// Created on savesnippets.com · https://savesnippets.com/LfDdwbAgRSAkC4
package main

import "fmt"

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

func Filter[T any](s []T, pred func(T) bool) []T {
    out := s[:0]                              // reuse backing array
    for _, v := range s {
        if pred(v) {
            out = append(out, v)
        }
    }
    return out
}

func Reduce[T, U any](s []T, init U, f func(U, T) U) U {
    acc := init
    for _, v := range s {
        acc = f(acc, v)
    }
    return acc
}

func main() {
    nums := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
    squares := Map(nums, func(n int) int { return n * n })
    evens   := Filter(nums, func(n int) bool { return n%2 == 0 })
    sum     := Reduce(nums, 0, func(acc, n int) int { return acc + n })
    fmt.Println(squares, evens, sum)
}