doc/manual: put back the manual in the main repository

[nit.git] / doc / manual / genericity.md

diff --git a/doc/manual/genericity.md b/doc/manual/genericity.md
new file mode 100644 (file)
index 0000000..0b96f92
--- /dev/null
+++ b/doc/manual/genericity.md
@@ -0,0 +1,27 @@
+# Generic Classes
+
+Generic classes are defined with formal generic parameters
+declared within brackets. Formal generic parameters can then be used as
+a regular type inside the class. Generic classes must always be
+qualified when used.
+
+~~~
+class Pair[E]
+    var first: E
+    var second: E
+    fun is_same: Bool
+    do
+        return self.first == self.second
+    end
+end
+var p1 = new Pair[Int](1, 2)
+print p1.second * 10 # outputs "20"
+print p1.is_same # outputs "false"
+var p2 = new Pair[String]("hello", "world")
+p2.first = "world"
+print p2.is_same # outputs "true"
+~~~
+
+Unlike many object-oriented languages, generic classes in Nit yield a
+kind of sub-typing. For example, `Pair[Int]` is a subtype of
+`Pair[Object]`.