pjstadig · June 8, 2021 13:22 · Apr 17, 2019 · Apr 16, 2019 · Apr 16, 2019
diff --git a/transducers.md b/transducers.md
@@ -26,7 +26,7 @@ One somewhat esoteric use of this is to transpose a matrix:
 ## The Turn
 However, transducers can only be applied to a single source...or can they?
 
-I noticed that `map`'s transducer takes a fourth arity with a variable number of collections.  This really surprised me since as far as I could remember there were no transducing contexts that would actually work with multiple sources...or where there?
+I noticed that `map`'s transducer takes a fourth arity with a variable number of items.  This really surprised me since as far as I could remember there were no transducing contexts that would actually work with multiple sources...or where there?
 
 I looked through `transduce`, `eduction`, `into`, and `sequence` and found that `sequnece` can take multiple collections.
 

diff --git a/transducers.md b/transducers.md
@@ -10,6 +10,7 @@ One somewhat practical use of this is if you want to compare two sequences, pair
 
 ```clojure
 (every? true? (map < (range 5) (range 5 10)))
+=> true
 ```
 
 One somewhat esoteric use of this is to transpose a matrix:
@@ -29,7 +30,7 @@ I noticed that `map`'s transducer takes a fourth arity with a variable number of
 
 I looked through `transduce`, `eduction`, `into`, and `sequence` and found that `sequnece` can take multiple collections.
 
-Neither `map'`s fourth arity nor `sequence` taking multiple collections is mentioned in any of the transducer documentation.  The only hint is `sequence`'s docstring mentions multiple collections, but you'd have look at the source for `map` to see the fourth arity.
+Neither `map`'s fourth arity nor `sequence` taking multiple collections is mentioned in any of the transducer documentation.  The only hint is `sequence`'s docstring mentions multiple collections, but you'd have look at the source for `map` to see the fourth arity.
 
 ## The Prestige
 `map` is the only `clojure.core` transducer with a fourth arity, and `sequence` is the only `clojure.core` transducer context to supply multiple sources, so this seems to be of limited use.  However, as long as `map` is the first stage of your transducer pipeline you can use other transducers with a multi-collection invocation of `sequence`:

diff --git a/transducers.md b/transducers.md
@@ -0,0 +1,46 @@
+## The Pledge
+One thing that always made me a little sad about transducers was how `map` lost its ability to iterate multiple collections in parallel.  This is actually my favorite feature of `map`.  For example:
+
+```clojure
+(map + (range 5) (range 5 10))
+=> (5 7 9 11 13)
+```
+
+One somewhat practical use of this is if you want to compare two sequences, pairwise, using a comparator.  Though I wish that `every?` took multiple collections, this is an adequate substitute:
+
+```clojure
+(every? true? (map < (range 5) (range 5 10)))
+```
+
+One somewhat esoteric use of this is to transpose a matrix:
+
+```clojure
+(apply map vector [[1 2 3]
+                   [4 5 6]])
+=> [[1 4]
+    [2 5]
+    [3 6]]
+```
+
+## The Turn
+However, transducers can only be applied to a single source...or can they?
+
+I noticed that `map`'s transducer takes a fourth arity with a variable number of collections.  This really surprised me since as far as I could remember there were no transducing contexts that would actually work with multiple sources...or where there?
+
+I looked through `transduce`, `eduction`, `into`, and `sequence` and found that `sequnece` can take multiple collections.
+
+Neither `map'`s fourth arity nor `sequence` taking multiple collections is mentioned in any of the transducer documentation.  The only hint is `sequence`'s docstring mentions multiple collections, but you'd have look at the source for `map` to see the fourth arity.
+
+## The Prestige
+`map` is the only `clojure.core` transducer with a fourth arity, and `sequence` is the only `clojure.core` transducer context to supply multiple sources, so this seems to be of limited use.  However, as long as `map` is the first stage of your transducer pipeline you can use other transducers with a multi-collection invocation of `sequence`:
+
+```clojure
+(sequence (comp (map +)
+                (filter even?))
+          (range 5)
+          (range 5 10)
+          (range 10 15))
+=> (18 24)
+```
+
+This makes me happy!
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