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143 lines
3.4 KiB
Plaintext
143 lines
3.4 KiB
Plaintext
# An example implementation of functional, lazy
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# sequences, as in clojure. The lazy seq is essentially
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# A lazy linked list, where the next value is a function
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# that must be called (realizing it), and the memoized.
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# Use with (import "./path/to/this/file" :prefix "seq/")
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(defmacro delay [& forms]
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"Lazily evaluate a series of expressions. Returns a function that
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returns the result of the last expression. Will only evaluate the
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body once, and then memoizes the result."
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(def $state (gensym))
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(def $loaded (gensym))
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(tuple 'do
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(tuple 'var $state nil)
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(tuple 'var $loaded nil)
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(tuple 'fn (array)
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(tuple 'if $loaded
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$state
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(tuple 'do
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(tuple ':= $loaded true)
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(tuple ':= $state (tuple-prepend forms 'do)))))))
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# Use tuples instead of structs to save memory
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(def HEAD :private 0)
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(def TAIL :private 1)
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(defn empty-seq
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"The empty sequence."
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[] nil)
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(defmacro cons
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"Create a new sequence by prepending a value to the original sequence."
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[h t]
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(def x (tuple h t))
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(fn [] x))
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(defn empty?
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"Check if a sequence is empty."
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[s]
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(not (s)))
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(defn head
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"Get the next value of the sequence."
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[s]
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(get (s) HEAD))
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(defn tail
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"Get the rest of a sequence"
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[s]
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(get (s) TAIL))
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(defn range2
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"Return a sequence of integers [start, end)."
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[start end]
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(if (< start end)
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(delay (tuple start (range2 (+ 1 start) end)))
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empty-seq))
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(defn range
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"Return a sequence of integers [0, end)."
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[end]
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(range2 0 end))
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(defn map
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"Return a sequence that is the result of applying f to each value in s."
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[f s]
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(delay
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(def x (s))
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(if x (tuple (f (get x HEAD)) (map f (get x TAIL))))))
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(defn realize
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"Force evaluation of a lazy sequence."
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[s]
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(when (s) (realize (tail s))))
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(defn realize-map [f s]
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"Evaluate f on each member of the sequence. Forces evaluation."
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(when (s) (f (head s)) (realize-map f (tail s))))
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(defn drop
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"Ignores the first n values of the sequence and returns the rest."
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[n s]
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(delay
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(def x (s))
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(if (and x (pos? n)) ((drop (- n 1) (get x TAIL))))))
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(defn take
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"Returns at most the first n values of s."
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[n s]
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(delay
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(def x (s))
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(if (and x (pos? n))
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(tuple (get x HEAD) (take (- n 1) (get x TAIL))))))
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(defn randseq
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"Return a sequence of random numbers."
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[]
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(delay (tuple (random) (randseq))))
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(defn take-while
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"Returns a sequence of values until the predicate is false."
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[pred s]
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(delay
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(def x (s))
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(when x
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(def thehead (get HEAD x))
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(if thehead (tuple thehead (take-while pred (get TAIL x)))))))
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# Iterators are a concept that looks a lot like lazy seq
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# The following functions turn iterators to lazy seq and vice versa
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(defn- iter-self
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[next more]
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(delay
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(if (more) (tuple (next) (iter-self next more)))))
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(defn iter2lazy
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"Create a lazy sequence from an iterator"
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[iter]
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(def {:more more :next next} iter)
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(iter-self next more))
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(defn lazy2iter
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"turn a lazy-seq to an iterator"
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[lazy-seq]
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(var node lazy-seq)
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{:more (fn [] (node))
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:next (fn []
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(when-let [n (node)]
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(:= node (get n 1))
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(get n 0)))})
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# Now we can use the non-functional filter from boot.dst
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# to write a filter version that returns a lazy sequence
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# Be careful when creating lazy sequences from mutable
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# data structures as their values are references to this
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# data structures. Same is true for iterators
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(defn filter2 [pred coll] (iter2lazy (filter pred coll)))
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# be careful with the filter function. First element in (filter pos? arr) is nil
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# last element is false
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