0.6.0 x/reductions and extend behavior of x/for

README.md

@@ -4,7 +4,7 @@ More transducers and reducing functions for Clojure!
 
 [![Build Status](https://travis-ci.org/cgrand/xforms.png?branch=master)](https://travis-ci.org/cgrand/xforms)
 
-Transducers: `reduce`, `into`, `last`, `count`, `avg`, `min`, `minimum`, `max`, `maximum`, `str`, `by-key`, `partition`, `for`, `multiplex`, `transjuxt`, `window` and `window-by-time`.
+Transducers: `reduce`, `into`, `last`, `count`, `avg`, `min`, `minimum`, `max`, `maximum`, `str`, `by-key`, `partition`, `reductions`, `for`, `multiplex`, `transjuxt`, `window` and `window-by-time`.
 
 Reducing functions (in `net.cgrand.xforms.rfs`): `min`, `minimum`, `max`, `maximum`, `str`, `str!`, `avg`, `juxt` and `last`.
 
@@ -15,7 +15,7 @@ Transducing context: `transjuxt` (for performing several transductions in a sing
 Add this dependency to your project:
 
 ```clj
-[net.cgrand/xforms "0.5.1"]
+[net.cgrand/xforms "0.6.0"]
 ```
 
 ```clj
@@ -199,6 +199,11 @@ Evaluation count : 24 in 6 samples of 4 calls.
     Execution time std-deviation : 364,932996 µs
 ```
 
+## Changelog
+### 0.6.0
+
+ * Added `x/reductions`
+ * Now if the first collection expression in `x/for` is not a placeholder then `x/for` works like `x/for` but returns an eduction and performs all iterations using reduce. 
 
 ## License
 

project.clj

@@ -1,4 +1,4 @@
-(defproject net.cgrand/xforms "0.5.1"
+(defproject net.cgrand/xforms "0.6.0"
   :description "Extra transducers for Clojure"
   #_#_:url "http://example.com/FIXME"
   :license {:name "Eclipse Public License"

src/net/cgrand/xforms.clj

@@ -1,16 +1,17 @@
 (ns net.cgrand.xforms
   "Extra transducers for Clojure"
   {:author "Christophe Grand"}
-  (:refer-clojure :exclude [reduce into count for partition str last keys vals min max])
+  (:refer-clojure :exclude [reduce reductions into count for partition str last keys vals min max])
   (:require [clojure.core :as clj]
     [net.cgrand.xforms.rfs :as rf]))
 
 (defmacro for
-  "Like clojure.core/for with the first expression being replaced by % (or _). Returns a transducer."
+  "Like clojure.core/for with the first expression being replaced by % (or _). Returns a transducer.
+   When the first expression is not % (or _) returns an eduction."
   [[binding %or_ & seq-exprs] body-expr]
-  (assert (and (symbol? %or_) (#{"%" "_"} (name %or_)))
-    "The second element of the comprehension vector must be % or _.")
-  (let [rf (gensym 'rf)
+  (if-not (and (symbol? %or_) (#{"%" "_"} (name %or_)))
+    `(eduction (for [~binding ~'% ~@seq-exprs] ~body-expr) ~%or_)
+    (let [rf (gensym 'rf)
         acc (gensym 'acc)
         pair? #(and (vector? %) (= 2 (clj/count %)))
         destructuring-pair? (every-pred pair?
@@ -47,7 +48,7 @@
            ~(if (destructuring-pair? binding)
               `([~acc ~@(map #(vary-meta % dissoc :tag) binding)] ~body)
               `([~acc k# v#]
-                 (let [~binding (clojure.lang.MapEntry. k# v#)] ~body))))))))
+                 (let [~binding (clojure.lang.MapEntry. k# v#)] ~body)))))))))
 
 (defprotocol KvRfable "Protocol for reducing fns that accept key and val as separate arguments."
   (some-kvrf [f] "Returns a kvrf or nil"))
@@ -310,6 +311,30 @@
                   acc)
                 acc))))))))
 
+(defn reductions
+  "Transient version of reductions. There's a difference in behavior when init is not provided: (f) is used.
+   So x/reductions works like x/reduce or transduce, not like reduce and reductions when no init and 1-item input."
+  ([f] (reductions f (f)))
+  ([f init]
+    (fn [rf]
+      (let [prev (volatile! nil)]
+        (vreset! prev prev) ; cheap sentinel to detect the first call, this is done to avoid having a 1-item delay
+        (fn
+          ([] (rf)) ; no you can't emit init there since there's no guarantee that this arity is going to be called
+          ([acc] (if (identical? @prev prev)
+                   (rf (unreduced (rf acc init)))
+                   (rf acc)))
+          ([acc x]
+            (if (identical? @prev prev)
+              (let [acc (rf acc (vreset! prev init))]
+                (if (reduced? acc)
+                  acc
+                  (recur acc x)))
+              (let [curr (vswap! prev f x)]
+                (if (reduced? curr)
+                  (ensure-reduced (rf acc @curr))
+                  (rf acc curr))))))))))
+
 (def avg (reduce rf/avg))
 
 (defn window

test/net/cgrand/xforms_test.clj

@@ -61,6 +61,10 @@
           8 (range 16)))
     (is (trial (x/partition 3 (take 2))
           8 (range 2)))
+    (is (trial (x/reductions conj [])
+          8 (range 2)))
+    (is (trial (x/reductions conj)
+          8 (range 2)))
     (is (trial (x/into [])
           4 (range 16)))
     (is (trial (x/for [x % y (range x)] [x y])
@@ -68,6 +72,16 @@
     (is (trial (x/reduce +)
           4 (range 16)))))
 
+(deftest reductions
+  (is (= (into [] (x/reductions +) (range 10)) [0 0 1 3 6 10 15 21 28 36 45]))
+  (is (= (into [] (x/reductions +) (range 0)) [0]))
+  (is (= (into [] (x/reductions +) (range 1)) [0 0]))
+  (is (= (into [] (x/reductions +) (range 2)) [0 0 1]))
+  (is (= (into [] (comp (x/reductions +) (take 2)) (range)) [0 0]))
+  (is (= (into [] (comp (x/reductions +) (take 3)) (range)) [0 0 1]))
+  (is (= (into [] (comp (take 3) (x/reductions +)) (range)) [0 0 1 3]))
+  (is (= (into [] (x/reductions (constantly (reduced 42)) 0) (range)) [0 42])))
+
 (deftest window-by-time
   (is (= (into 
            []