Golang FP basics: filter, map, flatMap

fp.go

func Map[A any, B any](things []A, mapper func (x A) B) []B {
	result := make([] B, len(things))
	for i := range(things) {
		result[i] = mapper(things[i])
	}
	return result
} 

func Filter[A any](things []A, predicate func (x A) bool) []A {
	result := make([] A, len(things))
	count := 0
	for i := range(things) {
		if predicate(things[i]) {
			result[count] = things[i]
			count++
		}
	}
	return result[0:count]
}

func FlatMap[A any, B any](things []A, mapper func (x A) []B) []B {
	return Flatten(Map(things, mapper))
} 

func Flatten[A any](things [][]A) []A {
	count := 0
	for i := range(things) {
		count += len(things[i])
	}
	result := make([] A, count)
	index := 0
	for i := range(things) {
		for j := range(things[i]) {
			result[index] = things[i][j]
			index++
		}
	}
	return result
}