Example of a generic FP map function [T=>U], and filter(), that golang doesn't have out of the box

mapf.go

package main

import (
	"fmt"
	"strconv"
)

// Maps over collection of T and applies function f to each item
// Returns a new slice with the transformed items
func mapf[T any, U any](items []T, f func(T) U) []U {
	transformed := make([]U, 0, len(items))
	for _, n := range items {
		transformed = append(transformed, f(n))
	}
	return transformed
}

// for which the provided function returns true.
func filter[T any](items []T, predicate func(T) bool) []T {
	var result []T
	for _, item := range items {
		if predicate(item) {
			result = append(result, item)
		}
	}
	return result
}

func main() {

	func1 := func(n int) int {
		return n + 1
	}

	numbers := []int{1, 2, 3, 4}

	fmt.Println(mapf(numbers, func1))

	func2 := func(n int) int {
		return n * 2
	}

	fmt.Println(mapf(numbers, func2))

	strings := []string{"1", "2", "3", "oink"}

	// Using an anonymous function

	fmt.Println(mapf(strings, func(st string) string { return st + "!!" }))

	fmt.Println(mapf(strings, func(st string) int {
		i, err := strconv.Atoi(st)
		if err != nil {
			return 99 // or handle the error differently
		}
		return i
	}))
	
	fmt.Println(filter(numbers, func(n int) bool { return n%2 == 0 }))
	
	fmt.Println(filter(mapf(strings, func(st string) int {
		i, _ := strconv.Atoi(st)
		return i
	}), func(n int) bool { return n > 0 }))
}

Output

[2 3 4 5]
[2 4 6 8]
[1!! 2!! 3!! oink!!]
[1 2 3 99]
[2 4]
[1 2 3]