Enrich error message for reduce function

Signed-off-by: Dinesh <dineshudt17@gmail.com>

Author: dineshba

reduce.go

@@ -22,13 +22,13 @@ func Reduce(in, out, reduceFn interface{}) error {
 		outputKind := output.Elem().Kind()
 		reducerFnType := reducer.Type()
 		if outputKind != reducerFnType.In(0).Kind() {
-			return fmt.Errorf("reducer function's first argument has to be the type of out")
+			return fmt.Errorf("reduceFn's first argument's type(%s) has to be the type of out(%s)", reducerFnType.In(0).Kind(), outputKind)
 		}
 		if input.Type().Elem().Kind() != reducerFnType.In(1).Kind() {
-			return fmt.Errorf("reducer function's second argument has to be the type of element of input slice")
+			return fmt.Errorf("reduceFn's second argument's type(%s) has to be the type of element of input slice(%s)", reducerFnType.In(1).Kind(), input.Type().Elem().Kind())
 		}
 		if outputKind != reducerFnType.Out(0).Kind() {
-			return fmt.Errorf("reducer function's return type has to be the type of out")
+			return fmt.Errorf("reduceFn's return type(%s) has to be the type of out(%s)", reducerFnType.Out(0).Kind(), outputKind)
 		}
 
 		result := output.Elem()
@@ -48,13 +48,13 @@ func Reduce(in, out, reduceFn interface{}) error {
 func validateReducer(reducer reflect.Value) error {
 	reducerFnType := reducer.Type()
 	if reducer.Kind() != reflect.Func {
-		return fmt.Errorf("reduceFn has to be a function")
+		return fmt.Errorf("reduceFn has to be a (func) and not (%s)", reducer.Kind())
 	}
 	if reducerFnType.NumIn() != 2 {
-		return fmt.Errorf("reducer function has to take exactly two argument")
+		return fmt.Errorf("reduceFn has to take exactly 2 arguments and not %d argument(s)", reducerFnType.NumIn())
 	}
 	if reducerFnType.NumOut() != 1 {
-		return fmt.Errorf("reducer function should return only one return value")
+		return fmt.Errorf("reduceFn should have only one return value and not %d return type(s)", reducerFnType.NumOut())
 	}
 	return nil
 }

reduce_test.go

@@ -83,7 +83,7 @@ func TestReduce(t *testing.T) {
 
 		err := godash.Reduce(in, &out, 7)
 
-		assert.EqualError(t, err, "reduceFn has to be a function")
+		assert.EqualError(t, err, "reduceFn has to be a (func) and not (int)")
 	})
 
 	t.Run("should not accept reducer function that do not take exactly two argument", func(t *testing.T) {
@@ -92,12 +92,12 @@ func TestReduce(t *testing.T) {
 
 		{
 			err := godash.Reduce(in, &out, func() int { return 0 })
-			assert.EqualError(t, err, "reducer function has to take exactly two argument")
+			assert.EqualError(t, err, "reduceFn has to take exactly 2 arguments and not 0 argument(s)")
 		}
 
 		{
 			err := godash.Reduce(in, &out, func(int) int { return 0 })
-			assert.EqualError(t, err, "reducer function has to take exactly two argument")
+			assert.EqualError(t, err, "reduceFn has to take exactly 2 arguments and not 1 argument(s)")
 		}
 	})
 
@@ -107,12 +107,12 @@ func TestReduce(t *testing.T) {
 
 		{
 			err := godash.Reduce(in, &out, func(int, int) {})
-			assert.EqualError(t, err, "reducer function should return only one return value")
+			assert.EqualError(t, err, "reduceFn should have only one return value and not 0 return type(s)")
 		}
 
 		{
 			err := godash.Reduce(in, &out, func(int, int) (int, int) { return 0, 0 })
-			assert.EqualError(t, err, "reducer function should return only one return value")
+			assert.EqualError(t, err, "reduceFn should have only one return value and not 2 return type(s)")
 		}
 	})
 
@@ -122,7 +122,7 @@ func TestReduce(t *testing.T) {
 
 		{
 			err := godash.Reduce(in, &out, func(string, int) int { return 0 })
-			assert.EqualError(t, err, "reducer function's first argument has to be the type of out")
+			assert.EqualError(t, err, "reduceFn's first argument's type(string) has to be the type of out(int)")
 		}
 
 		{
@@ -137,7 +137,7 @@ func TestReduce(t *testing.T) {
 
 		{
 			err := godash.Reduce(in, &out, func(string, string) string { return "" })
-			assert.EqualError(t, err, "reducer function's second argument has to be the type of element of input slice")
+			assert.EqualError(t, err, "reduceFn's second argument's type(string) has to be the type of element of input slice(int)")
 		}
 
 		{
@@ -152,7 +152,7 @@ func TestReduce(t *testing.T) {
 
 		{
 			err := godash.Reduce(in, &out, func(string, int) int { return 0 })
-			assert.EqualError(t, err, "reducer function's return type has to be the type of out")
+			assert.EqualError(t, err, "reduceFn's return type(int) has to be the type of out(string)")
 		}
 
 		{