feature: add advanced_errs2

New exercise to demonstrate traits that make it easier for other code
to consume our custom error types.

exercises/advanced_errors/advanced_errs2.rs

@@ -0,0 +1,203 @@
+// advanced_errs2.rs
+
+// This exercise demonstrates a few traits that are useful for custom error
+// types to implement, especially so that other code can consume the custom
+// error type more usefully.
+
+// Make this compile, and make the tests pass!
+// Execute `rustlings hint advanced_errs2` for hints.
+
+// Steps:
+// 1. Implement a missing trait so that `main()` will compile.
+// 2. Complete the partial implementation of `From` for
+//    `ParseClimateError`.
+// 3. Handle the missing error cases in the `FromStr` implementation for
+//    `Climate`.
+// 4. Complete the partial implementation of `Display` for
+//    `ParseClimateError`.
+
+// I AM NOT DONE
+
+use std::error::Error;
+use std::fmt::{self, Display, Formatter};
+use std::num::{ParseFloatError, ParseIntError};
+use std::str::FromStr;
+
+// This is the custom error type that we will be using for the parser for
+// `Climate`.
+#[derive(Debug, PartialEq)]
+enum ParseClimateError {
+    Empty,
+    BadLen,
+    NoCity,
+    ParseInt(ParseIntError),
+    ParseFloat(ParseFloatError),
+}
+
+// This `From` implementation allows the `?` operator to work on
+// `ParseIntError` values.
+impl From<ParseIntError> for ParseClimateError {
+    fn from(e: ParseIntError) -> Self {
+        Self::ParseInt(e)
+    }
+}
+
+// This `From` implementation allows the `?` operator to work on
+// `ParseFloatError` values.
+impl From<ParseFloatError> for ParseClimateError {
+    fn from(e: ParseFloatError) -> Self {
+        // TODO: Complete this function
+    }
+}
+
+// TODO: Implement a missing trait so that `main()` below will compile. It
+// is not necessary to implement any methods inside the missing trait.
+
+// The `Display` trait allows for other code to obtain the error formatted
+// as a user-visible string.
+impl Display for ParseClimateError {
+    // TODO: Complete this function so that it produces the correct strings
+    // for each error variant.
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        // Imports the variants to make the following code more compact.
+        use ParseClimateError::*;
+        match self {
+            NoCity => write!(f, "no city name"),
+            ParseFloat(e) => write!(f, "error parsing temperature: {}", e),
+            _ => write!(f, "unhandled error!"),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq)]
+struct Climate {
+    city: String,
+    year: u32,
+    temp: f32,
+}
+
+// Parser for `Climate`.
+// 1. Split the input string into 3 fields: city, year, temp.
+// 2. Return an error if the string is empty or has the wrong number of
+//    fields.
+// 3. Return an error if the city name is empty.
+// 4. Parse the year as a `u32` and return an error if that fails.
+// 5. Parse the temp as a `f32` and return an error if that fails.
+// 6. Return an `Ok` value containing the completed `Climate` value.
+impl FromStr for Climate {
+    type Err = ParseClimateError;
+    // TODO: Complete this function by making it handle the missing error
+    // cases.
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let v: Vec<_> = s.split(',').collect();
+        let (city, year, temp) = match &v[..] {
+            [city, year, temp] => (city.to_string(), year, temp),
+            _ => return Err(ParseClimateError::BadLen),
+        };
+        let year: u32 = year.parse()?;
+        let temp: f32 = temp.parse()?;
+        Ok(Climate { city, year, temp })
+    }
+}
+
+// Don't change anything below this line (other than to enable ignored
+// tests).
+
+fn main() -> Result<(), Box<dyn Error>> {
+    println!("{:?}", "Hong Kong,1999,25.7".parse::<Climate>()?);
+    println!("{:?}", "".parse::<Climate>()?);
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn test_empty() {
+        let res = "".parse::<Climate>();
+        assert_eq!(res, Err(ParseClimateError::Empty));
+        assert_eq!(res.unwrap_err().to_string(), "empty input");
+    }
+    #[test]
+    fn test_short() {
+        let res = "Boston,1991".parse::<Climate>();
+        assert_eq!(res, Err(ParseClimateError::BadLen));
+        assert_eq!(res.unwrap_err().to_string(), "incorrect number of fields");
+    }
+    #[test]
+    fn test_long() {
+        let res = "Paris,1920,17.2,extra".parse::<Climate>();
+        assert_eq!(res, Err(ParseClimateError::BadLen));
+        assert_eq!(res.unwrap_err().to_string(), "incorrect number of fields");
+    }
+    #[test]
+    fn test_no_city() {
+        let res = ",1997,20.5".parse::<Climate>();
+        assert_eq!(res, Err(ParseClimateError::NoCity));
+        assert_eq!(res.unwrap_err().to_string(), "no city name");
+    }
+    #[test]
+    fn test_parse_int_neg() {
+        let res = "Barcelona,-25,22.3".parse::<Climate>();
+        assert!(matches!(res, Err(ParseClimateError::ParseInt(_))));
+        let err = res.unwrap_err();
+        if let ParseClimateError::ParseInt(ref inner) = err {
+            assert_eq!(
+                err.to_string(),
+                format!("error parsing year: {}", inner.to_string())
+            );
+        } else {
+            unreachable!();
+        };
+    }
+    #[test]
+    fn test_parse_int_bad() {
+        let res = "Beijing,foo,15.0".parse::<Climate>();
+        assert!(matches!(res, Err(ParseClimateError::ParseInt(_))));
+        let err = res.unwrap_err();
+        if let ParseClimateError::ParseInt(ref inner) = err {
+            assert_eq!(
+                err.to_string(),
+                format!("error parsing year: {}", inner.to_string())
+            );
+        } else {
+            unreachable!();
+        };
+    }
+    #[test]
+    fn test_parse_float() {
+        let res = "Manila,2001,bar".parse::<Climate>();
+        assert!(matches!(res, Err(ParseClimateError::ParseFloat(_))));
+        let err = res.unwrap_err();
+        if let ParseClimateError::ParseFloat(ref inner) = err {
+            assert_eq!(
+                err.to_string(),
+                format!("error parsing temperature: {}", inner.to_string())
+            );
+        } else {
+            unreachable!();
+        };
+    }
+    #[test]
+    fn test_parse_good() {
+        let res = "Munich,2015,23.1".parse::<Climate>();
+        assert_eq!(
+            res,
+            Ok(Climate {
+                city: "Munich".to_string(),
+                year: 2015,
+                temp: 23.1,
+            })
+        );
+    }
+    #[test]
+    #[ignore]
+    fn test_downcast() {
+        let res = "São Paulo,-21,28.5".parse::<Climate>();
+        assert!(matches!(res, Err(ParseClimateError::ParseInt(_))));
+        let err = res.unwrap_err();
+        let inner: Option<&(dyn Error + 'static)> = err.source();
+        assert!(inner.is_some());
+        assert!(inner.unwrap().is::<ParseIntError>());
+    }
+}

info.toml

@@ -994,3 +994,35 @@ it to the error type of the return type of the surrounding function.
 Hint: You will need to write another implementation of `From` that has a
 different input type.
 """
+
+[[exercises]]
+name = "advanced_errs2"
+path = "exercises/advanced_errors/advanced_errs2.rs"
+mode = "test"
+hint = """
+This exercise demonstrates a few traits that are useful for custom error
+types to implement. These traits make it easier for other code to consume
+the custom error type.
+
+Follow the steps in the comment near the top of the file. You will have to
+supply a missing trait implementation, and complete a few incomplete ones.
+
+You may find these pages to be helpful references:
+https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/define_error_type.html
+https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/boxing_errors.html
+https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/wrap_error.html
+
+Hint: What trait must our error type have for `main()` to return the return
+type that it returns?
+
+Another hint: It's not necessary to implement any methods inside the missing
+trait. (Some methods have default implementations that are supplied by the
+trait.)
+
+Another hint: Consult the tests to determine which error variants (and which
+error message text) to produce for certain error conditions.
+
+Challenge: There is one test that is marked `#[ignore]`. Can you supply the
+missing code that will make it pass? You may want to consult the standard
+library documentation for a certain trait for more hints.
+"""