A small naive implementation of map, filter and reduce in racket, implemented by myself.

map_filter_reduce.rkt

#lang racket

;; This file departs from the book* a little bit, because, reading the book
;; made some older things I had read in the Simply Scheme book "click in".
;; In particular, I finally understood how I could go around and implement
;; map-filter-reduce on my own, so, this is my stab at it.
;;
;; *where book == The Little Schemer

;; Map works as follows: It takes a function argument and a list of atoms, and
;; returns a new list that is the result of applying the function argument to
;; each of the lists atoms.
(define (map fn lat)
  (cond
    ([null? lat] '())
    (else
     (cons (fn (car lat)) (map fn (cdr lat))))))

;; Filter works as follows: It takes a function argument that acts as a predicate
;; and a list of atoms, and constructs a new list that only returns the values in
;; the original list that pass the predicate test.
(define (filter fn lat)
  (cond
    ([null? lat] '())
    (else
     (if (fn (car lat))
         (cons (car lat) (filter fn (cdr lat)))
         (filter fn (cdr lat))))))

;; Reduce works as follows: It takes a function argument and a list of atoms
;; and recursively applies the function to the lists atoms until they have been
;; reduced to single value dictated by the evaluation of the function argument.
(define (reduce fn lat)
  (cond
    ([null? (cdr lat)] (car lat))
    (else
     (fn (car lat) (reduce fn (cdr lat))))))