function: catch earlier programming error (#8)

Place at first line of the function body a function type safe guard
check.

function/function.go

@@ -11,10 +11,12 @@ import (
 
 // After creates a function that invokes func once it's called n or more times
 func After(n int, fn interface{}) func(args ...interface{}) []reflect.Value {
+	// Catch programming error while constructing the closure
+	MustBeFunction(fn)
 	return func(args ...interface{}) []reflect.Value {
 		n--
 		if n < 1 {
-			return invokeFunc(fn, args...)
+			return unsafeInvokeFunc(fn, args...)
 		}
 		return nil
 	}
@@ -22,11 +24,12 @@ func After(n int, fn interface{}) func(args ...interface{}) []reflect.Value {
 
 // Before creates a function that invokes func once it's called less than n times
 func Before(n int, fn interface{}) func(args ...interface{}) []reflect.Value {
+	// Catch programming error while constructing the closure
+	MustBeFunction(fn)
 	var res []reflect.Value
-
 	return func(args ...interface{}) []reflect.Value {
 		if n > 0 {
-			res = invokeFunc(fn, args...)
+			res = unsafeInvokeFunc(fn, args...)
 		}
 		if n <= 0 {
 			fn = nil
@@ -69,11 +72,12 @@ func Delay(delay time.Duration, fn interface{}, args ...interface{}) {
 
 // Schedule invoke function every duration time, util close the returned bool chan
 func Schedule(d time.Duration, fn interface{}, args ...interface{}) chan bool {
+	// Catch programming error while constructing the closure
+	MustBeFunction(fn)
 	quit := make(chan bool)
-
 	go func() {
 		for {
-			invokeFunc(fn, args...)
+			unsafeInvokeFunc(fn, args...)
 			select {
 			case <-time.After(d):
 			case <-quit:

function/function_util.go

@@ -14,6 +14,15 @@ func invokeFunc(fn interface{}, args ...interface{}) []reflect.Value {
 	return fv.Call(params)
 }
 
+func unsafeInvokeFunc(fn interface{}, args ...interface{}) []reflect.Value {
+	fv := reflect.ValueOf(fn)
+	params := make([]reflect.Value, len(args))
+	for i, item := range args {
+		params[i] = reflect.ValueOf(item)
+	}
+	return fv.Call(params)
+}
+
 func functionValue(function interface{}) reflect.Value {
 	v := reflect.ValueOf(function)
 	if v.Kind() != reflect.Func {
@@ -21,3 +30,10 @@ func functionValue(function interface{}) reflect.Value {
 	}
 	return v
 }
+
+func MustBeFunction(function interface{}) {
+	v := reflect.ValueOf(function)
+	if v.Kind() != reflect.Func {
+		panic(fmt.Sprintf("Invalid function type, value of type %T", function))
+	}
+}