Trying to Unmarshal a map[Priority]string

I have been playing around with the Golang json package, and been running into a issue, i am simply trying to unmarshal a map[string]string into map[Priority]string the easiest way, but it seems like my UnmarshalJSON isn't even running, the following is my code, i would've like to made a playground where it could run, but i am using the OS package so:

import (
"encoding/json"
"fmt"
"os"
)
type Priority int
const (
VERYLOW = Priority(iota)
LOW
MEDIUM
HIGH
VERYHIGH
)
const (
verylow  = "verylow"
low      = "low"
medium   = "medium"
high     = "high"
veryhigh = "veryhigh"
// ANSI color constants.
RESET      = "\x1b[0m"
BRIGHT     = "\x1b[1m"
DIM        = "\x1b[2m"
UNDERSCORE = "\x1b[4m"
BLINK      = "\x1b[5m"
REVERSE    = "\x1b[7m"
HIDDEN     = "\x1b[8m"
FGBLACK    = "\x1b[30m"
FGRED      = "\x1b[31m"
FGGREEN    = "\x1b[32m"
FGYELLOW   = "\x1b[33m"
FGBLUE     = "\x1b[34m"
FGMAGENTA  = "\x1b[35m"
FGCYAN     = "\x1b[36m"
FGWHITE    = "\x1b[37m"
BGBLACK    = "\x1b[40m"
BGRED      = "\x1b[41m"
BGGREEN    = "\x1b[42m"
BGYELLOW   = "\x1b[43m"
BGBLUE     = "\x1b[44m"
BGMAGENTA  = "\x1b[45m"
BGCYAN     = "\x1b[46m"
BGWHITE    = "\x1b[47m"
TITLE_COLOUR = BRIGHT + FGGREEN
NUMBER_COLOR = FGGREEN
//color constants
BLACK         = "BLACK"
RED           = "RED"
GREEN         = "GREEN"
YELLOW        = "YELLOW"
BLUE          = "BLUE"
MAGENTA       = "MAGENTA"
CYAN          = "CYAN"
WHITE         = "WHITE"
BRIGHTBLACK   = "BRIGHTBLACK"
BRIGHTRED     = "BRIGHTRED"
BRIGHTGREEN   = "BRIGHTGREEN"
BRIGHTYELLOW  = "BRIGHTYELLOW"
BRIGHTBLUE    = "BRIGHTBLUE"
BRIGHTMAGENTA = "BRIGHTMAGENTA"
BRIGHTCYAN    = "BRIGHTCYAN"
BRIGHTWHITE   = "BRIGHTWHITE"
NOCOLOR       = ""
)
type Config struct {
Name     string
FGColors map[Priority]string
}
type MarshalableConfig struct {
Name     string
FGColors map[string]string
}
var priorityMapFromString = map[string]Priority{
veryhigh: VERYHIGH,
high:     HIGH,
medium:   MEDIUM,
low:      LOW,
verylow:  VERYLOW,
}
var priorityToString = map[Priority]string{
VERYHIGH: veryhigh,
HIGH:     high,
MEDIUM:   medium,
VERYLOW:  verylow,
LOW:      low,
}
func (p Priority) String() string {
return priorityToString

}
func PriorityFromString(priority string) Priority {
if p, ok := priorityMapFromString[priority]; ok {
return p
}
return MEDIUM
}
func (priority *Priority) UnmarshalJSON(data []byte) error {
var s string
if err := json.Unmarshal(data, &s); err != nil {
return err
}
*priority = PriorityFromString(s)
return nil
}
func main() {
fgColors := map[string]string{
verylow:  RED,
low:      BLUE,
medium:   GREEN,
high:     BRIGHTBLUE,
veryhigh: BRIGHTMAGENTA,
}
m := MarshalableConfig{"Alice", fgColors}
b, err := json.Marshal(m)
if err != nil {
panic(err)
}
file, err := os.Create("testfile.json")
if err != nil {
panic(err)
}
file.Write(b)
emptyConfig := Config{}
err = json.Unmarshal(b, &emptyConfig)
if err != nil {
fmt.Printf("Got a error from unmarshalling: %+v \n", err)
panic(err)
}
fmt.Printf("%+v \n", emptyConfig)
var x interface{} = emptyConfig.FGColors[VERYHIGH]
fmt.Println(x.(string))
}

I get the error:

json: cannot unmarshal number high into Go value of type main.Priority  

So the impression i get is that it never calls my UnmarshalJSON method on the priority, and i don't know if thats the way its suppose to work anyway now that i have nested the Priority type inside of the map.

So i thouth maybe i have to do a alias type for the whole map like:

type FGColor map[Priority]string 

and implement unmarshal on that instead, but that doesn't seem logical, because nested structs would be a nightmare to unmarshal i would think.

What am i doing wrong??

As AniSkywalker and putu have mentioned in the comments, you could impliment encoding.TextUnmarshaler.

However I think in your case since you need to use Priority as a json map key, it's much easier to just use Priority as a type string.

type Priority string
const (
VERYLOW = "verylow"
LOW = "low"
MEDIUM = "medium"
HIGH = "high"
VERYHIGH = "veryhigh" )

This way you can discard the use of the following as well.

const (
verylow  = "verylow"
low      = "low"
medium   = "medium"
high     = "high"
veryhigh = "veryhigh"
)

I updated my code, as per the comment from @AniSkywalker to the following code, using the UnmarshalText:

package main
import (
"encoding/json"
"fmt"
)
type Priority int
const (
VERYLOW = Priority(iota)
LOW
MEDIUM
HIGH
VERYHIGH
)
const (
verylow  = "verylow"
low      = "low"
medium   = "medium"
high     = "high"
veryhigh = "veryhigh"
// ANSI color constants.
RESET      = "\x1b[0m"
BRIGHT     = "\x1b[1m"
DIM        = "\x1b[2m"
UNDERSCORE = "\x1b[4m"
BLINK      = "\x1b[5m"
REVERSE    = "\x1b[7m"
HIDDEN     = "\x1b[8m"
FGBLACK    = "\x1b[30m"
FGRED      = "\x1b[31m"
FGGREEN    = "\x1b[32m"
FGYELLOW   = "\x1b[33m"
FGBLUE     = "\x1b[34m"
FGMAGENTA  = "\x1b[35m"
FGCYAN     = "\x1b[36m"
FGWHITE    = "\x1b[37m"
BGBLACK    = "\x1b[40m"
BGRED      = "\x1b[41m"
BGGREEN    = "\x1b[42m"
BGYELLOW   = "\x1b[43m"
BGBLUE     = "\x1b[44m"
BGMAGENTA  = "\x1b[45m"
BGCYAN     = "\x1b[46m"
BGWHITE    = "\x1b[47m"
TITLE_COLOUR = BRIGHT + FGGREEN
NUMBER_COLOR = FGGREEN
//color constants
BLACK         = "BLACK"
RED           = "RED"
GREEN         = "GREEN"
YELLOW        = "YELLOW"
BLUE          = "BLUE"
MAGENTA       = "MAGENTA"
CYAN          = "CYAN"
WHITE         = "WHITE"
BRIGHTBLACK   = "BRIGHTBLACK"
BRIGHTRED     = "BRIGHTRED"
BRIGHTGREEN   = "BRIGHTGREEN"
BRIGHTYELLOW  = "BRIGHTYELLOW"
BRIGHTBLUE    = "BRIGHTBLUE"
BRIGHTMAGENTA = "BRIGHTMAGENTA"
BRIGHTCYAN    = "BRIGHTCYAN"
BRIGHTWHITE   = "BRIGHTWHITE"
NOCOLOR       = ""
)
type Config struct {
Name     string
FGColors map[Priority]string
}
type MarshalableConfig struct {
Name     string
FGColors map[string]string
}
var priorityMapFromString = map[string]Priority{
veryhigh: VERYHIGH,
high:     HIGH,
medium:   MEDIUM,
low:      LOW,
verylow:  VERYLOW,
}
var priorityToString = map[Priority]string{
VERYHIGH: veryhigh,
HIGH:     high,
MEDIUM:   medium,
VERYLOW:  verylow,
LOW:      low,
}
func (p Priority) String() string {
return priorityToString

}
func PriorityFromString(priority string) Priority {
if p, ok := priorityMapFromString[priority]; ok {
return p
}
return MEDIUM
}
func (priority *Priority) UnmarshalText(data []byte) error {
*priority = PriorityFromString(string(data))
return nil
}
func main() {
fgColors := map[string]string{
verylow:  RED,
low:      BLUE,
medium:   GREEN,
high:     BRIGHTBLUE,
veryhigh: BRIGHTMAGENTA,
}
m := MarshalableConfig{"Alice", fgColors}
b, err := json.Marshal(m)
if err != nil {
panic(err)
}
emptyConfig := Config{}
err = json.Unmarshal(b, &emptyConfig)
if err != nil {
fmt.Printf("Got a error from unmarshalling: %+v \n", err)
panic(err)
}
fmt.Printf("%+v \n", emptyConfig)
var x interface{} = emptyConfig.FGColors[VERYHIGH]
fmt.Println(x.(string))
}