Golang Pointer Understanding - go

Question

I am learning go and i need to understand something. I am getting few errors. I have created a Product struct and attached a func with it. I also got a product lists as a slice. Actually I am following one example. I was just trying add different endpoints to it.
I have added question in comment in code. Please explain. I need to return the json single object as a response to user. Please guide me.
package data
type Product struct {
ID int `json:"id"`
Name string `json:"name"`
Description string `json:"description"`
Price float32 `json:"price"`
SKU string `json:"sku"`
CreatedOn string `json:"-"`
UpdatedOn string `json:"-"`
DeletedOn string `json:"-"`
}
type Products []*Product
func (p *Products) ToJSON(w io.Writer) error {
e := json.NewEncoder(w)
return e.Encode(p)
}
func (p *Product) FromJSON(r io.Reader) error {
d := json.NewDecoder(r)
return d.Decode(p)
}
var ErrProductNotFound = fmt.Errorf("Product not found")
func GetProduct(id int) (*Product, error) { // this is returning *Product & err. When I use this in GetProduct in handler func it is giving error
for _, p := range productList {
if p.ID == id {
fmt.Println(p)
return p, nil
}
}
return nil, ErrProductNotFound
}
var productList = []*Product{ **// Why in example the teacher doing it like this.** []*Product{&Product{}, &Product{}} **what it the reason? Please explain.
&Product{ // this gives warning : redundant type from array, slice, or map composite literal. need to understand why**
ID: 1,
Name: "Latte",
Description: "chai",
Price: 2.45,
SKU: "abc123",
CreatedOn: time.Now().UTC().String(),
UpdatedOn: time.Now().UTC().String(),
},
&Product{
ID: 2,
Name: "Tea",
Description: "chai",
Price: 1.45,
SKU: "abc1234",
CreatedOn: time.Now().UTC().String(),
UpdatedOn: time.Now().UTC().String(),
},
}
package handlers
func (p *Product) GetProduct(rw http.ResponseWriter, r *http.Request) {
vars := mux.Vars(r)
id, _ := strconv.Atoi(vars["id"])
p, errr := data.GetProduct(id) **// cannot use data.GetProduct(id) (value of type *data.Product) as *Product value in assignment**
errr = p.ToJSON(rw) // **p.ToJSON undefined (type *Product has no field or method ToJSON)**
if errr != nil {
http.Error(rw, "could not locate the product", http.StatusBadGateway)
}
}

Answer 1

cannot use data.GetProduct(id) (value of type *data.Product) as *Product value in assignment
p.ToJSON undefined (type *Product has no field or method ToJSON)
The problem here is that inside the GetProduct handler the variable p already has a type (*handlers.Product) that is different from the one returned by the data.GetProduct function (*data.Product). So what you can do is to use a different name for the variable that will store the result of data.GetProduct.
Why in example the teacher doing it like this. []*Product{&Product{}, &Product{}} what it the reason? Please explain.
In general because that's one of the available methods for how to initialize a slice of structs, as per the language spec. Why the teacher used this method specifically? Unless the teacher confided the reason to someone, then no one would know, I certainly don't.
this gives warning : redundant type from array, slice, or map composite literal. need to understand why
Because it's true, it is redundant, as per the language spec, in a composite literal expression you can elide the types of the elements and keys.
For example a non-redundant version of the following composite literal:
[]*Product{&Product{}, &Product{}}
would look like this:
[]*Product{{}, {}}
and the result of these two expressions would be the same.