Add IsPredicate helper

- Write tests for `IsPredicate`
- Write missing tests for `IsFunction`

Author: vellotis

helpers.go

@@ -75,7 +75,7 @@ func PtrOf(itf interface{}) interface{} {
 func IsFunction(in interface{}, num ...int) bool {
 	funcType := reflect.TypeOf(in)
 
-	result := funcType.Kind() == reflect.Func
+	result := funcType != nil && funcType.Kind() == reflect.Func
 
 	if len(num) >= 1 {
 		result = result && funcType.NumIn() == num[0]
@@ -88,6 +88,27 @@ func IsFunction(in interface{}, num ...int) bool {
 	return result
 }
 
+// IsPredicate returns if the argument is a predicate function.
+func IsPredicate(in interface{}, inTypes ...reflect.Type) bool {
+	if len(inTypes) == 0 {
+		inTypes = append(inTypes, nil)
+	}
+
+	funcType := reflect.TypeOf(in)
+
+	result := funcType != nil && funcType.Kind() == reflect.Func
+
+	result = result && funcType.NumOut() == 1 && funcType.Out(0).Kind() == reflect.Bool
+	result = result && funcType.NumIn() == len(inTypes)
+
+	for i := 0; result && i < len(inTypes); i++ {
+		inType := inTypes[i]
+		result = inType == nil || inType.ConvertibleTo(funcType.In(i))
+	}
+
+	return result
+}
+
 // IsEqual returns if the two objects are equal
 func IsEqual(expected interface{}, actual interface{}) bool {
 	if expected == nil || actual == nil {

helpers_test.go

@@ -269,3 +269,32 @@ func TestIsIteratee(t *testing.T) {
 
 	is.False(IsIteratee(nil))
 }
+
+func TestIsFunction(t *testing.T) {
+	is := assert.New(t)
+
+	is.False(IsFunction(nil))
+	is.False(IsFunction(""))
+	is.True(IsFunction(func() {}))
+	is.True(IsFunction(func(string, string, string) bool { return false }, 3))
+	is.False(IsFunction(func(string, string, string) bool { return false }, 3, 0))
+	is.True(IsFunction(func(string, string, string) (bool, error) { return false, nil }, 3, 2))
+}
+
+func TestIsPredicate(t *testing.T) {
+	is := assert.New(t)
+
+	is.False(IsPredicate(nil))
+	is.False(IsPredicate(""))
+	is.False(IsPredicate(func() {}))
+	is.False(IsPredicate(func() bool { return false}))
+	is.True(IsPredicate(func(int) bool { return false}))
+	is.True(IsPredicate(func(int) bool { return false}, nil))
+	is.False(IsPredicate(func(int) bool { return false}, reflect.TypeOf("")))
+	is.True(IsPredicate(func(int) bool { return false}, reflect.TypeOf(0)))
+	is.False(IsPredicate(func(int, string) bool { return false}, reflect.TypeOf("")))
+	is.False(IsPredicate(func(int, string) bool { return false}, reflect.TypeOf(""), reflect.TypeOf(0)))
+	is.True(IsPredicate(func(int, string) bool { return false}, reflect.TypeOf(0), reflect.TypeOf("")))
+	is.False(IsPredicate(func(struct{}, string) bool { return false}, reflect.TypeOf(0), reflect.TypeOf("")))
+	is.True(IsPredicate(func(struct{}, string) bool { return false}, nil, reflect.TypeOf("")))
+}