[libgee] Add Gee.Lazy class (call-by-need)

[Maciej Marcin Piechotka <mpiechotka src gnome org>]

commit 92762a63df1cffdb8b370907e2a30296694ea82c
Author: Maciej Piechotka <uzytkownik2 gmail com>
Date:   Sat Apr 30 10:14:19 2011 +0200

    Add Gee.Lazy class (call-by-need)

 gee/Makefile.am |    1 +
 gee/lazy.vala   |   60 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 0 deletions(-)
---
diff --git a/gee/Makefile.am b/gee/Makefile.am
index b1639a3..e7eff2d 100644
--- a/gee/Makefile.am
+++ b/gee/Makefile.am
@@ -28,6 +28,7 @@ libgee_la_SOURCES = \
 	hashset.vala \
 	iterable.vala \
 	iterator.vala \
+	lazy.vala \
 	linkedlist.vala \
 	list.vala \
 	listiterator.vala \
diff --git a/gee/lazy.vala b/gee/lazy.vala
new file mode 100644
index 0000000..36c1d3d
--- /dev/null
+++ b/gee/lazy.vala
@@ -0,0 +1,60 @@
+/* lazy.vala
+ *
+ * Copyright (C) 2011  Maciej Piechotka
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301  USA
+ *
+ * Author:
+ * 	Maciej Piechotka <uzytkownik2 gmail com>
+ */
+
+namespace Gee {
+	public delegate G LazyFunc<G> ();
+}
+
+/**
+ * Represents a lazy value. I.e. value that is computed on demand.
+ */
+public class Gee.Lazy<G> {
+	public Lazy (owned LazyFunc<G> func) {
+		_func = (owned)func;
+	}
+
+	public Lazy.from_value (G item) {
+		_value = item;
+	}
+
+	public void eval () {
+		if (_func != null) {
+			_value = _func ();
+			_func = null;
+		}
+	}
+
+	public new G get () {
+		eval ();
+		return _value;
+	}
+
+	public new G value {
+		get {
+			eval ();
+			return _value;
+		}
+	}
+
+	private LazyFunc<G>? _func;
+	private G? _value;
+}