Fix nil pointer on watch function (#568)

* Added a new `DecoderConfigOption` type allowing the user to write custom
  functions that can override the default mapstructure.DecoderConfig
  settings

* Added a new `DecodeHook` function which returns
  a `DecoderConfigOption`. This allows the user to easily set their own
  Decode hooks when Unmarshaling

* Updated Unmarshal, UnmarshalKey and defaultDecoderConfig to support variadic
  trailing `DecoderConfigOption` functions to allow for customisation of
  the default  mapstructure.DecoderConfig

* Added a test case with example usage

.travis.yml

@@ -2,8 +2,8 @@ go_import_path: github.com/spf13/viper
 
 language: go
 go:
-  - 1.7.5
-  - 1.8
+  - 1.9.x
+  - 1.10.x
   - tip
 
 os:

README.md

@@ -185,13 +185,13 @@ with ENV:
  * `AutomaticEnv()`
  * `BindEnv(string...) : error`
  * `SetEnvPrefix(string)`
- * `SetEnvReplacer(string...) *strings.Replacer`
+ * `SetEnvKeyReplacer(string...) *strings.Replacer`
 
 _When working with ENV variables, it’s important to recognize that Viper
 treats ENV variables as case sensitive._
 
 Viper provides a mechanism to try to ensure that ENV variables are unique. By
-using `SetEnvPrefix`, you can tell Viper to use add a prefix while reading from
+using `SetEnvPrefix`, you can tell Viper to use a prefix while reading from
 the environment variables. Both `BindEnv` and `AutomaticEnv` will use this
 prefix.
 
@@ -212,7 +212,7 @@ time a `viper.Get` request is made. It will apply the following rules. It will
 check for a environment variable with a name matching the key uppercased and
 prefixed with the `EnvPrefix` if set.
 
-`SetEnvReplacer` allows you to use a `strings.Replacer` object to rewrite Env
+`SetEnvKeyReplacer` allows you to use a `strings.Replacer` object to rewrite Env
 keys to an extent. This is useful if you want to use `-` or something in your
 `Get()` calls, but want your environmental variables to use `_` delimiters. An
 example of using it can be found in `viper_test.go`.
@@ -437,6 +437,7 @@ The following functions and methods exist:
  * `GetTime(key string) : time.Time`
  * `GetDuration(key string) : time.Duration`
  * `IsSet(key string) : bool`
+ * `AllSettings() : map[string]interface{}`
 
 One important thing to recognize is that each Get function will return a zero
 value if it’s not found. To check if a given key exists, the `IsSet()` method
@@ -590,6 +591,27 @@ if err != nil {
 }
 ```
 
+### Marshalling to string
+
+You may need to marhsal all the settings held in viper into a string rather than write them to a file. 
+You can use your favorite format's marshaller with the config returned by `AllSettings()`.
+
+```go
+import (
+    yaml "gopkg.in/yaml.v2"
+    // ...
+) 
+
+func yamlStringSettings() string {
+    c := viper.AllSettings()
+	bs, err := yaml.Marshal(c)
+	if err != nil {
+        t.Fatalf("unable to marshal config to YAML: %v", err)
+    }
+	return string(bs)
+}
+```
+
 ## Viper or Vipers?
 
 Viper comes ready to use out of the box. There is no configuration or

nohup.out

@@ -1 +0,0 @@
-QProcess::start: Process is already running

remote/remote.go

@@ -8,10 +8,11 @@ package remote
 
 import (
 	"bytes"
-	"github.com/spf13/viper"
-	crypt "github.com/xordataexchange/crypt/config"
 	"io"
 	"os"
+
+	"github.com/spf13/viper"
+	crypt "github.com/xordataexchange/crypt/config"
 )
 
 type remoteConfigProvider struct{}

util.go

@@ -11,22 +11,16 @@
 package viper
 
 import (
-	"bytes"
-	"encoding/json"
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"runtime"
 	"strings"
 	"unicode"
 
-	"github.com/hashicorp/hcl"
-	"github.com/magiconair/properties"
-	toml "github.com/pelletier/go-toml"
+	"github.com/spf13/afero"
 	"github.com/spf13/cast"
 	jww "github.com/spf13/jwalterweatherman"
-	"gopkg.in/yaml.v2"
 )
 
 // ConfigParseError denotes failing to parse configuration file.
@@ -121,8 +115,8 @@ func absPathify(inPath string) string {
 }
 
 // Check if File / Directory Exists
-func exists(path string) (bool, error) {
-	_, err := v.fs.Stat(path)
+func exists(fs afero.Fs, path string) (bool, error) {
+	_, err := fs.Stat(path)
 	if err == nil {
 		return true, nil
 	}
@@ -152,61 +146,6 @@ func userHomeDir() string {
 	return os.Getenv("HOME")
 }
 
-func unmarshallConfigReader(in io.Reader, c map[string]interface{}, configType string) error {
-	buf := new(bytes.Buffer)
-	buf.ReadFrom(in)
-
-	switch strings.ToLower(configType) {
-	case "yaml", "yml":
-		if err := yaml.Unmarshal(buf.Bytes(), &c); err != nil {
-			return ConfigParseError{err}
-		}
-
-	case "json":
-		if err := json.Unmarshal(buf.Bytes(), &c); err != nil {
-			return ConfigParseError{err}
-		}
-
-	case "hcl":
-		obj, err := hcl.Parse(string(buf.Bytes()))
-		if err != nil {
-			return ConfigParseError{err}
-		}
-		if err = hcl.DecodeObject(&c, obj); err != nil {
-			return ConfigParseError{err}
-		}
-
-	case "toml":
-		tree, err := toml.LoadReader(buf)
-		if err != nil {
-			return ConfigParseError{err}
-		}
-		tmap := tree.ToMap()
-		for k, v := range tmap {
-			c[k] = v
-		}
-
-	case "properties", "props", "prop":
-		var p *properties.Properties
-		var err error
-		if p, err = properties.Load(buf.Bytes(), properties.UTF8); err != nil {
-			return ConfigParseError{err}
-		}
-		for _, key := range p.Keys() {
-			value, _ := p.Get(key)
-			// recursively build nested maps
-			path := strings.Split(key, ".")
-			lastKey := strings.ToLower(path[len(path)-1])
-			deepestMap := deepSearch(c, path[0:len(path)-1])
-			// set innermost value
-			deepestMap[lastKey] = value
-		}
-	}
-
-	insensitiviseMap(c)
-	return nil
-}
-
 func safeMul(a, b uint) uint {
 	c := a * b
 	if a > 1 && b > 1 && c/b != a {

viper.go

@@ -22,6 +22,7 @@ package viper
 import (
 	"bytes"
 	"encoding/csv"
+	"encoding/json"
 	"fmt"
 	"io"
 	"log"
@@ -31,14 +32,30 @@ import (
 	"strings"
 	"time"
 
+	yaml "gopkg.in/yaml.v2"
+
 	"github.com/fsnotify/fsnotify"
+	"github.com/hashicorp/hcl"
+	"github.com/hashicorp/hcl/hcl/printer"
+	"github.com/magiconair/properties"
 	"github.com/mitchellh/mapstructure"
+	toml "github.com/pelletier/go-toml"
 	"github.com/spf13/afero"
 	"github.com/spf13/cast"
 	jww "github.com/spf13/jwalterweatherman"
 	"github.com/spf13/pflag"
 )
 
+// ConfigMarshalError happens when failing to marshal the configuration.
+type ConfigMarshalError struct {
+	err error
+}
+
+// Error returns the formatted configuration error.
+func (e ConfigMarshalError) Error() string {
+	return fmt.Sprintf("While marshaling config: %s", e.err.Error())
+}
+
 var v *Viper
 
 type RemoteResponse struct {
@@ -96,6 +113,23 @@ func (fnfe ConfigFileNotFoundError) Error() string {
 	return fmt.Sprintf("Config File %q Not Found in %q", fnfe.name, fnfe.locations)
 }
 
+// A DecoderConfigOption can be passed to viper.Unmarshal to configure
+// mapstructure.DecoderConfig options
+type DecoderConfigOption func(*mapstructure.DecoderConfig)
+
+// DecodeHook returns a DecoderConfigOption which overrides the default
+// DecoderConfig.DecodeHook value, the default is:
+//
+//  mapstructure.ComposeDecodeHookFunc(
+//		mapstructure.StringToTimeDurationHookFunc(),
+//		mapstructure.StringToSliceHookFunc(","),
+//	)
+func DecodeHook(hook mapstructure.DecodeHookFunc) DecoderConfigOption {
+	return func(c *mapstructure.DecoderConfig) {
+		c.DecodeHook = hook
+	}
+}
+
 // Viper is a prioritized configuration registry. It
 // maintains a set of configuration sources, fetches
 // values to populate those, and provides them according
@@ -162,6 +196,10 @@ type Viper struct {
 	aliases        map[string]string
 	typeByDefValue bool
 
+	// Store read properties on the object so that we can write back in order with comments.
+	// This will only be used if the configuration read is a properties file.
+	properties *properties.Properties
+
 	onConfigChange func(fsnotify.Event)
 }
 
@@ -188,7 +226,7 @@ func New() *Viper {
 // can use it in their testing as well.
 func Reset() {
 	v = New()
-	SupportedExts = []string{"json", "toml", "yaml", "yml", "hcl"}
+	SupportedExts = []string{"json", "toml", "yaml", "yml", "properties", "props", "prop", "hcl"}
 	SupportedRemoteProviders = []string{"etcd", "consul"}
 }
 
@@ -268,7 +306,9 @@ func (v *Viper) WatchConfig() {
 							if err != nil {
 								log.Println("error:", err)
 							}
-							v.onConfigChange(event)
+							if v.onConfigChange != nil {
+								v.onConfigChange(event)
+							}
 						}
 					}
 				case err := <-watcher.Errors:
@@ -661,6 +701,12 @@ func (v *Viper) GetInt(key string) int {
 	return cast.ToInt(v.Get(key))
 }
 
+// GetInt32 returns the value associated with the key as an integer.
+func GetInt32(key string) int32 { return v.GetInt32(key) }
+func (v *Viper) GetInt32(key string) int32 {
+	return cast.ToInt32(v.Get(key))
+}
+
 // GetInt64 returns the value associated with the key as an integer.
 func GetInt64(key string) int64 { return v.GetInt64(key) }
 func (v *Viper) GetInt64(key string) int64 {
@@ -718,9 +764,11 @@ func (v *Viper) GetSizeInBytes(key string) uint {
 }
 
 // UnmarshalKey takes a single key and unmarshals it into a Struct.
-func UnmarshalKey(key string, rawVal interface{}) error { return v.UnmarshalKey(key, rawVal) }
-func (v *Viper) UnmarshalKey(key string, rawVal interface{}) error {
-	err := decode(v.Get(key), defaultDecoderConfig(rawVal))
+func UnmarshalKey(key string, rawVal interface{}, opts ...DecoderConfigOption) error {
+	return v.UnmarshalKey(key, rawVal, opts...)
+}
+func (v *Viper) UnmarshalKey(key string, rawVal interface{}, opts ...DecoderConfigOption) error {
+	err := decode(v.Get(key), defaultDecoderConfig(rawVal, opts...))
 
 	if err != nil {
 		return err
@@ -733,9 +781,11 @@ func (v *Viper) UnmarshalKey(key string, rawVal interface{}) error {
 
 // Unmarshal unmarshals the config into a Struct. Make sure that the tags
 // on the fields of the structure are properly set.
-func Unmarshal(rawVal interface{}) error { return v.Unmarshal(rawVal) }
-func (v *Viper) Unmarshal(rawVal interface{}) error {
-	err := decode(v.AllSettings(), defaultDecoderConfig(rawVal))
+func Unmarshal(rawVal interface{}, opts ...DecoderConfigOption) error {
+	return v.Unmarshal(rawVal, opts...)
+}
+func (v *Viper) Unmarshal(rawVal interface{}, opts ...DecoderConfigOption) error {
+	err := decode(v.AllSettings(), defaultDecoderConfig(rawVal, opts...))
 
 	if err != nil {
 		return err
@@ -747,14 +797,21 @@ func (v *Viper) Unmarshal(rawVal interface{}) error {
 }
 
 // defaultDecoderConfig returns default mapsstructure.DecoderConfig with suppot
-// of time.Duration values
-func defaultDecoderConfig(output interface{}) *mapstructure.DecoderConfig {
-	return &mapstructure.DecoderConfig{
+// of time.Duration values & string slices
+func defaultDecoderConfig(output interface{}, opts ...DecoderConfigOption) *mapstructure.DecoderConfig {
+	c := &mapstructure.DecoderConfig{
 		Metadata:         nil,
 		Result:           output,
 		WeaklyTypedInput: true,
-		DecodeHook:       mapstructure.StringToTimeDurationHookFunc(),
+		DecodeHook: mapstructure.ComposeDecodeHookFunc(
+			mapstructure.StringToTimeDurationHookFunc(),
+			mapstructure.StringToSliceHookFunc(","),
+		),
 	}
+	for _, opt := range opts {
+		opt(c)
+	}
+	return c
 }
 
 // A wrapper around mapstructure.Decode that mimics the WeakDecode functionality
@@ -1116,6 +1173,7 @@ func (v *Viper) ReadInConfig() error {
 		return UnsupportedConfigError(v.getConfigType())
 	}
 
+	jww.DEBUG.Println("Reading file: ", filename)
 	file, err := afero.ReadFile(v.fs, filename)
 	if err != nil {
 		return err
@@ -1175,6 +1233,195 @@ func (v *Viper) MergeConfig(in io.Reader) error {
 	return nil
 }
 
+// WriteConfig writes the current configuration to a file.
+func WriteConfig() error { return v.WriteConfig() }
+func (v *Viper) WriteConfig() error {
+	filename, err := v.getConfigFile()
+	if err != nil {
+		return err
+	}
+	return v.writeConfig(filename, true)
+}
+
+// SafeWriteConfig writes current configuration to file only if the file does not exist.
+func SafeWriteConfig() error { return v.SafeWriteConfig() }
+func (v *Viper) SafeWriteConfig() error {
+	filename, err := v.getConfigFile()
+	if err != nil {
+		return err
+	}
+	return v.writeConfig(filename, false)
+}
+
+// WriteConfigAs writes current configuration to a given filename.
+func WriteConfigAs(filename string) error { return v.WriteConfigAs(filename) }
+func (v *Viper) WriteConfigAs(filename string) error {
+	return v.writeConfig(filename, true)
+}
+
+// SafeWriteConfigAs writes current configuration to a given filename if it does not exist.
+func SafeWriteConfigAs(filename string) error { return v.SafeWriteConfigAs(filename) }
+func (v *Viper) SafeWriteConfigAs(filename string) error {
+	return v.writeConfig(filename, false)
+}
+
+func writeConfig(filename string, force bool) error { return v.writeConfig(filename, force) }
+func (v *Viper) writeConfig(filename string, force bool) error {
+	jww.INFO.Println("Attempting to write configuration to file.")
+	ext := filepath.Ext(filename)
+	if len(ext) <= 1 {
+		return fmt.Errorf("Filename: %s requires valid extension.", filename)
+	}
+	configType := ext[1:]
+	if !stringInSlice(configType, SupportedExts) {
+		return UnsupportedConfigError(configType)
+	}
+	if v.config == nil {
+		v.config = make(map[string]interface{})
+	}
+	var flags int
+	if force == true {
+		flags = os.O_CREATE | os.O_TRUNC | os.O_WRONLY
+	} else {
+		if _, err := os.Stat(filename); os.IsNotExist(err) {
+			flags = os.O_WRONLY
+		} else {
+			return fmt.Errorf("File: %s exists. Use WriteConfig to overwrite.", filename)
+		}
+	}
+	f, err := v.fs.OpenFile(filename, flags, os.FileMode(0644))
+	if err != nil {
+		return err
+	}
+	return v.marshalWriter(f, configType)
+}
+
+// Unmarshal a Reader into a map.
+// Should probably be an unexported function.
+func unmarshalReader(in io.Reader, c map[string]interface{}) error {
+	return v.unmarshalReader(in, c)
+}
+func (v *Viper) unmarshalReader(in io.Reader, c map[string]interface{}) error {
+	buf := new(bytes.Buffer)
+	buf.ReadFrom(in)
+
+	switch strings.ToLower(v.getConfigType()) {
+	case "yaml", "yml":
+		if err := yaml.Unmarshal(buf.Bytes(), &c); err != nil {
+			return ConfigParseError{err}
+		}
+
+	case "json":
+		if err := json.Unmarshal(buf.Bytes(), &c); err != nil {
+			return ConfigParseError{err}
+		}
+
+	case "hcl":
+		obj, err := hcl.Parse(string(buf.Bytes()))
+		if err != nil {
+			return ConfigParseError{err}
+		}
+		if err = hcl.DecodeObject(&c, obj); err != nil {
+			return ConfigParseError{err}
+		}
+
+	case "toml":
+		tree, err := toml.LoadReader(buf)
+		if err != nil {
+			return ConfigParseError{err}
+		}
+		tmap := tree.ToMap()
+		for k, v := range tmap {
+			c[k] = v
+		}
+
+	case "properties", "props", "prop":
+		v.properties = properties.NewProperties()
+		var err error
+		if v.properties, err = properties.Load(buf.Bytes(), properties.UTF8); err != nil {
+			return ConfigParseError{err}
+		}
+		for _, key := range v.properties.Keys() {
+			value, _ := v.properties.Get(key)
+			// recursively build nested maps
+			path := strings.Split(key, ".")
+			lastKey := strings.ToLower(path[len(path)-1])
+			deepestMap := deepSearch(c, path[0:len(path)-1])
+			// set innermost value
+			deepestMap[lastKey] = value
+		}
+	}
+
+	insensitiviseMap(c)
+	return nil
+}
+
+// Marshal a map into Writer.
+func marshalWriter(f afero.File, configType string) error {
+	return v.marshalWriter(f, configType)
+}
+func (v *Viper) marshalWriter(f afero.File, configType string) error {
+	c := v.AllSettings()
+	switch configType {
+	case "json":
+		b, err := json.MarshalIndent(c, "", "  ")
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+		_, err = f.WriteString(string(b))
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+
+	case "hcl":
+		b, err := json.Marshal(c)
+		ast, err := hcl.Parse(string(b))
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+		err = printer.Fprint(f, ast.Node)
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+
+	case "prop", "props", "properties":
+		if v.properties == nil {
+			v.properties = properties.NewProperties()
+		}
+		p := v.properties
+		for _, key := range v.AllKeys() {
+			_, _, err := p.Set(key, v.GetString(key))
+			if err != nil {
+				return ConfigMarshalError{err}
+			}
+		}
+		_, err := p.WriteComment(f, "#", properties.UTF8)
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+
+	case "toml":
+		t, err := toml.TreeFromMap(c)
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+		s := t.String()
+		if _, err := f.WriteString(s); err != nil {
+			return ConfigMarshalError{err}
+		}
+
+	case "yaml", "yml":
+		b, err := yaml.Marshal(c)
+		if err != nil {
+			return ConfigMarshalError{err}
+		}
+		if _, err = f.WriteString(string(b)); err != nil {
+			return ConfigMarshalError{err}
+		}
+	}
+	return nil
+}
+
 func keyExists(k string, m map[string]interface{}) string {
 	lk := strings.ToLower(k)
 	for mk := range m {
@@ -1287,16 +1534,6 @@ func (v *Viper) WatchRemoteConfigOnChannel() error {
 	return v.watchKeyValueConfigOnChannel()
 }
 
-// Unmarshal a Reader into a map.
-// Should probably be an unexported function.
-func unmarshalReader(in io.Reader, c map[string]interface{}) error {
-	return v.unmarshalReader(in, c)
-}
-
-func (v *Viper) unmarshalReader(in io.Reader, c map[string]interface{}) error {
-	return unmarshallConfigReader(in, c, v.getConfigType())
-}
-
 func (v *Viper) insensitiviseMaps() {
 	insensitiviseMap(v.config)
 	insensitiviseMap(v.defaults)
@@ -1516,25 +1753,21 @@ func (v *Viper) getConfigType() string {
 }
 
 func (v *Viper) getConfigFile() (string, error) {
-	// if explicitly set, then use it
-	if v.configFile != "" {
-		return v.configFile, nil
+	if v.configFile == "" {
+		cf, err := v.findConfigFile()
+		if err != nil {
+			return "", err
+		}
+		v.configFile = cf
 	}
-
-	cf, err := v.findConfigFile()
-	if err != nil {
-		return "", err
-	}
-
-	v.configFile = cf
-	return v.getConfigFile()
+	return v.configFile, nil
 }
 
 func (v *Viper) searchInPath(in string) (filename string) {
 	jww.DEBUG.Println("Searching for config in ", in)
 	for _, ext := range SupportedExts {
 		jww.DEBUG.Println("Checking for", filepath.Join(in, v.configName+"."+ext))
-		if b, _ := exists(filepath.Join(in, v.configName+"."+ext)); b {
+		if b, _ := exists(v.fs, filepath.Join(in, v.configName+"."+ext)); b {
 			jww.DEBUG.Println("Found: ", filepath.Join(in, v.configName+"."+ext))
 			return filepath.Join(in, v.configName+"."+ext)
 		}

viper_test.go

@@ -7,6 +7,7 @@ package viper
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -18,6 +19,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/mitchellh/mapstructure"
+	"github.com/spf13/afero"
 	"github.com/spf13/cast"
 
 	"github.com/spf13/pflag"
@@ -262,7 +265,7 @@ func TestDefault(t *testing.T) {
 	assert.Equal(t, "leather", Get("clothing.jacket"))
 }
 
-func TestUnmarshalling(t *testing.T) {
+func TestUnmarshaling(t *testing.T) {
 	SetConfigType("yaml")
 	r := bytes.NewReader(yamlExample)
 
@@ -417,7 +420,7 @@ func TestAutoEnvWithPrefix(t *testing.T) {
 	assert.Equal(t, "13", Get("bar"))
 }
 
-func TestSetEnvReplacer(t *testing.T) {
+func TestSetEnvKeyReplacer(t *testing.T) {
 	Reset()
 
 	AutomaticEnv()
@@ -502,6 +505,42 @@ func TestUnmarshal(t *testing.T) {
 	assert.Equal(t, &config{Name: "Steve", Port: 1234, Duration: time.Second + time.Millisecond}, &C)
 }
 
+func TestUnmarshalWithDecoderOptions(t *testing.T) {
+	Set("credentials", "{\"foo\":\"bar\"}")
+
+	opt := DecodeHook(mapstructure.ComposeDecodeHookFunc(
+		mapstructure.StringToTimeDurationHookFunc(),
+		mapstructure.StringToSliceHookFunc(","),
+		// Custom Decode Hook Function
+		func(rf reflect.Kind, rt reflect.Kind, data interface{}) (interface{}, error) {
+			if rf != reflect.String || rt != reflect.Map {
+				return data, nil
+			}
+			m := map[string]string{}
+			raw := data.(string)
+			if raw == "" {
+				return m, nil
+			}
+			return m, json.Unmarshal([]byte(raw), &m)
+		},
+	))
+
+	type config struct {
+		Credentials map[string]string
+	}
+
+	var C config
+
+	err := Unmarshal(&C, opt)
+	if err != nil {
+		t.Fatalf("unable to decode into struct, %v", err)
+	}
+
+	assert.Equal(t, &config{
+		Credentials: map[string]string{"foo": "bar"},
+	}, &C)
+}
+
 func TestBindPFlags(t *testing.T) {
 	v := New() // create independent Viper object
 	flagSet := pflag.NewFlagSet("test", pflag.ContinueOnError)
@@ -847,6 +886,190 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, (*Viper)(nil), subv)
 }
 
+var hclWriteExpected = []byte(`"foos" = {
+  "foo" = {
+    "key" = 1
+  }
+
+  "foo" = {
+    "key" = 2
+  }
+
+  "foo" = {
+    "key" = 3
+  }
+
+  "foo" = {
+    "key" = 4
+  }
+}
+
+"id" = "0001"
+
+"name" = "Cake"
+
+"ppu" = 0.55
+
+"type" = "donut"`)
+
+func TestWriteConfigHCL(t *testing.T) {
+	v := New()
+	fs := afero.NewMemMapFs()
+	v.SetFs(fs)
+	v.SetConfigName("c")
+	v.SetConfigType("hcl")
+	err := v.ReadConfig(bytes.NewBuffer(hclExample))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := v.WriteConfigAs("c.hcl"); err != nil {
+		t.Fatal(err)
+	}
+	read, err := afero.ReadFile(fs, "c.hcl")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, hclWriteExpected, read)
+}
+
+var jsonWriteExpected = []byte(`{
+  "batters": {
+    "batter": [
+      {
+        "type": "Regular"
+      },
+      {
+        "type": "Chocolate"
+      },
+      {
+        "type": "Blueberry"
+      },
+      {
+        "type": "Devil's Food"
+      }
+    ]
+  },
+  "id": "0001",
+  "name": "Cake",
+  "ppu": 0.55,
+  "type": "donut"
+}`)
+
+func TestWriteConfigJson(t *testing.T) {
+	v := New()
+	fs := afero.NewMemMapFs()
+	v.SetFs(fs)
+	v.SetConfigName("c")
+	v.SetConfigType("json")
+	err := v.ReadConfig(bytes.NewBuffer(jsonExample))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := v.WriteConfigAs("c.json"); err != nil {
+		t.Fatal(err)
+	}
+	read, err := afero.ReadFile(fs, "c.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, jsonWriteExpected, read)
+}
+
+var propertiesWriteExpected = []byte(`p_id = 0001
+p_type = donut
+p_name = Cake
+p_ppu = 0.55
+p_batters.batter.type = Regular
+`)
+
+func TestWriteConfigProperties(t *testing.T) {
+	v := New()
+	fs := afero.NewMemMapFs()
+	v.SetFs(fs)
+	v.SetConfigName("c")
+	v.SetConfigType("properties")
+	err := v.ReadConfig(bytes.NewBuffer(propertiesExample))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := v.WriteConfigAs("c.properties"); err != nil {
+		t.Fatal(err)
+	}
+	read, err := afero.ReadFile(fs, "c.properties")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, propertiesWriteExpected, read)
+}
+
+func TestWriteConfigTOML(t *testing.T) {
+	fs := afero.NewMemMapFs()
+	v := New()
+	v.SetFs(fs)
+	v.SetConfigName("c")
+	v.SetConfigType("toml")
+	err := v.ReadConfig(bytes.NewBuffer(tomlExample))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := v.WriteConfigAs("c.toml"); err != nil {
+		t.Fatal(err)
+	}
+
+	// The TOML String method does not order the contents.
+	// Therefore, we must read the generated file and compare the data.
+	v2 := New()
+	v2.SetFs(fs)
+	v2.SetConfigName("c")
+	v2.SetConfigType("toml")
+	v2.SetConfigFile("c.toml")
+	err = v2.ReadInConfig()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assert.Equal(t, v.GetString("title"), v2.GetString("title"))
+	assert.Equal(t, v.GetString("owner.bio"), v2.GetString("owner.bio"))
+	assert.Equal(t, v.GetString("owner.dob"), v2.GetString("owner.dob"))
+	assert.Equal(t, v.GetString("owner.organization"), v2.GetString("owner.organization"))
+}
+
+var yamlWriteExpected = []byte(`age: 35
+beard: true
+clothing:
+  jacket: leather
+  pants:
+    size: large
+  trousers: denim
+eyes: brown
+hacker: true
+hobbies:
+- skateboarding
+- snowboarding
+- go
+name: steve
+`)
+
+func TestWriteConfigYAML(t *testing.T) {
+	v := New()
+	fs := afero.NewMemMapFs()
+	v.SetFs(fs)
+	v.SetConfigName("c")
+	v.SetConfigType("yaml")
+	err := v.ReadConfig(bytes.NewBuffer(yamlExample))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := v.WriteConfigAs("c.yaml"); err != nil {
+		t.Fatal(err)
+	}
+	read, err := afero.ReadFile(fs, "c.yaml")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, yamlWriteExpected, read)
+}
+
 var yamlMergeExampleTgt = []byte(`
 hello:
     pop: 37890
@@ -883,6 +1106,10 @@ func TestMergeConfig(t *testing.T) {
 		t.Fatalf("lagrenum != 765432101234567, = %d", pop)
 	}
 
+	if pop := v.GetInt32("hello.pop"); pop != int32(37890) {
+		t.Fatalf("pop != 37890, = %d", pop)
+	}
+
 	if pop := v.GetInt64("hello.lagrenum"); pop != int64(765432101234567) {
 		t.Fatalf("int64 lagrenum != 765432101234567, = %d", pop)
 	}
@@ -907,6 +1134,10 @@ func TestMergeConfig(t *testing.T) {
 		t.Fatalf("lagrenum != 7654321001234567, = %d", pop)
 	}
 
+	if pop := v.GetInt32("hello.pop"); pop != int32(45000) {
+		t.Fatalf("pop != 45000, = %d", pop)
+	}
+
 	if pop := v.GetInt64("hello.lagrenum"); pop != int64(7654321001234567) {
 		t.Fatalf("int64 lagrenum != 7654321001234567, = %d", pop)
 	}