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+;; Stream utilities
+
+;; by Konrad Hinsen
+;; last updated January 25, 2009
+
+;; Note: this module is at least as experimental as Clojure's stream
+;; facilities. It may change significantly, change name, or disappear
+;; in the future. Don't rely on it for your applications. I put this
+;; module in clojure.contrib so that it can serve as a testground and
+;; a basis for discussions ont the group.
+
+;; Copyright (c) Konrad Hinsen, 2009. All rights reserved.  The use
+;; and distribution terms for this software are covered by the Eclipse
+;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
+;; which can be found in the file epl-v10.html at the root of this
+;; distribution.  By using this software in any fashion, you are
+;; agreeing to be bound by the terms of this license.  You must not
+;; remove this notice, or any other, from this software.
+
+(ns clojure.contrib.stream-utils
+  (:use [clojure.contrib.monads]))
+
+;; Streams
+
+; Stream of integers
+(defn int-stream
+  "Return a stream of integers starting from init and incrementing by step
+   until end, the first value not returned. The defaults are 0, 1, and nil,
+   where nil stands for an infinite stream."
+  ([] (int-stream 0 1 nil))
+  ([init] (int-stream init 1 nil))
+  ([init step] (int-stream init step nil))
+  ([init step end]
+   (let [n (into-array [init])]
+     (stream
+      (fn [eos]
+	(let [current (aget n 0)]
+	  (if (or (nil? end) (< current end))
+	    (do
+	      (aset n 0 (+ current step))
+	      current)
+	    eos)))))))
+
+;; Stream transformers
+
+(defn drop-first
+  "Return a stream that returns the elements of s except for the first n ones."
+  [n s]
+  (let [eos (Object.)
+	iter (stream-iter s)
+	gen (fn [eos] (next! iter eos))]
+    (doseq [_ (range n)] (gen eos))
+    (stream gen)))
+
+;; Monadic stream transformers
+
+; Generator monad
+;
+; This monad represents computations with generators, as defined for
+; Clojure streams: a function of one argument, an end-of-stream value,
+; that returns at each call the "next" value of the stream, until the
+; end of the stream is reached, and from then on it returns the supplied
+; eos value forever. This monad permits the simple composition of such
+; generators.
+;
+; The use of generators instead of streams as the basis of this monad
+; has two reasons:
+; 1) It is not possible to pass around a stream such that each
+;    monadic computation step takes one item of it, because there can
+;    be only one iterator per stream.
+; 2) One could use iterators instead of generators as the monadic
+;    values, even with minimal changes, but each computational step
+;    in the monad would then create a new stream and a new iter,
+;    adding overhead for no apparent benefit.
+
+(defmonad generator
+   "Monad describing stream-transforming computations. The monadic
+    values are generator functions."
+   [m-result (fn m-result-generator [v]
+	       (fn [eos] v))
+    m-bind   (fn m-bind-generator [mv f]
+               (fn [eos]
+		 (let [v (mv eos)]
+		   (if (identical? v eos)
+		     eos
+		     ((f v) eos)))))
+    ])
+
+; A macro for defining stream transformers using the generator monad.
+; It wraps the input streams into iters-that-look-like-generators,
+; and converts the resulting generator into a stream.
+(defmacro stream-transformer
+  [stream-args & body]
+  `(stream (let [~stream-args (map stream-iter ~stream-args)]
+	     (with-monad generator ~@body))))
+
+
+; Derive a variant of the generator monad that permits the
+; use of :when conditions. Invalid values in the output are
+; represented by ::skip, which is filtered out in the end
+; by applying remove-skip. The macro stream-transformer-cond
+; takes care of all these administrative details.
+(def generator-cond (maybe-t generator ::skip :m-plus-from-maybe))
+
+(defn remove-skip [g]
+  (fn [eos]
+    (loop [v (g eos)]
+      (if (identical? v ::skip)
+	(recur (g eos))
+	v))))
+
+(defmacro stream-transformer-cond
+  [stream-args & body]
+  `(stream (remove-skip
+    (let [~stream-args (map stream-iter ~stream-args)]
+      (with-monad generator-cond ~@body)))))
+
+
+;; Some examples of monadic stream transformers
+
+(defn skip-s [s]
+  "Return a stream that skips every second item of s"
+  (stream-transformer [s]
+    (domonad
+      [x1 s
+       x2 s]
+      x2)))
+
+(defn partition-s [n s]
+  "Return a stream in which each element is a sequence of n consecutive
+   elements of the input stream s."
+  (stream-transformer [s]
+    (m-seq (replicate n s))))
+
+; Map a function on streams.
+; Map has to be defined by an explicit expression for every number of
+; arguments because the m-lift macro requires a constant as its first
+; argument.
+(defn map-s
+  "Return a stream of the results of applying f on one item of each
+   input sequence."
+  ([f s]
+   (stream-transformer [s]
+     ((m-lift 1 f) s)))
+  ([f s1 s2]
+   (stream-transformer [s1 s2]
+     ((m-lift 2 f) s1 s2)))
+  ([f s1 s2 s3]
+   (stream-transformer [s1 s2 s3]
+     ((m-lift 3 f) s1 s2 s3)))
+  ([f s1 s2 s3 s4]
+   (stream-transformer [s1 s2 s3 s4]
+     ((m-lift 4 f) s1 s2 s3 s4)))
+  ([f s1 s2 s3 s4 s5]
+   (stream-transformer [s1 s2 s3 s4 s5]
+     ((m-lift 5 f) s1 s2 s3 s4 s5))))
+
+(defn filter-s [p s]
+  "Return a stream consisting of the elements of s that satisfy predicate p."
+  (stream-transformer-cond [s]
+    (domonad
+     [x s :when (p x)]
+     x)))
+
+(comment
+
+; Some example expressions
+
+(take 10 (drop-first 5 (int-stream)))
+
+(take 10 (skip-s (int-stream)))
+
+(take 10 (partition-s 3 (int-stream)))
+
+(take 10 (map-s inc (int-stream)))
+(take 10 (map-s + (int-stream) (map-s inc (int-stream))))
+
+(take 10 (filter-s #(zero? (rem % 3)) (int-stream)))
+
+)
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