Fix map function's validation of output return type

Author: aswinkarthik

map.go

@@ -5,7 +5,15 @@ import (
 	"reflect"
 )
 
-// Map applies mapperFn on each item of in and puts it in out
+// Map applies mapperFn on each item of in and puts it in out.
+// Currently, input and output for type slice is supported.
+//
+// Validations:
+// 1. Mapper function should take in one argument and return one argument
+// 2. Mapper function's argument should be of the same type of each element of input slice.
+// 3. Mapper function's output should be of the same type of each element of output slice.
+//
+// Validation failures are returned as error by the godash.Map to the caller.
 func Map(in, out, mapperFn interface{}) error {
 	input := reflect.ValueOf(in)
 	output := reflect.ValueOf(out)
@@ -28,14 +36,14 @@ func Map(in, out, mapperFn interface{}) error {
 	}
 
 	if input.Kind() == reflect.Slice {
-		if output.Kind() != reflect.Slice {
+		if output.Elem().Kind() != reflect.Slice {
 			return fmt.Errorf("output should be a slice for input of type slice")
 		}
-		if input.Type().Elem().Kind() != mapper.Type().In(0).Kind() {
-			return fmt.Errorf("mapper function's first argument has to be the type of element of input slice")
+		if input.Type().Elem() != mapper.Type().In(0) {
+			return fmt.Errorf("mapper function's first argument (%s) has to be (%s)", mapper.Type().In(0), input.Type().Elem())
 		}
-		if output.Type().Elem().Elem().Kind() != mapper.Type().Out(0).Kind() {
-			return fmt.Errorf("mapper function's return type has to be the type of element of output slice")
+		if output.Elem().Type().Elem() != mapper.Type().Out(0) {
+			return fmt.Errorf("mapper function's return type has to be (%s) but is (%s)", mapper.Type().Out(0), output.Elem().Type().Elem())
 		}
 
 		result := reflect.MakeSlice(output.Elem().Type(), 0, input.Len())

map_test.go

@@ -1,6 +1,7 @@
 package godash_test
 
 import (
+	"fmt"
 	"github.com/stretchr/testify/assert"
 	"github.com/thecasualcoder/godash"
 	"testing"
@@ -116,7 +117,7 @@ func TestMap(t *testing.T) {
 
 		{
 			err := godash.Map(in, &out, func(string) string { return "" })
-			assert.EqualError(t, err, "mapper function's first argument has to be the type of element of input slice")
+			assert.EqualError(t, err, "mapper function's first argument (string) has to be (int)")
 		}
 
 		{
@@ -131,7 +132,7 @@ func TestMap(t *testing.T) {
 
 		{
 			err := godash.Map(in, &out, func(int) int { return 0 })
-			assert.EqualError(t, err, "mapper function's return type has to be the type of element of output slice")
+			assert.EqualError(t, err, "mapper function's return type has to be (int) but is (string)")
 		}
 
 		{
@@ -140,3 +141,16 @@ func TestMap(t *testing.T) {
 		}
 	})
 }
+
+func ExampleMap() {
+	input := []int{0, 1, 2, 3, 4}
+	var output []string
+
+	_ = godash.Map(input, &output, func(num int) string {
+		return fmt.Sprintf("%d", num*num)
+	})
+
+	fmt.Println(output)
+
+	// Output: [0 1 4 9 16]
+}