Initial commit

solutions/23_conversions/from_str.rs

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+// This is similar to the previous `from_into` exercise. But this time, we'll
+// implement `FromStr` and return errors instead of falling back to a default
+// value. Additionally, upon implementing `FromStr`, you can use the `parse`
+// method on strings to generate an object of the implementor type. You can read
+// more about it in the documentation:
+// https://doc.rust-lang.org/std/str/trait.FromStr.html
+
+use std::num::ParseIntError;
+use std::str::FromStr;
+
+#[derive(Debug, PartialEq)]
+struct Person {
+    name: String,
+    age: u8,
+}
+
+// We will use this error type for the `FromStr` implementation.
+#[derive(Debug, PartialEq)]
+enum ParsePersonError {
+    // Incorrect number of fields
+    BadLen,
+    // Empty name field
+    NoName,
+    // Wrapped error from parse::<u8>()
+    ParseInt(ParseIntError),
+}
+
+impl FromStr for Person {
+    type Err = ParsePersonError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut split = s.split(',');
+        let (Some(name), Some(age), None) = (split.next(), split.next(), split.next()) else {
+            //                      ^^^^ there should be no third element
+            return Err(ParsePersonError::BadLen);
+        };
+
+        if name.is_empty() {
+            return Err(ParsePersonError::NoName);
+        }
+
+        let age = age.parse().map_err(ParsePersonError::ParseInt)?;
+
+        Ok(Self {
+            name: name.into(),
+            age,
+        })
+    }
+}
+
+fn main() {
+    let p = "Mark,20".parse::<Person>();
+    println!("{p:?}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ParsePersonError::*;
+
+    #[test]
+    fn empty_input() {
+        assert_eq!("".parse::<Person>(), Err(BadLen));
+    }
+
+    #[test]
+    fn good_input() {
+        let p = "John,32".parse::<Person>();
+        assert!(p.is_ok());
+        let p = p.unwrap();
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 32);
+    }
+
+    #[test]
+    fn missing_age() {
+        assert!(matches!("John,".parse::<Person>(), Err(ParseInt(_))));
+    }
+
+    #[test]
+    fn invalid_age() {
+        assert!(matches!("John,twenty".parse::<Person>(), Err(ParseInt(_))));
+    }
+
+    #[test]
+    fn missing_comma_and_age() {
+        assert_eq!("John".parse::<Person>(), Err(BadLen));
+    }
+
+    #[test]
+    fn missing_name() {
+        assert_eq!(",1".parse::<Person>(), Err(NoName));
+    }
+
+    #[test]
+    fn missing_name_and_age() {
+        assert!(matches!(",".parse::<Person>(), Err(NoName | ParseInt(_))));
+    }
+
+    #[test]
+    fn missing_name_and_invalid_age() {
+        assert!(matches!(
+            ",one".parse::<Person>(),
+            Err(NoName | ParseInt(_)),
+        ));
+    }
+
+    #[test]
+    fn trailing_comma() {
+        assert_eq!("John,32,".parse::<Person>(), Err(BadLen));
+    }
+
+    #[test]
+    fn trailing_comma_and_some_string() {
+        assert_eq!("John,32,man".parse::<Person>(), Err(BadLen));
+    }
+}