feat(macros): add macro to create custom enums to use as types

examples/13a_EnumValuesAutomatic.rs

@@ -0,0 +1,39 @@
+// You can use clap's value_t! macro with a custom enum by implementing the std::str::FromStr
+// trait which is very straight forward. There are two ways to do this, for simple enums you
+// can use clap's simple_enum! macro, but if you require additional functionality you can
+// create and implement the trait manually.
+//
+// In the following example we will create an enum with 4 values, assign a positional argument
+// that accepts only one of those values, and use clap to parse the argument.
+//
+// Start with bringing the trait into scope.
+
+// Add clap like normal
+#[macro_use]
+extern crate clap;
+
+use clap::{App, Arg};
+
+// Define your enum, the simple_num! macro takes a enum name followed by => and each value
+// separated by a ','
+simple_enum!{ Vals => Foo, Bar, Baz, Qux }
+
+fn main() {
+    // Create the application like normal
+    let m = App::new("myapp")
+                    // Use a single positional argument that is required
+                    .arg(Arg::from_usage("<type> 'The type to use'")
+                            // Define the list of possible values
+                            .possible_values(vec!["Foo", "Bar", "Baz", "Qux"]))
+                    .get_matches();
+
+    let t = value_t_or_exit!(m.value_of("type"), Vals);
+
+    // Now we can use our enum like normal.
+    match t {
+        Vals::Foo => println!("Found a Foo"),
+        Vals::Bar => println!("Found a Bar"),
+        Vals::Baz => println!("Found a Baz"),
+        Vals::Qux => println!("Found a Qux")
+    }
+}

examples/13b_EnumValuesManual.rs

@@ -1,5 +1,5 @@
-// You can use clap's value_t! macro with a custom enum by implementing the std::str::FromStr
-// trait which is very straight forward.
+// If you require more complex configuration than simple_enum! provides, you can implement the
+// trait manually, as in the following example.
 //
 // In the following example we will create an enum with 4 values, assign a positional argument
 // that accepts only one of those values, and use clap to parse the argument.

src/macros.rs

@@ -213,3 +213,45 @@ macro_rules! value_t_or_exit {
 		}
 	};
 }
+
+/// Convenience macro generated a simple enum with variants to be used as a type when parsing
+/// arguments.
+///
+/// # Example
+///
+/// ```no_run
+/// # #[macro_use]
+/// # extern crate clap;
+/// # use clap::{App, Arg};
+/// simple_enum!{Foo => Bar, Baz, Qux}
+/// // Foo enum can now be used via Foo::Bar, or Foo::Baz, etc
+/// // and implements std::str::FromStr to use with the value_t! macros
+/// fn main() {
+/// 	let m = App::new("app")
+///					.arg(Arg::from_usage("<foo> 'the foo'")
+///						.possible_values(vec!["Bar", "Baz", "Qux"]))
+///					.get_matches();
+/// 	let f = value_t_or_exit!(m.value_of("foo"), Foo);
+///
+///		// Use f like any other Foo variant...
+/// }
+/// ```
+#[macro_export]
+macro_rules! simple_enum {
+	($e:ident => $($v:ident),+) => {
+		enum $e {
+			$($v),+
+		}
+
+		impl<'a> std::str::FromStr for $e {
+			type Err = &'a str;
+
+			fn from_str(s: &str) -> Result<Self,<Self as std::str::FromStr>::Err> {
+				match s {
+					$(stringify!($v) => Ok($e::$v),)+
+					_                => Err("no match")
+				}
+			}
+		}
+	};
+}