Don't Treat append Like a Pure Function

In Go, we add elements to a slice by calling the built-in append function, one typical use case is:

names = append(names, "satoru")

Unlike programming languages like Python or Java in which we call some append method on a list and update it in place, in Go, we have to assign the returned value of append back to the variable itself.

Judging from the function signature, append seems like a pure function, but this is not the case.

Let’s quote a statement in the official document of append:

If it has sufficient capacity, the destination is resliced to accommodate the new elements.

What does that mean? Let’s look at an example:

a := make([]int, 0, 4)
a = append(a, 1)
a = append(a, 2)
x := append(a, 3)
y := append(a, 4)
fmt.Println(a)  // => [1 2]
fmt.Println(x)  // => [1 2 4]
fmt.Println(y)  // => [1 2 4]

Both x and y would end up being [1 2 4], why’s that?

The slice a has a capacity of 4 and a length of 2, it has sufficient capacity for two more elements. When we later call append(a, 3) and append(a, 4), the array underlying a is actually reused. This would be clear if I print some more information in the example:

a := make([]int, 0, 4)
a = append(a, 1)
a = append(a, 2)
fmt.Printf("Address: %p, Len: %d, Cap: %d\n", &a[0], len(a), cap(a))
x := append(a, 3)
fmt.Printf("Address: %p, Len: %d, Cap: %d\n", &x[0], len(x), cap(x))
y := append(a, 4)
fmt.Printf("Address: %p, Len: %d, Cap: %d\n", &y[0], len(y), cap(y))
fmt.Println(a)
fmt.Println(x)
fmt.Println(y)

The output would be like (you may see different array addresses):

Address: 0xc000098000, Len: 2, Cap: 4
Address: 0xc000098000, Len: 3, Cap: 4
Address: 0xc000098000, Len: 3, Cap: 4
4
[1 2]
[1 2 4]
[1 2 4]

As you can tell from the identical addresses of the first element, the three slices are pointing to the same underlying array. When we call append to add a third element to a, it’s actually like setting the third element of the underlying array, that’s why x also becomes [1 2 4].

So append is not actually a pure function, don’t treat it like one! If you want to play with the code yourself, here’s a link to the playground.