Add some examples for threading macros and a couple of simple transdu…

…cer koans

resources/koans.clj

@@ -259,11 +259,24 @@
                    "Giants"]}]
 
  ["24_macros" {"__" [~(first form)
-                  ~(nth form 2)
-                  form
-                  (drop 2 form)
-                  "Hello, Macros!"
-                  10
-                  '(+ 9 1)]}]
+                     ~(nth form 2)
+                     form
+                     (drop 2 form)
+                     "Hello, Macros!"
+                     10
+                     '(+ 9 1)]}]
+
+ ["25_threading_macros" {"__" [{:a 1}
+                               "Hello world, and moon, and stars"
+                               "String with a trailing space"
+                               6
+                               1
+                               [2 3 4]
+                               12
+                               [1 2 3]]}]
 
+ ["26_transducers" {"__" [[2 4]
+                          [2 4]
+                          [2 4]
+                          6]}]
 ]

src/koans/25_threading_macros.clj

@@ -0,0 +1,70 @@
+(ns koans.25-threading-macros
+  (:require [koan-engine.core :refer :all]))
+
+(def a-list
+  '(1 2 3 4 5))
+
+(def a-list-with-maps
+  '({:a 1} {:a 2} {:a 3}))
+
+(defn function-that-takes-a-map [m a b]
+  (do
+    (println (str "Other unused arguments: " a " " b))
+    (get m :a)))
+
+(defn function-that-takes-a-coll [a b coll]
+  (do
+    (println (str "Other unused arguments: " a " " b))
+    (map :a coll)))
+
+(meditations
+ "We can use thread first for more readable sequential operations"
+ (= __
+    (-> {}
+        (assoc :a 1)))
+
+ "Consider also the case of strings"
+ (= __
+    (-> "Hello world"
+        (str ", and moon")
+        (str ", and stars")))
+
+ "When a function has no arguments to partially apply, just reference it"
+ (= __
+    (-> "String with a trailing space "
+        clojure.string/trim))
+
+ "Most operations that take a scalar value as an argument can be threaded-first"
+ (= __
+    (-> {}
+        (assoc :a 1)
+        (assoc :b 2)
+        (assoc :c {:d 4
+                   :e 5})
+        (update-in [:c :e] inc)
+        (get-in [:c :e])))
+
+ "We can use functions we have written ourselves that follow this pattern"
+ (= __
+    (-> {}
+        (assoc :a 1)
+        (function-that-takes-a-map "hello" "there")))
+
+ "We can also thread last using ->>"
+ (= __
+    (->> [1 2 3]
+         (map inc)))
+
+ "Most operations that take a collection can be threaded-last"
+ (= __
+    (->> a-list
+         (map inc)
+         (filter even?)
+         (into [])
+         (reduce +)))
+
+ "We can use funtions we have written ourselves that follow this pattern"
+ (= __
+    (->> a-list-with-maps
+         (function-that-takes-a-coll "hello" "there")
+         (into []))))

src/koans/26_transducers.clj

@@ -0,0 +1,23 @@
+(ns koans.26-transducers
+  (:require [koan-engine.core :refer :all]))
+
+(def xfms
+  (comp (map inc)
+  (filter even?)))
+
+(meditations
+ "Consider that sequence operations can be used as transducers"
+ (= __
+    (transduce xfms conj [1 2 3]))
+
+ "We can do this eagerly"
+ (= __
+    (into [] xfms [1 2 3]))
+
+ "Or lazily"
+ (= __
+    (sequence xfms [1 2 3]))
+
+ "The transduce function can combine mapping and reduction"
+ (= __
+    (transduce xfms + [1 2 3])))