Unmarshal hcl to struct using viper - go

Question

Trying to Unmarshal a hcl config file to a struct, using viper, this error is returned: 1 error(s) decoding:\n\n* 'NATS' expected a map, got 'slice'. What is missing?
The code:
func lab() {
var c conf
// config file
viper.SetConfigName("draft")
viper.AddConfigPath(".")
viper.SetConfigType("hcl")
if err := viper.ReadInConfig(); err != nil {
log.Error(err)
return
}
log.Info(viper.Get("NATS")) // gives [map[port:10041 username:cl1 password:__Psw__4433__ http_port:10044]]
if err := viper.Unmarshal(&c); err != nil {
log.Error(err)
return
}
log.Infow("got conf", "conf", c)
}
type conf struct {
NATS struct {
HTTPPort int
Port int
Username string
Password string
}
}
And the config file (draft.hcl inside current directory):
NATS {
HTTPPort = 10044
Port = 10041
Username = "cl1"
Password = "__Psw__4433__"
}
Edit
Have checked this struct with hcl package and it gets marshaled/unmarshalled correctly. Also this works correctly with yaml and viper.
There is a difference between these two where log.Info(viper.Get("NATS")) is called. While the hcl version returns a slice of maps, the yaml version returns a map: map[password:__psw__4433__ httpport:10044 port:10041 username:cl1].

Answer 1

Your conf struct is not matching the HCL. When converted to json the HCL looks like below
{
"NATS": [
{
"HTTPPort": 10044,
"Password": "__Psw__4433__",
"Port": 10041,
"Username": "cl1"
}
]
}
So the Conf Struct should look like this
type Conf struct {
NATS []struct{
HTTPPort int
Port int
Username string
Password string
}
}
Modified code
package main
import (
"log"
"github.com/spf13/viper"
"fmt"
)
type Conf struct {
NATS []struct{
HTTPPort int
Port int
Username string
Password string
}
}
func main() {
var c Conf
// config file
viper.SetConfigName("draft")
viper.AddConfigPath(".")
viper.SetConfigType("hcl")
if err := viper.ReadInConfig(); err != nil {
log.Fatal(err)
}
fmt.Println(viper.Get("NATS")) // gives [map[port:10041 username:cl1 password:__Psw__4433__ http_port:10044]]
if err := viper.Unmarshal(&c); err != nil {
log.Fatal(err)
}
fmt.Println(c.NATS[0].Username)
}

Answer 2

I know this question is more than two years old now, but I came across the same issue recently.
I'm using viper to be able to load different configuration files into a Go struct, allowing configuration in JSON, YAML, TOML, HCL, just pick your favourite :)
HCL file format does wrap a map into a slice because it allows redefining a section like:
section = {
key1 = "value"
}
section = {
key2 = "value"
}
which is something that is not supported by the other formats.
And here's how I fixed it:
My solution implies each new block will override any previous definition of the same key, and keep all the others. You can do some merging magic but I didn't need to.
You need to make a hook to convert a slice of maps into a map:
// sliceOfMapsToMapHookFunc merges a slice of maps to a map
func sliceOfMapsToMapHookFunc() mapstructure.DecodeHookFunc {
return func(from reflect.Type, to reflect.Type, data interface{}) (interface{}, error) {
if from.Kind() == reflect.Slice && from.Elem().Kind() == reflect.Map && (to.Kind() == reflect.Struct || to.Kind() == reflect.Map) {
source, ok := data.([]map[string]interface{})
if !ok {
return data, nil
}
if len(source) == 0 {
return data, nil
}
if len(source) == 1 {
return source[0], nil
}
// flatten the slice into one map
convert := make(map[string]interface{})
for _, mapItem := range source {
for key, value := range mapItem {
convert[key] = value
}
}
return convert, nil
}
return data, nil
}
}
then you need to create a DecodeHook:
configOption := viper.DecodeHook(mapstructure.ComposeDecodeHookFunc(
sliceOfMapsToMapHookFunc(),
mapstructure.StringToTimeDurationHookFunc(),
mapstructure.StringToSliceHookFunc(","),
))
the two other hooks are the default ones so you might want to keep them
then you pass the option to the Unmarshal method
viper.Unmarshal(&c, configOption)
With this method you don't need a slice around your structs or your maps. Also that makes it compatible with the other configuration file formats