# An example implementation of functional, lazy
# sequences, as in clojure. The lazy seq is essentially
# A lazy linked list, where the next value is a function
# that must be called (realizing it), and the memoized.
# Use with (import "./path/to/this/file" :prefix "seq/")

# This shows the need for syntax quoting D:
(defmacro delay 
 "Macro for lazy evaluation. Returns a function that will evaluate
the body when invoked. If called a second time, will return the first return value
that was memoized."
 [& forms]
 (def $state (gensym "state"))
 (def $loaded (gensym "loaded"))
 (def $temp (gensym "temp"))
 (tuple 'do
  (tuple 'var $state nil)
  (tuple 'var $loaded nil)
  (tuple 'fn []
   (tuple 'if $loaded $state
    (tuple 'do
      (tuple 'def $temp (tuple-prepend forms 'do))
      (tuple := $state $temp)
      (tuple := $loaded true)
      $temp)))))

# Use tuples instead of structs to save memory
(def HEAD :private 0)
(def TAIL :private 1)

(defn empty-seq 
 "The empty sequence." 
 [] nil)

(defn cons 
 "Create a new sequence by prepending a value to the original sequence."
 [h t]
 (delay (tuple h t)))

(defn empty? 
 "Check if a sequence is empty."
 [s]
 (not (s)))

(defn head
 "Get the next value of the sequence."
 [s]
 (get (s) HEAD))

(defn tail
 "Get the rest of a sequence"
 [s]
 (get (s) TAIL))

(defn range2 
 "Return a sequence of integers [start, end)."
 [start end]
 (if (< start end)
  (cons start (range2 (+ 1 start) end))
  empty-seq))

(defn range
 "Return a sequence of integers [0, end)."
 [end]
 (range2 0 end))

(defn map
 "Return a sequence that is the result of apply f to each value in s."
 [f s]
 (delay
  (def x (s))
  (if x (tuple (f (get x HEAD)) (map f (get x TAIL))))))

(defn realize
 "Force evaluation of a lazy sequence."
 [s]
 (when (s) (realize (tail s))))

(defn realize-map [f s]
 "Evaluate f on each member of the sequence. Forces evaluation."
 (when (s) (f (head s)) (realize-map f (tail s))))

(defn drop 
 "Ignores the first n values of the sequence and returns the rest."
 [n s]
 (delay
  (def x (s))
  (if (s) (if (zero? n) s (drop (- n 1) (tail s))) empty-seq)))

(defn take
 "Returns at most the first n values of s."
 [n s]
 (if (and (s) (pos? n))
  (cons (head s) (take (- n 1) (tail s)))
  empty-seq))

(defn randseq
 "Return a sequence of random numbers."
 []
 (delay (tuple (random) (randseq))))

(defn take-while
 "Returns a sequence of values until the predicate is false."
 [pred s]
 (delay 
  (def x (s))
  (when x
   (def thehead (get HEAD x))
   (if thehead (tuple thehead (take-while pred (get TAIL x)))))))