LState) SetLocal(dbg *Debug, no int, lv LValue) string {
 }
 
 func (ls *LState) GetUpvalue(fn *LFunction, no int) (string, LValue) {
-	if fn.IsG {
+	if fn.IsG() {
 		return "", LNil
 	}
 
@@ -1634,7 +1634,7 @@ func (ls *LState) GetUpvalue(fn *LFunction, no int) (string, LValue) {
 }
 
 func (ls *LState) SetUpvalue(fn *LFunction, no int, lv LValue) string {
-	if fn.IsG {
+	if fn.IsG() {
 		return ""
 	}
 
diff --git a/_vm.go b/_vm.go
index ee2be040..82d5484b 100644
--- a/_vm.go
+++ b/_vm.go
@@ -15,7 +15,7 @@ func mainLoop(L *LState, baseframe *callFrame) {
 	}
 
 	L.currentFrame = L.stack.Last()
-	if L.currentFrame.Fn.IsG {
+	if L.currentFrame.Fn.IsG() {
 		callGFunction(L, false)
 		return
 	}
@@ -39,7 +39,7 @@ func mainLoopWithContext(L *LState, baseframe *callFrame) {
 	}
 
 	L.currentFrame = L.stack.Last()
-	if L.currentFrame.Fn.IsG {
+	if L.currentFrame.Fn.IsG() {
 		callGFunction(L, false)
 		return
 	}
@@ -579,7 +579,7 @@ func init() {
 				callable, meta = L.metaCall(lv)
 			}
 			// +inline-call L.pushCallFrame callFrame{Fn:callable,Pc:0,Base:RA,LocalBase:RA+1,ReturnBase:RA,NArgs:nargs,NRet:nret,Parent:cf,TailCall:0} lv meta
-			if callable.IsG && callGFunction(L, false) {
+			if callable.IsG() && callGFunction(L, false) {
 				return 1
 			}
 			return 0
@@ -608,7 +608,7 @@ func init() {
 				L.RaiseError("attempt to call a non-function object")
 			}
 			// +inline-call L.closeUpvalues lbase
-			if callable.IsG {
+			if callable.IsG() {
 				luaframe := cf
 				L.pushCallFrame(callFrame{
 					Fn:         callable,
@@ -624,7 +624,7 @@ func init() {
 				if callGFunction(L, true) {
 					return 1
 				}
-				if L.currentFrame == nil || L.currentFrame.Fn.IsG || luaframe == baseframe {
+				if L.currentFrame == nil || L.currentFrame.Fn.IsG() || luaframe == baseframe {
 					return 1
 				}
 			} else {
@@ -674,7 +674,7 @@ func init() {
 			islast := baseframe == L.stack.Pop() || L.stack.IsEmpty()
 			// +inline-call copyReturnValues L cf.ReturnBase RA n B
 			L.currentFrame = L.stack.Last()
-			if islast || L.currentFrame == nil || L.currentFrame.Fn.IsG {
+			if islast || L.currentFrame == nil || L.currentFrame.Fn.IsG() {
 				return 1
 			}
 			return 0
diff --git a/baselib.go b/baselib.go
index aa2c08a9..5d98f314 100644
--- a/baselib.go
+++ b/baselib.go
@@ -97,7 +97,7 @@ func baseGetFEnv(L *LState) int {
 	}
 
 	if fn, ok := value.(*LFunction); ok {
-		if !fn.IsG {
+		if !fn.IsG() {
 			L.Push(fn.Env)
 		} else {
 			L.Push(L.G.Global)
@@ -114,7 +114,7 @@ func baseGetFEnv(L *LState) int {
 			for i := 0; i < level && cf != nil; i++ {
 				cf = cf.Parent
 			}
-			if cf == nil || cf.Fn.IsG {
+			if cf == nil || cf.Fn.IsG() {
 				L.Push(L.G.Global)
 			} else {
 				L.Push(cf.Fn.Env)
@@ -351,7 +351,7 @@ func baseSetFEnv(L *LState) int {
 	env := L.CheckTable(2)
 
 	if fn, ok := value.(*LFunction); ok {
-		if fn.IsG {
+		if fn.IsG() {
 			L.RaiseError("cannot change the environment of given object")
 		} else {
 			fn.Env = env
@@ -371,7 +371,7 @@ func baseSetFEnv(L *LState) int {
 		for i := 0; i < level && cf != nil; i++ {
 			cf = cf.Parent
 		}
-		if cf == nil || cf.Fn.IsG {
+		if cf == nil || cf.Fn.IsG() {
 			L.RaiseError("cannot change the environment of given object")
 		} else {
 			cf.Fn.Env = env
@@ -506,7 +506,7 @@ func loModule(L *LState) int {
 	caller := L.currentFrame.Parent
 	if caller == nil {
 		L.RaiseError("no calling stack.")
-	} else if caller.Fn.IsG {
+	} else if caller.Fn.IsG() {
 		L.RaiseError("module() can not be called from GFunctions.")
 	}
 	L.SetFEnv(caller.Fn, tb)
diff --git a/function.go b/function.go
index 169e5407..1340291a 100644
--- a/function.go
+++ b/function.go
@@ -154,7 +154,6 @@ func (fp *FunctionProto) str(level int, count int) string {
 
 func newLFunctionL(proto *FunctionProto, env *LTable, nupvalue int) *LFunction {
 	return &LFunction{
-		IsG: false,
 		Env: env,
 
 		Proto:     proto,
@@ -165,7 +164,6 @@ func newLFunctionL(proto *FunctionProto, env *LTable, nupvalue int) *LFunction {
 
 func newLFunctionG(gfunc LGFunction, env *LTable, nupvalue int) *LFunction {
 	return &LFunction{
-		IsG: true,
 		Env: env,
 
 		Proto:     nil,
@@ -175,7 +173,7 @@ func newLFunctionG(gfunc LGFunction, env *LTable, nupvalue int) *LFunction {
 }
 
 func (fn *LFunction) LocalName(regno, pc int) (string, bool) {
-	if fn.IsG {
+	if fn.IsG() {
 		return "", false
 	}
 	p := fn.Proto
diff --git a/state.go b/state.go
index 292f93b4..79cca372 100644
--- a/state.go
+++ b/state.go
@@ -715,7 +715,7 @@ func (ls *LState) printCallStack() {
 		if frame == nil {
 			break
 		}
-		if frame.Fn.IsG {
+		if frame.Fn.IsG() {
 			println("IsG:", true, "Frame:", frame, "Fn:", frame.Fn)
 		} else {
 			println("IsG:", false, "Frame:", frame, "Fn:", frame.Fn, "pc:", frame.Pc)
@@ -726,7 +726,7 @@ func (ls *LState) printCallStack() {
 
 func (ls *LState) closeAllUpvalues() { // +inline-start
 	for cf := ls.currentFrame; cf != nil; cf = cf.Parent {
-		if !cf.Fn.IsG {
+		if !cf.Fn.IsG() {
 			ls.closeUpvalues(cf.LocalBase)
 		}
 	}
@@ -753,7 +753,7 @@ func (ls *LState) raiseError(level int, format string, args ...interface{}) {
 
 func (ls *LState) findLocal(frame *callFrame, no int) string {
 	fn := frame.Fn
-	if !fn.IsG {
+	if !fn.IsG() {
 		if name, ok := fn.LocalName(no, frame.Pc-1); ok {
 			return name
 		}
@@ -800,7 +800,7 @@ func (ls *LState) stackTrace(level int) string {
 		for dbg, ok := ls.GetStack(i); ok; dbg, ok = ls.GetStack(i) {
 			cf := dbg.frame
 			buf = append(buf, fmt.Sprintf("\t%v in %v", ls.Where(i), ls.formattedFrameFuncName(cf)))
-			if !cf.Fn.IsG && cf.TailCall > 0 {
+			if !cf.Fn.IsG() && cf.TailCall > 0 {
 				for tc := cf.TailCall; tc > 0; tc-- {
 					buf = append(buf, "\t(tailcall): ?")
 					i++
@@ -846,19 +846,19 @@ func (ls *LState) frameFuncName(fr *callFrame) (string, bool) {
 			return "corountine", true
 		}
 	}
-	if !frame.Fn.IsG {
+	if !frame.Fn.IsG() {
 		pc := frame.Pc - 1
 		for _, call := range frame.Fn.Proto.DbgCalls {
 			if call.Pc == pc {
 				name := call.Name
-				if (name == "?" || fr.TailCall > 0) && !fr.Fn.IsG {
+				if (name == "?" || fr.TailCall > 0) && !fr.Fn.IsG() {
 					name = fmt.Sprintf("<%v:%v>", fr.Fn.Proto.SourceName, fr.Fn.Proto.LineDefined)
 				}
 				return name, false
 			}
 		}
 	}
-	if !fr.Fn.IsG {
+	if !fr.Fn.IsG() {
 		return fmt.Sprintf("<%v:%v>", fr.Fn.Proto.SourceName, fr.Fn.Proto.LineDefined), false
 	}
 	return "(anonymous)", false
@@ -1034,7 +1034,7 @@ func (ls *LState) metaCall(lvalue LValue) (*LFunction, bool) {
 }
 
 func (ls *LState) initCallFrame(cf *callFrame) { // +inline-start
-	if cf.Fn.IsG {
+	if cf.Fn.IsG() {
 		ls.reg.SetTop(cf.LocalBase + cf.NArgs)
 	} else {
 		proto := cf.Fn.Proto
@@ -1146,7 +1146,7 @@ func (ls *LState) pushCallFrame(cf callFrame, fn LValue, meta bool) { // +inline
 	// source function is 'func (ls *LState) initCallFrame(cf *callFrame) ' in '_state.go'
 	{
 		cf := newcf
-		if cf.Fn.IsG {
+		if cf.Fn.IsG() {
 			ls.reg.SetTop(cf.LocalBase + cf.NArgs)
 		} else {
 			proto := cf.Fn.Proto
@@ -1762,20 +1762,20 @@ func (ls *LState) GetInfo(what string, dbg *Debug, fn LValue) (LValue, error) {
 		case 'S':
 			if dbg.frame != nil && dbg.frame.Parent == nil {
 				dbg.What = "main"
-			} else if f.IsG {
+			} else if f.IsG() {
 				dbg.What = "G"
 			} else if dbg.frame != nil && dbg.frame.TailCall > 0 {
 				dbg.What = "tail"
 			} else {
 				dbg.What = "Lua"
 			}
-			if !f.IsG {
+			if !f.IsG() {
 				dbg.Source = f.Proto.SourceName
 				dbg.LineDefined = f.Proto.LineDefined
 				dbg.LastLineDefined = f.Proto.LastLineDefined
 			}
 		case 'l':
-			if !f.IsG && dbg.frame != nil {
+			if !f.IsG() && dbg.frame != nil {
 				if dbg.frame.Pc > 0 {
 					dbg.CurrentLine = f.Proto.DbgSourcePositions[dbg.frame.Pc-1]
 				}
@@ -1804,7 +1804,7 @@ func (ls *LState) GetStack(level int) (*Debug, bool) {
 	frame := ls.currentFrame
 	for ; level > 0 && frame != nil; frame = frame.Parent {
 		level--
-		if !frame.Fn.IsG {
+		if !frame.Fn.IsG() {
 			level -= frame.TailCall
 		}
 	}
@@ -1835,7 +1835,7 @@ func (ls *LState) SetLocal(dbg *Debug, no int, lv LValue) string {
 }
 
 func (ls *LState) GetUpvalue(fn *LFunction, no int) (string, LValue) {
-	if fn.IsG {
+	if fn.IsG() {
 		return "", LNil
 	}
 
@@ -1847,7 +1847,7 @@ func (ls *LState) GetUpvalue(fn *LFunction, no int) (string, LValue) {
 }
 
 func (ls *LState) SetUpvalue(fn *LFunction, no int, lv LValue) string {
-	if fn.IsG {
+	if fn.IsG() {
 		return ""
 	}
 
diff --git a/value.go b/value.go
index 4156e9d5..fc83dac1 100644
--- a/value.go
+++ b/value.go
@@ -154,7 +154,6 @@ func (tb *LTable) String() string   { return fmt.Sprintf("table: %p", tb) }
 func (tb *LTable) Type() LValueType { return LTTable }
 
 type LFunction struct {
-	IsG       bool
 	Env       *LTable
 	Proto     *FunctionProto
 	GFunction LGFunction
@@ -164,6 +163,7 @@ type LGFunction func(*LState) int
 
 func (fn *LFunction) String() string   { return fmt.Sprintf("function: %p", fn) }
 func (fn *LFunction) Type() LValueType { return LTFunction }
+func (fn *LFunction) IsG() bool        { return fn.GFunction != nil }
 
 type Global struct {
 	MainThread    *LState
diff --git a/vm.go b/vm.go
index 97335a75..02c61532 100644
--- a/vm.go
+++ b/vm.go
@@ -19,7 +19,7 @@ func mainLoop(L *LState, baseframe *callFrame) {
 	}
 
 	L.currentFrame = L.stack.Last()
-	if L.currentFrame.Fn.IsG {
+	if L.currentFrame.Fn.IsG() {
 		callGFunction(L, false)
 		return
 	}
@@ -43,7 +43,7 @@ func mainLoopWithContext(L *LState, baseframe *callFrame) {
 	}
 
 	L.currentFrame = L.stack.Last()
-	if L.currentFrame.Fn.IsG {
+	if L.currentFrame.Fn.IsG() {
 		callGFunction(L, false)
 		return
 	}
@@ -1209,7 +1209,7 @@ func init() {
 				// source function is 'func (ls *LState) initCallFrame(cf *callFrame) ' in '_state.go'
 				{
 					cf := newcf
-					if cf.Fn.IsG {
+					if cf.Fn.IsG() {
 						ls.reg.SetTop(cf.LocalBase + cf.NArgs)
 					} else {
 						proto := cf.Fn.Proto
@@ -1305,7 +1305,7 @@ func init() {
 				}
 				ls.currentFrame = newcf
 			}
-			if callable.IsG && callGFunction(L, false) {
+			if callable.IsG() && callGFunction(L, false) {
 				return 1
 			}
 			return 0
@@ -1353,7 +1353,7 @@ func init() {
 					}
 				}
 			}
-			if callable.IsG {
+			if callable.IsG() {
 				luaframe := cf
 				L.pushCallFrame(callFrame{
 					Fn:         callable,
@@ -1369,7 +1369,7 @@ func init() {
 				if callGFunction(L, true) {
 					return 1
 				}
-				if L.currentFrame == nil || L.currentFrame.Fn.IsG || luaframe == baseframe {
+				if L.currentFrame == nil || L.currentFrame.Fn.IsG() || luaframe == baseframe {
 					return 1
 				}
 			} else {
@@ -1391,7 +1391,7 @@ func init() {
 				// source function is 'func (ls *LState) initCallFrame(cf *callFrame) ' in '_state.go'
 				{
 					ls := L
-					if cf.Fn.IsG {
+					if cf.Fn.IsG() {
 						ls.reg.SetTop(cf.LocalBase + cf.NArgs)
 					} else {
 						proto := cf.Fn.Proto
@@ -1784,7 +1784,7 @@ func init() {
 				}
 			}
 			L.currentFrame = L.stack.Last()
-			if islast || L.currentFrame == nil || L.currentFrame.Fn.IsG {
+			if islast || L.currentFrame == nil || L.currentFrame.Fn.IsG() {
 				return 1
 			}
 			return 0