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

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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

go higher-order-functions generics
1 Answers

In Go, arrays are inflexible (because their length is encoded in their type) and costly to pass to functions (because a function operates on copies 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:

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.

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