Add a chapter on custom formatting to the stagin area

examples/staging/custom-fmt/input.md

@@ -0,0 +1,12 @@
+The `std::fmt` module defines a number of traits each for one of the formatting
+argument types.
+
+This example demonstrates how to implement the `Show` trait used for
+the default formatting argument: `{}`. Implemention of the other traits
+is exactly the same - the `fmt` function is all that is needed.
+
+See the docs for
+[full list](http://doc.rust-lang.org/std/fmt/index.html#formatting-traits)
+of formatting traits.
+
+{print-custom.play}

examples/staging/custom-fmt/print-custom.rs

@@ -0,0 +1,33 @@
+use std::fmt;
+use std::num::abs;
+
+struct City {
+    name: &'static str,
+    // Latitude
+    lat: f32,
+    // Longitude
+    lon: f32,
+}
+
+impl fmt::Show for City {
+    // Will output City's name as well as its location in a nice format
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let lat_c = if self.lat >= 0.0 {'N'} else {'S'};
+        let lon_c = if self.lon >= 0.0 {'E'} else {'W'};
+
+        // `write!` writes formatted string into a `io::Writer`
+        // and handles `Result` propagation
+        write!(f, "{} ({:.3f}° {}, {:.3f}° {})",
+               self.name, abs(self.lat), lat_c, abs(self.lon), lon_c)
+    }
+}
+
+fn main() {
+    for city in [
+        City{name: "Vancouver", lat: 49.25, lon: -123.1,},
+        City{name: "Dublin", lat: 53.347778, lon: -6.259722,},
+        City{name: "Oslo", lat: 59.95, lon: 10.75,},
+    ].iter() {
+        println!("{}", city);
+    }
+}

examples/structure.json

@@ -107,7 +107,8 @@
     { "id": "rand", "title": "Random", "children": null  },
     { "id": "simd", "title": "SIMD", "children": null  },
     { "id": "test", "title": "Testing", "children": null  },
-    { "id": "unsafe", "title": "Unsafe operations", "children": null  }
+    { "id": "unsafe", "title": "Unsafe operations", "children": null  },
+    { "id": "custom-fmt", "title": "Custom formatters", "children": null }
   ] },
   { "id": "todo", "title": "TODO", "children": [
     { "id": "arg", "title": "Program arguments", "children": null },