1 files changed, 208 insertions, 167 deletions

diff --git a/src/clojure/contrib/stream_utils.clj b/src/clojure/contrib/stream_utils.clj
index fd4e6a04..605426db 100644
--- a/src/clojure/contrib/stream_utils.clj
+++ b/src/clojure/contrib/stream_utils.clj
@@ -1,13 +1,7 @@
 ;; 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.
+;; last updated February 23, 2009
 
 ;; Copyright (c) Konrad Hinsen, 2009. All rights reserved.  The use
 ;; and distribution terms for this software are covered by the Eclipse
@@ -18,163 +12,210 @@
 ;; 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)))
-
-)
\ No newline at end of file
+  "Functions for setting up computational pipelines via data streams.
+
+   NOTE: This library is experimental. It may change significantly
+   with future release.
+
+   This library defines:
+   - the multimethod stream-next as consumer interface to data streams
+   - implementations of stream-next for three data stream types:
+     1) Clojure seqs, sequences, and vectors
+     2) Stream generator functions
+     3) nil representing an empty stream
+   - the monad stream-m for writing stream transformers
+   - macros defst-seq and defst-gen for writing monad-based stream
+     transformers with a seq or a generator function interface
+   - various utility functions for working with data streams
+
+   Stream generator functions are called with a single argument representing
+   the end-of-stream sentinel value. At each call, they return the next
+   element of their data stream and the new stream state. When the end of
+   the stream is reached, they return the passed-in end-of-stream object.
+   The new stream state is typically a closure. While stream generator
+   functions are less flexible than lazy seqs (because they cannot be used
+   with Clojure's seq-handling functions), they have a few advantages:
+   - The data stream is never cached.
+   - The state of a stream can be stored in any Clojure data structure,
+     and the stream can be re-generated from it any number of times.
+   Nothing prevents a generator function from storing the stream state
+   in a mutable data structure and just return itself as the new state,
+   but such functions are neither thread-safe nor safe to be used to
+   reproduce their stream more than once.
+   
+   Stream transformers take any number of input streams and produce one
+   output stream. They are typically written using the stream-m
+   monad. Input streams can be defined by any value that stream-next
+   can handle. The output stream can have a (lazy) seq interface or
+   a generator function interface. In the definition of a stream
+   transformer, (pick s) returns the next value of stream argument s,
+   whereas pick-all returns the next value of all stream arguments
+   in the form of a vector."
+
+  (:use [clojure.contrib.monads])
+  (:use [clojure.contrib.macros :only (letfn)]))
+
+
+(let [eos (Object.)
+      skip (Object.)]
+
+  (defn stream-eos?
+    "Returns true if x is the special end-of-stream value used by
+     stream-next and the stream-m monad."
+    [x]
+    (identical? x eos))
+
+  (defn stream-skip?
+    "Returns true if x is the special skip value used by the stream-m monad."
+    [x]
+    (identical? x skip))
+
+  (defmulti stream-next
+    "Returns a vector of length two whose first element is the next
+     item in the data stream defined by stream-state and whose second
+     element is the new state of the stream. At the end of the stream,
+     the returned value is a special end-of-stream object for which
+     stream-eos? returns true."
+    {:arglists '([stream-state])}
+    class)
+
+  (defmethod stream-next nil
+    [s]
+    [eos nil])
+
+  (defmethod stream-next clojure.lang.ISeq
+    [s]
+    (if (seq s)
+      [(first s) (rest s)]
+      [eos nil]))
+
+  (defmethod stream-next clojure.lang.IPersistentVector
+    [v]
+    (stream-next (seq v)))
+
+  (defmethod stream-next clojure.lang.Fn
+    [g]
+    (g eos))
+
+  (defmonad stream-m
+     "Monad describing stream computations. The monadic values can be
+      of any type handled by stream-next."
+     [m-result  (fn m-result-stream [v]
+  	        (fn [s] [v s]))
+      m-bind    (fn m-bind-stream [mv f]
+  		(fn [s]
+  		  (let [[v ss] (mv s)]
+  		    (if (or (stream-eos? v) (stream-skip? v))
+  		      [v ss]
+  		      ((f v) ss)))))
+      m-zero     (fn [s] [skip s])
+      ])
+
+  (defn pick
+    "Return the next value of stream argument n inside a stream
+     transformer. When used inside of defst, the name of the stream
+     argument can be used instead of its index n."
+    [n]
+    (fn [streams]
+      (let [[v ns] (stream-next (streams n))]
+	[v (assoc streams n ns)])))
+
+  (defn pick-all
+    "Return a vector containing the next value of each stream argument
+     inside a stream transformer."
+    [streams]
+    (if (some nil? streams)
+      [eos streams]
+      (let [next    (map stream-next streams)
+	    values  (map first next)
+	    streams (vec (map second next))]
+	(if (some stream-eos? values)
+	  [eos streams]
+	  [values streams]))))
+
+  (defn st-as-seq
+    "Takes a stream transformer expression st (typically written using the
+     stream-m monad) and a vector of stream arguments and returns a lazy
+     seq representing the output stream of the transformer."
+    [st streams]
+    (lazy-seq
+     (loop [s streams]
+       (let [[v ns] (st s)]
+	 (cond (stream-eos? v) nil
+	       (stream-skip? v) (recur ns)
+	       :else (cons v (st-as-seq st ns)))))))
+
+  (defn st-as-generator
+    "Takes a stream transformer expression st (typically written using the
+     stream-m monad) and a vector of stream arguments and returns a stream
+     generator function representing the output stream of the transformer."
+    [st streams]
+    (letfn [make-gen [s]
+	    (fn [eos]
+	      (loop [s s]
+		(let [[v ns] (st s)]
+		  (cond (stream-eos? v) [eos nil]
+			(stream-skip? v) (recur ns)
+			:else [v (make-gen ns)]))))]
+      (make-gen streams)))
+)
+
+(defn- defst [wrapper name args streams body]
+  (if (= (first streams) '&)
+    `(defn ~name ~(vec (concat args streams))
+       (let [~'st (with-monad stream-m ~@body)]
+	 (~wrapper ~'st ~(second streams))))
+    `(defn ~name ~(vec (concat args streams))
+       (let [~'st (with-monad stream-m
+	  	    (let [~streams (range ~(count streams))]
+		      ~@body))]
+	 (~wrapper ~'st ~streams)))))
+
+(defmacro defst-seq
+  "Define the seq-returning stream transformer name by body.
+   The non-stream arguments args and the stream arguments streams
+   are given separately, with args being possibly empty."
+  [name args streams & body]
+  (defst 'st-as-seq name args streams body))
+
+(defmacro defst-gen
+  "Define the generator-returning stream transformer name by body.
+   The non-stream arguments args and the stream arguments streams
+   are given separately, with args being possibly empty."
+  [name args streams & body]
+  (defst 'st-as-generator name args streams body))
+
+(defn stream-drop
+  "Return a stream containing all but the first n elements of stream."
+  [n stream]
+  (if (zero? n)
+    stream
+    (let [[_ s] (stream-next stream)]
+      (recur (dec n) s))))
+
+(defn stream-as-seq
+  "Return a lazy seq of the stream s."
+  [s]
+  (lazy-seq
+   (let [[v ns] (stream-next s)]
+     (if (stream-eos? v)
+       nil
+       (cons v (stream-as-seq ns))))))
+
+(defn stream-flatten
+  "Converts a stream of sequences into a stream of the elements of the
+   sequences. Flattening is not recursive, only one level of sequences
+   will be removed."
+  [s]
+  (letfn [buffer-gen [buffer stream]
+	  (fn [eos]
+	    (loop [buffer buffer
+		   stream stream]
+	      (if (nil? buffer)
+		(let [[v new-stream] (stream-next stream)]
+		  (cond (stream-eos? v) [eos nil]
+			(empty? v) (recur nil new-stream)
+			:else (recur v new-stream)))
+		[(first buffer) (buffer-gen (next buffer) stream)])))]
+    (buffer-gen nil s)))
+