field: add Set() method for setting fields

README.md

@@ -95,6 +95,9 @@ name := s.Field("Name")
 // Get the underlying value,  value => "gopher"
 value := name.Value().(string)
 
+// Set the field's value
+name.Set("another gopher")
+
 // Check if the field is exported or not
 if name.IsExported() {
 	fmt.Println("Name field is exported")

field.go

@@ -1,6 +1,15 @@
 package structs
 
-import "reflect"
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+var (
+	errNotExported = errors.New("field is not exported")
+	errNotSettable = errors.New("field is not settable")
+)
 
 // Field represents a single struct field that encapsulates high level
 // functions around the field.
@@ -45,6 +54,32 @@ func (f *Field) Name() string {
 	return f.field.Name
 }
 
+// Set sets the field to given value v. It retuns an error if the field is not
+// settable (not addresable or not exported) or if the given value's type
+// doesn't match the fields type.
+func (f *Field) Set(val interface{}) error {
+	// needed to make the field settable
+	v := reflect.Indirect(f.value)
+
+	if !f.IsExported() {
+		return errNotExported
+	}
+
+	// do we get here? not sure...
+	if !v.CanSet() {
+		return errNotSettable
+	}
+
+	given := reflect.ValueOf(val)
+
+	if v.Kind() != given.Kind() {
+		return fmt.Errorf("wrong kind: %s want: %s", given.Kind(), v.Kind())
+	}
+
+	v.Set(given)
+	return nil
+}
+
 // Field returns the field from a nested struct. It panics if the nested struct
 // is not exported or if the field was not found.
 func (f *Field) Field(name string) *Field {

field_test.go

@@ -9,7 +9,9 @@ type Foo struct {
 	C    bool   `json:"c"`
 	d    string // not exported
 	x    string `xml:"x"` // not exported, with tag
-	*Bar        // embedded
+	Y    []string
+	Z    map[string]interface{}
+	*Bar // embedded
 }
 
 type Bar struct {
@@ -30,12 +32,70 @@ func newStruct() *Struct {
 		A: "gopher",
 		C: true,
 		d: "small",
+		Y: []string{"example"},
+		Z: nil,
 	}
 	f.Bar = b
 
 	return New(f)
 }
 
+func TestField_Set(t *testing.T) {
+	s := newStruct()
+
+	f := s.Field("A")
+	err := f.Set("fatih")
+	if err != nil {
+		t.Error(err)
+	}
+
+	if f.Value().(string) != "fatih" {
+		t.Errorf("Setted value is wrong: %s want: %s", f.Value().(string), "fatih")
+	}
+
+	f = s.Field("Y")
+	err = f.Set([]string{"override", "with", "this"})
+	if err != nil {
+		t.Error(err)
+	}
+
+	sliceLen := len(f.Value().([]string))
+	if sliceLen != 3 {
+		t.Errorf("Setted values slice length is wrong: %d, want: %d", sliceLen, 3)
+	}
+
+	f = s.Field("C")
+	err = f.Set(false)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if f.Value().(bool) {
+		t.Errorf("Setted value is wrong: %s want: %s", f.Value().(bool), false)
+	}
+
+	// let's pass a different type
+	f = s.Field("A")
+	err = f.Set(123) // Field A is of type string, but we are going to pass an integer
+	if err == nil {
+		t.Error("Setting a field's value with a different type than the field's type should return an error")
+	}
+
+	// old value should be still there :)
+	if f.Value().(string) != "fatih" {
+		t.Errorf("Setted value is wrong: %s want: %s", f.Value().(string), "fatih")
+	}
+
+	// let's access an unexported field, which should give an error
+	f = s.Field("d")
+	err = f.Set("large")
+	if err != errNotExported {
+		t.Error(err)
+	}
+
+	// TODO: let's access a non addresable field, which should give an error
+}
+
 func TestField(t *testing.T) {
 	s := newStruct()