Use infallible conversion to teach From trait

solutions/23_conversions/conversions2.rs

@@ -2,55 +2,38 @@
 // implemented, an implementation of `Into` is automatically provided.
 // You can read more about it in the documentation:
 // https://doc.rust-lang.org/std/convert/trait.From.html
+//
+// Frank the fairy would like to buy some truffles from Grace the gnome, a
+// world-renowned chocolatier. The truffles are priced in GnomeCoin though, and
+// Frank only has FairyCredit. Help Frank by providing a `From` implementation
+// to convert his FairyCredit to GnomeCoin. At the current exchange rate, one
+// FairyCredit is valued at 100 GnomeCoin.
 
 #[derive(Debug)]
-struct Person {
-    name: String,
-    age: u8,
-}
+struct FairyCredit(u32);
 
-// We implement the Default trait to use it as a fallback when the provided
-// string is not convertible into a `Person` object.
-impl Default for Person {
-    fn default() -> Self {
-        Self {
-            name: String::from("John"),
-            age: 30,
-        }
+#[derive(Debug, PartialEq)]
+struct GnomeCoin(u64);
+
+impl From<FairyCredit> for GnomeCoin {
+    fn from(value: FairyCredit) -> Self {
+        Self(value.0 as u64 * 100)
     }
 }
 
-impl From<&str> for Person {
-    fn from(s: &str) -> Self {
-        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 Self::default();
-        };
-
-        if name.is_empty() {
-            return Self::default();
-        }
-
-        let Ok(age) = age.parse() else {
-            return Self::default();
-        };
-
-        Self {
-            name: name.into(),
-            age,
-        }
-    }
-}
+// Note that we shouldn't provide the opposite conversion: from GnomeCoin to
+// FairyCredits. That's because less than 100 GnomeCoins cannot be represented
+// as FairyCredits, which would make the conversion lossy. The `From` trait is
+// only appropriate for infallible and lossless conversions.
 
 fn main() {
     // Use the `from` function.
-    let p1 = Person::from("Mark,20");
-    println!("{p1:?}");
+    let g1 = GnomeCoin::from(FairyCredit(12));
+    println!("{g1:?}");
 
-    // Since `From` is implemented for Person, we are able to use `Into`.
-    let p2: Person = "Gerald,70".into();
-    println!("{p2:?}");
+    // Since `From` is implemented for GnomeCoin, we are able to use `Into`.
+    let g2: GnomeCoin = FairyCredit(9).into();
+    println!("{g2:?}");
 }
 
 #[cfg(test)]
@@ -58,79 +41,14 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_default() {
-        let dp = Person::default();
-        assert_eq!(dp.name, "John");
-        assert_eq!(dp.age, 30);
+    fn test_from() {
+        let g = GnomeCoin::from(FairyCredit(12));
+        assert_eq!(g, GnomeCoin(1200));
     }
 
     #[test]
-    fn test_bad_convert() {
-        let p = Person::from("");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_good_convert() {
-        let p = Person::from("Mark,20");
-        assert_eq!(p.name, "Mark");
-        assert_eq!(p.age, 20);
-    }
-
-    #[test]
-    fn test_bad_age() {
-        let p = Person::from("Mark,twenty");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_comma_and_age() {
-        let p: Person = Person::from("Mark");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_age() {
-        let p: Person = Person::from("Mark,");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name() {
-        let p: Person = Person::from(",1");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name_and_age() {
-        let p: Person = Person::from(",");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name_and_invalid_age() {
-        let p: Person = Person::from(",one");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_trailing_comma() {
-        let p: Person = Person::from("Mike,32,");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_trailing_comma_and_some_string() {
-        let p: Person = Person::from("Mike,32,dog");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
+    fn test_into() {
+        let g: GnomeCoin = FairyCredit(9).into();
+        assert_eq!(g, GnomeCoin(900));
     }
 }