mirror of
https://github.com/janet-lang/janet
synced 2024-11-18 06:34:48 +00:00
107 lines
2.4 KiB
Plaintext
107 lines
2.4 KiB
Plaintext
# 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
|
|
"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."
|
|
[& forms]
|
|
(def state (gensym))
|
|
(def loaded (gensym))
|
|
~(do
|
|
(var ,state nil)
|
|
(var ,loaded nil)
|
|
(fn []
|
|
(if ,loaded
|
|
,state
|
|
(do
|
|
(:= ,loaded true)
|
|
(:= ,state (do ;forms)))))))
|
|
|
|
# Use tuples instead of structs to save memory
|
|
(def- HEAD 0)
|
|
(def- TAIL 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 lazy-range
|
|
"Return a sequence of integers [start, end)."
|
|
@[start end]
|
|
(if end
|
|
(if (< start end)
|
|
(delay (tuple start (lazy-range (+ 1 start) end)))
|
|
empty-seq)
|
|
(lazy-range 0 start)))
|
|
|
|
(defn lazy-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
|
|
"Evaluate f on each member of the sequence. Forces evaluation."
|
|
[f s]
|
|
(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 (math.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)))))))
|