(define defmacro
  (macro (@name @params :rest @body)
     "Macro for creating macros with a more concise syntax."
     (eval (pair 'define (pair @name (pair (pair 'macro (pair @params @body)) nil))))))

(defmacro defun (@name @params :rest @body)
    "Macro for creating functions with a more concise syntax."
    (eval (pair 'define (pair @name (pair (pair 'lambda (pair @params @body)) nil)))))

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

(defun number? (x)
  "Checks if the argument is a number."
  (= (type x) :number))

(defun symbol? (x)
  "Checks if the argument is a symbol."
  (= (type x) :symbol))

(defun keyword? (x)
  "Checks if the argument is a keyword."
  (= (type x) :keyword))

(defun pair? (x)
  "Checks if the argument is a pair."
  (= (type x) :pair))

(defun string? (x)
  "Checks if the argument is a string."
  (= (type x) :string))

(defun dynamic-function? (x)
  "Checks if the argument is a function."
  (= (type x) :dynamic-function))

(defun dynamic-macro? (x)
  "Checks if the argument is a macro."
  (= (type x) :dynamic-macro))

(defun built-in-function? (x)
  "Checks if the argument is a built-in function."
  (= (type x) :built-in-function))

(defun apply (fun seq)
  "Applies the funciton to the sequence, as in calls the funciton with
ithe sequence as arguemens."
  (eval (pair fun seq)))

(defmacro when (@test :rest @body)
  "Executes the code in :rest if test is true."
  (if (eval @test)
      (eval (pair prog @body))
    nil))

(defmacro unless (@test :rest @body)
  "Executes the code in :rest if test is false."
  (if (eval @test)
      nil
    (eval (pair prog @body))))

(defun end (seq)
  "Returns the last pair in the sqeuence."
  (if (or (nil? seq) (not (pair? (rest seq))))
      seq
    (end (rest seq))))

(defun last (seq)
  "Returns the (first) of the last (pair) of the given sequence."
  (first (end seq)))

(defun extend (seq elem)
  "Extends a list with the given element, by putting it in
the (rest) of the last element of the sequence."
  (when (pair? seq)
    (define e (end seq))
    (mutate e (pair (first e) elem)))
  seq)

(defun incr (val)
  "Adds one to the argument."
  (+ val 1))

(defun decr (val)
  "Subtracts one from the argument."
  (- val 1))

(defun append (seq elem)
  "Appends an element to a sequence, by extendeing the list
with (pair elem nil)."
  (extend seq (pair elem nil)))

(defun length (seq)
  "Returns the length of the given sequence."
  (if (nil? seq)
      0
    (incr (length (rest seq)))))

(defmacro n-times (@times @action)
  "Executes @action @times times."
  (unless (<= (eval @times) 0)
    (eval @action)
    (apply n-times (list (list - @times 1) @action))))

(defmacro for (@symbol @from @to :rest @for-body)
  "Designed to resemble a C style for loop. It takes a symbol as
well as its starting number and end number and executes the
@for-body with the defined symbol for all numbers between @from
to @to, where @to is exclusive."
  (if (< (eval @from) (eval @to))
      (macro-define @op incr)
    (if (> (eval @from) (eval @to))
        (macro-define @op decr)
      (macro-define @op nil)))
  (when @op
     (macro-define (eval @symbol) (eval @from))
     (eval (pair prog @for-body))
     (eval (extend (list for @symbol (@op @from) @to) @for-body))))

(defun range (: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'."
  (when (< from to)
      (pair from (range :from (+ 1 from) :to to))))

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

(defun reduce (fun seq)
  "Takes a function and a sequence 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."
  (apply fun seq))

(defun reduce-binary (fun seq)
  "Takes a function and a sequence 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 seq))
      (first seq)
    (fun (first seq)
         (reduce-binary fun (rest seq)))))

(defun filter (fun seq)
  "Takes a function and a sequence as arguments and applies the
function to every value in the sequence. If the result of that
funciton application returns a truthy value, the original value is
added to a list, which in the end is returned."
  (when seq
    (if (fun (first seq))
        (pair (first seq)
              (filter fun (rest seq)))
      (filter fun (rest seq)))))

(defun printf (:keys sep :defaults-to " " end :defaults-to "\n" :rest args)
  "A wrapper for the built-in (print) that accepts a variable number
of arguments and also provides keywords for specifying the printed
separators between the arguments and what should be printed after the
las argument."
  (defmacro printf-quoted (:keys @sep @end :rest @args)
    (if (nil? @args)
        (prog (print (eval @end)) nil)
      (prog
       (print (first @args))
       (unless (nil? (rest @args))
         (print (eval @sep)))
       (eval (pair printf-quoted
              (extend (list :@sep (eval @sep) :@end (eval @end)) (rest @args)))))))

  (eval (pair printf-quoted (extend (list :@sep (eval sep) :@end (eval end)) args))))

(defmacro pe (@expr)
  (printf @expr "evaluates to" (eval @expr)))