UniqueBy function for arrays/slices in Go

How to write a functional UniqueBy function in golang using generics, including a unit test. Stop writing this by hand every single time!

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UniqueBy applies a function to each element in the input slice and returns a new slice containing the unique elements based on the function’s return value. The function f takes an element of type A and returns a key of type K.

func UniqueBy[A any, K comparable](input []A, f func(A) K) []A {
	seen := make(map[K]bool)
	var result []A
	for _, v := range input {
		key := f(v)
		if !seen[key] {
			seen[key] = true
			result = append(result, v)
		}
	}
	return result
}

and this is a unittest for the function:

func TestUniqueBy(t *testing.T) {
	input := []string{"apple", "banana", "cherry", "date", "elderberry"}
	f := func(a string) int {
		return len(a)
	}
	expected := []string{"apple", "banana", "date", "elderberry"}
	result := util.UniqueBy(input, f)
	if !reflect.DeepEqual(expected, result) {
		t.Errorf("Expected %v, got %v", expected, result)
	}
}