janet/examples/lazyseqs.dst

# 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.")

(defmacro delay [& forms]
 "Lazily evaluate a series of expressions. Returns a function that
returns the result of the last expression. Will only evaluate the
body once, and then memoizes the result."
 (def $state (gensym))
 (def $loaded (gensym))
 (tuple 'do
  (tuple 'var $state nil)
  (tuple 'var $loaded nil)
  (tuple 'fn (array)
   (tuple 'if $loaded
    $state
    (tuple 'do
     (tuple ':= $loaded true)
     (tuple ':= $state (tuple.prepend forms 'do)))))))

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

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

(defmacro cons
 "Create a new sequence by prepending a value to the original sequence."
 [h t]
 (def x (tuple h t))
 (fn [] x))

(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)
  (delay (tuple 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 applying 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 (and x (pos? n)) ((drop (- n 1) (get x TAIL))))))

(defn take
 "Returns at most the first n values of s."
 [n s]
 (delay
  (def x (s))
  (if (and x (pos? n))
   (tuple (get x HEAD) (take (- n 1) (get x TAIL))))))

(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)))))))

# Iterators are a concept that looks a lot like lazy seq
# The following functions turn iterators to lazy seq and vice versa

(defn- iter-self
  [next more]
  (delay 
   (if (more) (tuple (next) (iter-self next more)))))

(defn iter2lazy
"Create a lazy sequence from an iterator"
  [iter]
  (def {:more more :next next} iter)
  (iter-self next more))

(defn lazy2iter
  "turn a lazy-seq to an iterator"
  [lazy-seq]
  (var node lazy-seq)
  {:more (fn [] (node))
   :next (fn []
       (when-let [n (node)]
        (:= node (get n 1))
        (get n 0)))})