go中使用var与不使用var声明变量之间的区别

Question

I was having some trouble with step 35 in the tour of Go exercise.

Here's what my code looks like:

package main

import "code.google.com/p/go-tour/pic"

func Pic(dx, dy int) [][]uint8 {
    var pic = make([][]uint8, dy)
    for y := 0; y < dy; y++ {
        pic[y] = make([]uint8, dx)
        for x := 0; y < dx; x++ {
            pic[y][x] = uint8(x*y)
        } 
    }
    return pic
}

When looking for a solution, I found PeterSO's code which works perfectly

func main() {
    pic.Show(Pic)
}

func Pic(dx, dy int) [][]uint8 {
    pixels := make([][]uint8, dy)
    for y := 0; y < dy; y++ {
        pixels[y] = make([]uint8, dx)
        for x := 0; x < dx; x++ {
            pixels[y][x] = uint8(x * y)
        }
    }
    return pixels
}

The only difference I can see, is that I'm defining the pic variable using the var keyword whereas his code is using the := assignment. Now, why does my code not work?

Answer 1

If you use the :=, the type of the variable is implied from the expression on the right of the sign. If you use =, no assumption is made and you need to specify the type yourself.

In this case, you should write it like this:

var pic [][]uint8 = make([][]uint8, dy)

but this is indeed better because shorter and as clear:

pic := make([][]uint8, dy)