add section about delegates

hands-on_dlang.md

@@ -67,6 +67,10 @@
             - [Other templates](#other-templates)
             - [Template value parameters](#template-value-parameters)
             - [Other template parameters](#other-template-parameters)
+        - [Delegates](#delegates)
+            - [Functions as arguments](#functions-as-arguments)
+            - [Local functions with context](#local-functions-with-context)
+            - [Anonymous functions and lambdas](#anonymous-functions-and-lambdas)
 
 ## Setup
 
@@ -1003,3 +1007,58 @@ void print(Args...)(Args args) {
 print(42, 3.5f, "foo");
 print(); // also valid, prints nothing
 ```
+
+### Delegates
+
+#### Functions as arguments
+
+Global functions can be referenced using the `function` type:
+
+```D
+void doSomething(int function(int, int) doer) {
+    // call passed function
+    doer(5,5);
+}
+
+auto add(int a, int b) {
+    return a + b;
+}
+
+doSomething(&add);
+```
+
+#### Local functions with context
+
+To reference member functions or local functions, `delegate`s have to be used. You
+can create _closures_ with this:
+
+```D
+auto foo() {
+    int x = 42;
+    auto local() {
+        return x;
+    }
+    return &local;
+}
+
+auto f = foo();
+static assert(is(typeof(f) == int delegate() pure nothrow @nogc @safe));
+writeln(f()); // 42
+```
+
+#### Anonymous functions and lambdas
+You can write anonymous functions and lambdas like this:
+
+```D
+auto add = (int lhs, int rhs) {
+    return lhs + rhs;
+};
+
+auto lambdaAdd = (int lhs, int rhs) => lhs + rhs;
+```
+
+These are often passed as template arguments in the functional parts of Phobos:
+
+```D
+[1, 2, 3].reduce!((a, b) => a + b)();
+```