These functions where coded to understand the difficulty of implementing these functions in golang. I do not recommend you use it. A simple for loop will do and work in most of the cases.
// Definition of Filter function
type filterFunc func(interface{}) bool
func Filter(fn filterFunc, array interface{}) []interface{}
Filter filters the array based on the predicate// Definition of Foldl function
type foldlFunc func(interface{}, interface{}) interface{}
func Foldl(fn foldrFunc, array interface{}, accumulator interface{}) interface{}
Folds left the array values (reduction) based on the function// Definition of Map function
type mapFunc func(interface{}) interface{}
func Map(fn mapFunc, array interface{}) []interface{}
Map maps the function onto the array