Golang mixed assignment and declaration

Question

I started working with go for a few weeks, and (once again) I stumbled across something that seems odd for me:

// Not working
a := 1
{
    a, b := 2, 3
}

//         


        

Answer 1

your Question has 2 parts:
first part:
= is just assignment
:= is define and assign for new vars(at least one new var) inside the function block(not global), working sample:

package main

import (
    "fmt"
)

func main() {
    var u1 uint32      //declare a variable and init with 0
    u1 = 32            //assign its value
    var u2 uint32 = 32 //declare a variable and assign its value at once
    //declare a new variable with defining data type:
    u3 := uint32(32)        //inside the function block this is equal to: var u3 uint32 = 32
    fmt.Println(u1, u2, u3) //32 32 32
    //u3 := 20//err: no new variables on left side of :=
    u3 = 20
    fmt.Println(u1, u2, u3)       //32 32 20
    u3, str4 := 100, "str"        // at least one new var
    fmt.Println(u1, u2, u3, str4) //32 32 100 str
}

second part:
An identifier declared in a block may be redeclared in an inner block.
Here 4 different working samples for Variable scoping and shadowing:

simple way to limit variables scope:

package main
import "fmt"
func main() {
    i := 1
    j := 2
    //new scope :
    {
        i := "hi" //new local var
        j++
        fmt.Println(i, j) //hi 3
    }
    fmt.Println(i, j) //1 3
}

limit variable scope using function calls:

package main
import "fmt"
func fun(i int, j *int) {
    i++                //+nice: use as local var without side effect
    *j++               //+nice: intentionally use as global var
    fmt.Println(i, *j) //11 21
}
func main() {
    i := 10 //scope: main
    j := 20
    fun(i, &j)
    fmt.Println(i, j) //10 21
}

using short-hand assignment inside statements:

package main
import "fmt"
func main() {
    i := 10 //scope: main
    j := 4
    for i := 'a'; i < 'b'; i++ {
        fmt.Println(i, j) //97 4
    }
    fmt.Println(i, j) //10 4

    if i := "test"; len(i) == j {
        fmt.Println(i, j) // i= test , j= 4
    } else {
        fmt.Println(i, j) //test 40
    }
    fmt.Println(i, j) //10 4
}

shadowing global vars:

package main
import "fmt"
var i int = 1 //global
func main() {
    j := 2
    fmt.Println(i, j) //1 2
    i := 10           //Shadowing global var
    fmt.Println(i, j) //10 2
    fun(i, j)         //10 2
}
func fun(i, j int) {
    //i := 100   //no new variables on left side of :=
    fmt.Println(i, j) //10 2
}