Add trait for ergonomic Options-as-Errors

src/errors.rs

@@ -0,0 +1,57 @@
+use std::fmt::{Debug, Display};
+use std::result::Result as StdResult;
+use failure::{Error, err_msg};
+
+/// A handy alias for `Result` that carries a generic error type.
+pub type Result<T> = StdResult<T, Error>;
+
+/// Treat `Option::None` as Error with context
+pub trait NoneErrorContext<T> {
+    /// Convert Option to Result by annotating what None means
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # extern crate quicli;
+    /// # use quicli::prelude::*;
+    /// # fn main() { assert!(run().is_err()); }
+    /// # fn run() -> Result<()> {
+    /// let xs = vec!["lorem", "ipsum"];
+    /// let x = xs.get(66); // will return None
+    /// let result = x.none_means("index not found")?;
+    /// # Ok(()) }
+    /// ```
+    fn none_means<E: Display + Debug + Sync + Send + 'static>(self, explanation: E) -> Result<T>;
+}
+
+impl<T> NoneErrorContext<T> for Option<T> {
+    fn none_means<E: Display + Debug + Sync + Send + 'static>(self, explanation: E) -> Result<T> {
+        match self {
+            Some(x) => Ok(x),
+            None => Err(err_msg(explanation)),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn none_means_error_message() {
+        use prelude::*;
+        run().unwrap();
+
+        fn run() -> Result<()> {
+            let xs = vec!["lorem", "ipsum"];
+            let x = xs.get(66); // will return None
+
+            let result = x.none_means("index not found");
+
+            assert!(result.is_err());
+            if let Err(error) = result {
+                assert_eq!(error.to_string(), "index not found".to_string());
+            }
+
+            Ok(())
+        }
+    }
+}

src/lib.rs

@@ -20,6 +20,7 @@ extern crate env_logger;
 
 pub mod fs;
 mod main_macro;
+mod errors;
 
 mod reexports {
     #[doc(hidden)] pub use serde_derive::*;
@@ -46,8 +47,7 @@ mod reexports {
 pub mod prelude {
     pub use reexports::*;
 
-    /// A handy alias for `Result` that carries a generic error type.
-    pub type Result<T> = ::std::result::Result<T, ::failure::Error>;
+    pub use errors::{Result, NoneErrorContext};
 
     pub use fs::{read_file, write_to_file};