Is there a way to map an array of objects in golang?

Issue

Coming from Nodejs, I could do something like:

// given an array `list` of objects with a field `fruit`:

fruits = list.map(el => el.fruit) # which will return an array of fruit strings 

Any way to do that in an elegant one liner in golang?

I know I can do it with a range loop, but I am looking for the possibility of a one liner solution

Solution

In Go, arrays are inflexible (because their length is encoded in their type) and costly to pass to functions (because a function operates on a copy of its array arguments). I’m assuming you’d like to operate on slices rather than on arrays.

Because methods cannot take additional type arguments, you cannot simply declare a generic Map method in Go. However, you can define Map as a generic top-level function:

func Map[T, U any](ts []T, f func(T) U) []U {
    us := make([]U, len(ts))
    for i := range ts {
        us[i] = f(ts[i])
    }
    return us
}

Then you can write the following code,

names := []string{"Alice", "Bob", "Carol"}
fmt.Println(Map(names, utf8.RuneCountInString))

which prints [5 3 5] to stdout (try it out in this Playground).

---

Go 1.18 saw the addition of a golang.org/x/exp/slices package, which provides many convenient operations on slices, but a Map function is noticeably absent from it. The omission of that function was the result of a long discussion in the GitHub issue dedicated to the golang.org/x/exp/slices proposal; concerns included the following:

• hidden cost (O(n)) of operations behind a one-liner
• uncertainty about error handling inside Map
• risk of encouraging a style that strays too far from Go’s traditional style

Russ Cox ultimately elected to drop Map from the proposal because it’s

probably better as part of a more comprehensive streams API somewhere else.

Answered By – jub0bs

Answer Checked By – Pedro (GoLangFix Volunteer)