Unable to write a generic function that can work on multiple Structs in Golang

Question

I am new to Go, and learning Interfaces and Structs. I am having 2 structs - ServiceSection and SliderSection and I am trying to accomplish the below 2 tasks with each of them-

1. GET the JSON response and unmarshal it.
2. Use the struct to create HTML using "html/template"

Therefore, I am trying to create a common function to perform both the tasks that can work for multiple Structs. The plan is to create 5-6 more such structs. Below is the code I have created-

package main
import (
            "bytes"
            "encoding/json"
            "fmt"
            "io/ioutil"
            "log"
            "net/http"
            "html/template"
        )
        
        const (
            getServicesAPI = "https://huyk1a44n6.execute-api.us-east-1.amazonaws.com/v1/services"
            getSliderAPI   = "https://huyk1a44n6.execute-api.us-east-1.amazonaws.com/v1/home-slider"
        
            servicesTemplate = `<div class="container">
                <div class="row">
                    <div class="col-md-12">
                        <div class="section-title">
                            <h5>{{.Heading1}}</h5>
                            <h2>{{.Heading2}}</h2>
                        </div>
                    </div>
                </div>
                <div class="row">
                  
                   {{range .Services}}
                    <div class="col-lg-4 col-md-6">
                        <a class="single-services-box" href="{{.Href}}">
                            <div class="services-img">
                                <img alt="Service Images"
                                     src="{{.Image}}">
                            </div>
                            <h3>{{.Heading}}</h3>
                        </a>
                    </div>
                    {{end}}
        
                </div>
            </div>`
        
            sliderTemplate = `    {{range .Slider}}
            <div class="home-slider flickity-dots-absolute" data-flickity='{ "bgLazyLoad": 1, "bgLazyLoad": true, "fade": true, "prevNextButtons": false, "autoPlay": 7000, "pauseAutoPlayOnHover": false }'>
                <div class="home-slider-single-item" data-flickity-bg-lazyload="{{.Image}}">
                    <div class="container">
                        <div class="row d-flex align-items-center">
                            <div class="col">
                                <div class="home-slider-content">
                                    <h1 class="home-slider-title">{{.title}}</h1>
                                    <div class="home-slider-description">
                                        <p>{{.description}}</p>
                                    </div>
                                    <div class="home-slider-btn-box">
                                        <a class="button home-btn-1 mr-15" href="{{.button1_href}}">{{.button1_title}}</a>
                                        <a class="button btn-primary" href="{{.button2_href}}">{{.button1_title}}</a>
                                    </div>
                                </div>
                            </div>
                        </div>
                    </div>
                </div>
            </div>
            {{end}}
        `
        )
        
        type ServiceSection struct {
            Heading1 string `json:"Heading1"`
            Heading2 string `json:"Heading2"`
            Services []struct {
                Image   string `json:"Image"`
                Href    string `json:"Href"`
                Heading string `json:"Heading"`
            } `json:"Services"`
        }
        
        type SliderStruct struct {
            Title        string `json:"title"`
            Description  string `json:"description"`
            Button1Title string `json:"button1_title"`
            Button1Href  string `json:"button1_href"`
            Button2Title string `json:"button2_title"`
            Button2Href  string `json:"button2_href"`
            Image        string `json:"Image"`
        }
        
        type SliderSection struct {
            Slider []SliderStruct
        }
        
        type MyInterface interface {
            populateHTML(string, string)
        }
        
        func (ss ServiceSection) populateHTML(endpoint string, tmpl string) {
            populateHTMLcommon(ss, endpoint, tmpl)
        }
        func (ss SliderSection) populateHTML(endpoint string, tmpl string) {
            populateHTMLcommon(ss, endpoint, tmpl)
        }
        
        func main() {
            println("WASM Go Initialized")
        
            ServiceSec := ServiceSection{}
            SliderSec := SliderSection{}
        
            ServiceSec.populateHTML(getServicesAPI, servicesTemplate)
            SliderSec.populateHTML(getSliderAPI, sliderTemplate)
        
        }
        
        func populateHTMLcommon(hs MyInterface, endpoint string, tmplStr string) {
            fmt.Println("struct ServiceSection", hs, endpoint)
        
            // GET API CALL
            fmt.Println("Inside getDataFromAPI()")
        
            response, err := http.Get(endpoint)
            if err != nil {
                log.Fatal(err)
            }
        
            responseData, err := ioutil.ReadAll(response.Body)
            if err != nil {
                log.Fatal(err)
            }
        
            err = json.Unmarshal(responseData, &hs)
            if err != nil {
                log.Fatal(err)
            }
        
            // Create HTML using template
            tmpl := template.Must(template.New("table").Parse(tmplStr))
        
            buf := new(bytes.Buffer)              // Buffer created to hold the final HTML
            err = tmpl.Execute(buf, responseData) // Populate the data in the HTML template
            if err != nil {
                log.Fatal(err)
            }
        
            tableHTMLString := buf.String()
            fmt.Println("tableHTMLString: ", tableHTMLString)
        }

On executing the above program I am getting the below error in the populateHTMLcommon() function while unmarshaling the JSON -

json: cannot unmarshal object into Go value of type main.MyInterface

This means it's not able to identify the appropriate struct from the MyInterface interface.

I don't understand how I can create a common function that could work for multiple structs. Any help is appreciated.

Answer 1

A couple of things:

• A Go interface is an abstraction - so it's rare, if ever, that you need to get the address of an interface
• if a struct's method needs to change the state of the struct (and persist the change), use a pointer receiver.

So to fix your immediate problems:

// err = json.Unmarshal(responseData, &hs) // address of an interface usually is not what you want
err = json.Unmarshal(responseData, hs)

and update your method signatures to use pointer-receivers:

func (ss *ServiceSection) populateHTML(endpoint string, tmpl string) {
    populateHTMLcommon(ss, endpoint, tmpl)
}
func (ss *SliderSection) populateHTML(endpoint string, tmpl string) {
    populateHTMLcommon(ss, endpoint, tmpl)
}

https://play.golang.org/p/cGmm3Cs5XTk