Generic slice filter in Go.

filter.go

package main

import (
	"fmt"
	"reflect"
	"time"
)

func Filter(slice interface{}, filter interface{}) interface{} {
	if slice == nil {
		panic("first argument is not valid or nil pointer")
	}

	sliceKind := reflect.TypeOf(slice).Kind()
	if sliceKind != reflect.Array && sliceKind != reflect.Slice {
		panic("first argument is not a slice or array")
	}

	if filter == nil || reflect.TypeOf(filter).Kind() != reflect.Func {
		panic("second argument is not a function or nil pointer")
	}

	filterValue := reflect.ValueOf(filter)
	filterType := filterValue.Type()

	filterArguments := filterType.NumIn() != 2 || filterType.NumOut() != 1
	if filterArguments || filterType.Out(0).Kind() != reflect.Bool {
		panic("second argument signature is not valid")
	}

	sliceValue := reflect.ValueOf(slice)
	sliceType := sliceValue.Type()

	if !sliceType.Elem().ConvertibleTo(filterType.In(0)) {
		panic("second argument signature does not match first argument type")
	}

	resultsType := reflect.SliceOf(sliceType.Elem())
	results := reflect.MakeSlice(resultsType, 0, 0)

	for i := 0; i < sliceValue.Len(); i++ {
		element := sliceValue.Index(i)

		filterResult := filterValue.Call([]reflect.Value{element, reflect.ValueOf(i)})[0]
		if filterResult.Interface().(bool) {
			results = reflect.Append(results, element)
		}
	}

	return results.Interface()
}

type Person struct {
	Name        string
	DateOfBirth time.Time
}

func main() {
	var t time.Time
	var p []*Person

	const dateFormat = `2006-01-02`

	t, _ = time.Parse(dateFormat, `1879-03-14`)
	p = append(p, &Person{"Albert Einstein", t})

	t, _ = time.Parse(dateFormat, `1856-07-10`)
	p = append(p, &Person{"Nikola Tesla", t})

	t, _ = time.Parse(dateFormat, `1887-08-12`)
	p = append(p, &Person{"Erwin Schrodinger", t})

	persons := Filter(p, func(p *Person, _ int) bool {
		t, _ := time.Parse(dateFormat, `1880-01-01`)
		return p.DateOfBirth.After(t)
	})

	fmt.Printf("%T\n", persons)

	for _, p := range persons.([]*Person) {
		fmt.Printf("%+v\n", &Person{p.Name, p.DateOfBirth})
	}

	strings := Filter([]string{"a", "b", "c"}, func(s string, _ int) bool {
		return s > "a"
	})

	fmt.Printf("%T\n", strings)
	fmt.Println(strings)
}