Auto generate argument enums

The argparse module we use for parsing events expects an enum in order
to convey what subcommand has been supplied as an argument. Such an enum
also needs to implement str::FromStr and, preferably, fmt::Display.
Manually writing down those definitions is error-prone, tedious, and
adds no value -- only lines of code.
As it turns out the proper definitions can be generated with relative
easy with a declarative macro, making the code much more concise. Hence,
with this change we use a newly introduced macro for generating those
enums.

1 files changed, 79 insertions, 0 deletions

diff --git a/nitrocli/src/arg_util.rs b/nitrocli/src/arg_util.rs
new file mode 100644
index 0000000..6dd14ed
--- /dev/null
+++ b/nitrocli/src/arg_util.rs
@@ -0,0 +1,79 @@
+// arg_util.rs
+
+// *************************************************************************
+// * Copyright (C) 2019 Daniel Mueller (deso@posteo.net)                   *
+// *                                                                       *
+// * This program is free software: you can redistribute it and/or modify  *
+// * it under the terms of the GNU General Public License as published by  *
+// * the Free Software Foundation, either version 3 of the License, or     *
+// * (at your option) any later version.                                   *
+// *                                                                       *
+// * This program 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 General Public License for more details.                          *
+// *                                                                       *
+// * You should have received a copy of the GNU General Public License     *
+// * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+// *************************************************************************
+
+/// A macro for generating an enum with a set of simple (i.e., no
+/// parameters) variants and their textual representations.
+// TODO: Right now we hard code the derives we create. We may want to
+//       make this set configurable.
+macro_rules! Enum {
+  ( $name:ident, [ $( $var:ident => $str:expr ), *] ) => {
+    #[derive(Clone, Copy, Debug, PartialEq)]
+    #[allow(unused)]
+    pub enum $name {
+      $(
+        $var,
+      )*
+    }
+
+    impl AsRef<str> for $name {
+      fn as_ref(&self) -> &'static str {
+        match *self {
+          $(
+            $name::$var => $str,
+          )*
+        }
+      }
+    }
+
+    impl ::std::fmt::Display for $name {
+      fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+        write!(f, "{}", self.as_ref())
+      }
+    }
+
+    impl ::std::str::FromStr for $name {
+      type Err = ();
+
+      fn from_str(s: &str) -> ::std::result::Result<Self, Self::Err> {
+        match s {
+          $(
+            $str => Ok($name::$var),
+          )*
+          _ => Err(()),
+        }
+      }
+    }
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  Enum! {Command, [
+    Var1 => "var1",
+    Var2 => "2",
+    Var3 => "crazy"
+  ]}
+
+  #[test]
+  fn text_representations() {
+    assert_eq!(Command::Var1.as_ref(), "var1");
+    assert_eq!(Command::Var2.as_ref(), "2");
+    assert_eq!(Command::Var3.as_ref(), "crazy");
+  }
+}