(define nil ())

(define nil?
  (lambda (x)
    "Checks if the argument is nil."
    (= x nil)))

(define append
  (lambda (obj sequence)
    (if (nil? sequence)
        (list obj)
      (if (nil? (rest sequence))
          ))))

(define range
  (lambda (:keys from :defaults-to 0 to)
    "Returns a sequence of numbers starting with the number
defined by the key 'from' and ends with the number defined in
'to'."
    (if (<= from to)
        (pair from (range :from (+ 1 from) :to to))
      nil)))

(define map
  (lambda (function sequence)
    "Takes a sequence and a function as arguments and returns a
new sequence which contains the results of using the first
sequences elemens as argument to that function."
    (if (nil? sequence)
        sequence
      (pair (function (first sequence))
            (map (rest sequence) function)))))

(define reduce
  (lambda (function sequence)
    "Takes a sequence and a function as arguments and applies the
function to the argument sequence. This only works correctly if the
given function accepts a variable amount of parameters. If your
funciton is limited to two arguments, use `reduce-binary' instead."
    (eval (pair function sequence))))

(define reduce-binary
  (lambda (function sequence)
    "Takes a sequence and a function as arguments and applies the
function to the argument sequence. reduce-binary applies the
arguments `pair-wise' which means it works with binary functions
as compared to `reduce'."
    (if (nil? (rest sequence))
        (first sequence)
      (function (first sequence)
                (reduce-binary (rest sequence) function)))))

(define filter
  (lambda (function sequence)
    (if (nil? sequence)
        nil
      (if (function (first sequence))
          (pair (first sequence)
                (filter (rest sequence) function))
        (filter (rest sequence) function)))))

(define printf
  (lambda (:keys sep :defaults-to " " end :defaults-to "\n" :rest args)
    (if (and (nil? (first args)) (nil? (rest args)))
        (print end)
      (prog
       (print (first args))
       (if (not (nil? (rest args)))
           (print sep))
       ;; TODO(Felix): later we should use `extend' here:
       ;; (eval (extend (quote (printf :sep sep :end end)) (rest args)))
       (eval (pair printf (pair :sep (pair sep (pair :end (pair end (rest args)))))))
       nil))))