Add monad iface test

Author: marijnh

src/test/run-pass/monad.rs

@@ -0,0 +1,31 @@
+iface monad<A> {
+    fn bind<B>(fn(A) -> self<B>) -> self<B>;
+}
+
+impl <A> of monad<A> for [A] {
+    fn bind<B>(f: fn(A) -> [B]) -> [B] {
+        let r = [];
+        for elt in self { r += f(elt); }
+        r
+    }
+}
+
+impl <A> of monad<A> for option<A> {
+    fn bind<B>(f: fn(A) -> option<B>) -> option<B> {
+        alt self {
+          some(a) { f(a) }
+          none { none }
+        }
+    }
+}
+
+fn transform(x: option<int>) -> option<str> {
+    x.bind {|n| some(n + 1)}.bind {|n| some(int::str(n))}
+}
+
+fn main() {
+    assert transform(some(10)) == some("11");
+    assert transform(none) == none;
+    assert ["hi"].bind {|x| [x, x + "!"]}.bind {|x| [x, x + "?"]} ==
+        ["hi", "hi?", "hi!", "hi!?"];
+}