具有返回值的go函数

Question

I saw this piece of code and trying to understand this line

func (ml *MarketList) UnmarshalJSON(b []byte) error 

In the above line, I believe the function name is UnmarshallJSON that takes a byte array as input and return error as output. What does

func (ml *MarketList)

I know * as a dereferenced operator but don't understand what is the context here. If someone can explain that would be great.

package main

import (
    "encoding/json"
    "fmt"
)

var jsonBytes = []byte(`
  {"MS": {
     "last":"25",
  "highestBid":"20"},
  "GE": {
      "last": "24",
      "highestBid": "22"}
  }`)

type Market struct {
    MarketName string
    Last       string
    HighestBid string
}

type MarketList []Market

func (ml *MarketList) UnmarshalJSON(b []byte) error {
    tmp := map[string]Market{}
    err := json.Unmarshal(b, &tmp)
    if err != nil {
        return err
    }

    var l MarketList
    for k, v := range tmp {
        v.MarketName = k
        l = append(l, v)
    }

    *ml = l
    return nil
}

func main() {
    ml := MarketList{}
    // Unmarshal directly into a []Market alias
    _ = json.Unmarshal(jsonBytes, &ml)

    fmt.Printf("%+v
", ml)
}

Answer 1

This is the way how you define a pointer Type. For example:

var a string 
var b *string // pointer to a string
b = &a

In your case *MarketList is the type pointer to a MarketList.

At all I would recommend you to make the tour of Go these things are explained there.