New library clojure.contrib.types implements algebraic data types

1 files changed, 81 insertions, 0 deletions

diff --git a/src/clojure/contrib/types/examples.clj b/src/clojure/contrib/types/examples.clj
new file mode 100644
index 00000000..26421697
--- /dev/null
+++ b/src/clojure/contrib/types/examples.clj
@@ -0,0 +1,81 @@
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;
+;; Application examples for algebraic data types
+;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(ns clojure.contrib.types.examples
+  (:use clojure.contrib.types))
+
+;
+; A simple tree structure
+;
+(deftype tree
+  empty-tree
+  (leaf value)
+  (node left-tree right-tree))
+
+(def a-tree (node (leaf :a) 
+		  (node (leaf :b)
+			(leaf :c))))
+
+(defn depth
+  [#^tree t]
+  (match t
+    empty-tree  0
+    (leaf n)    1
+    (node l r)  (inc (max (depth l) (depth r)))))
+
+(depth empty-tree)
+(depth a-tree)
+
+;
+; Algebraic data types with multimethods: Haskell-style functors
+;
+(defmulti fmap (fn [f s] (class s)))
+
+; Sequences
+(defmethod fmap clojure.lang.ISeq
+  [f s]
+  (map f s))
+
+; Vectors
+(defmethod fmap clojure.lang.IPersistentVector
+  [f v]
+  (into [] (map f v)))
+
+; Trees
+(defmethod fmap tree
+  [f t]
+  (match t
+    empty-tree  empty-tree
+    (leaf v)    (leaf (f v))
+    (node l r)  (node (fmap f l) (fmap f r))))
+
+(fmap str '(:a :b :c))
+(fmap str [:a :b :c])
+(fmap str a-tree)
+
+;
+; Nonsense examples to illustrate all the features of match
+;
+(deftype foo
+  (bar a b c))
+
+(defn foo-to-int
+  [a-foo]
+  (match a-foo
+    (bar x x x)  x
+    (bar 0 x y)  (+ x y)
+    (bar 1 2 3)  -1
+    (bar a b 1)  (* a b)
+    :else        42))
+
+(foo-to-int (bar 0 0 0))
+(foo-to-int (bar 0 5 6))
+(foo-to-int (bar 1 2 3))
+(foo-to-int (bar 3 3 1))
+(foo-to-int (bar 0 3 1))
+(foo-to-int (bar 10 20 30))