added error example

Author: BartMassey

README.md

@@ -13,3 +13,4 @@ in a controlled repository now.
 * `closures.rs`: closure examples
 * `lifetimes.rs`: simple lifetime example
 * `rc`: examples with `RefCell` and `Rc`
+* `error`: example of creating a custom error

error.rs

@@ -0,0 +1,57 @@
+use std::io;
+use std::str::FromStr;
+
+#[derive(Debug, PartialEq, Eq)]
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+#[derive(Debug)]
+enum PointError {
+    CoordDescErr(std::num::ParseIntError),
+    CoordCountErr(usize),
+}
+
+impl std::fmt::Display for PointError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match *self {
+            PointError::CoordDescErr(ref e) => write!(f, "coord parse error: {}", e),
+            PointError::CoordCountErr(n) => write!(f, "expected 2 coords, got {}", n),
+        }
+    }
+}
+
+impl std::error::Error for PointError {}
+
+impl FromStr for Point {
+    type Err = PointError;
+
+    fn from_str(s: &str) -> Result<Self, PointError> {
+        let coords = s
+            // XXX This will accept ((1,2)) etc.
+            .trim_matches(|p| p == '(' || p == ')')
+            .split(',')
+            .map(|s| s.trim().parse().map_err(PointError::CoordDescErr))
+            .collect::<Result<Vec<i32>, PointError>>()?;
+
+        if coords.len() != 2 {
+            return Err(PointError::CoordCountErr(coords.len()));
+        }
+
+        Ok(Point {
+            x: coords[0],
+            y: coords[1],
+        })
+    }
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let p = Point::from_str(
+        &std::env::args()
+            .nth(1)
+            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "missing program argument"))?,
+    )?;
+    println!("{:?}", p);
+    Ok(())
+}