Simplify story and increase difficulty

Conversion between Celsius and Fahrenheit should be understandable to
most. Inverting the formula is still not very hard, but a little harder
than only multiplying by 100.

exercises/23_conversions/conversions2.rs

@@ -3,50 +3,52 @@
 // 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
+// Representing units of measurements with separate types is a common practice.
-// world-renowned chocolatier. The truffles are priced in GnomeCoin though, and
+// It avoids accidentally mixing up values of different units of measurement.
-// 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 Celsius(f64);
-struct FairyCredit(u32);
 
-#[derive(Debug, PartialEq)]
+struct Fahrenheit(f64);
-struct GnomeCoin(u64);
 
-impl From<FairyCredit> for GnomeCoin {
+impl From<Celsius> for Fahrenheit {
-    // TODO: implement From<FairyCredit> for GnomeCoin
+    // TODO: Convert Celsius to Fahrenheit. Don't worry about floating-point
+    // precision. The formula is: F = C * 1.8 + 32
 }
 
-// Note that we shouldn't provide the opposite conversion: from GnomeCoin to
+impl From<Fahrenheit> for Celsius {
-// FairyCredits. That's because less than 100 GnomeCoins cannot be represented
+    // TODO: Convert Fahrenheit to Celsius.
-// 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.
+    // You can optionally experiment here.
-    let g1 = GnomeCoin::from(FairyCredit(12));
-    println!("{g1:?}");
-
-    // Since `From` is implemented for GnomeCoin, we are able to use `Into`.
-    let g2: GnomeCoin = FairyCredit(9).into();
-    println!("{g2:?}");
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    const CASES: [(f64, f64); 6] = [
+        (-50.0, -58.0),
+        (0.0, 32.0),
+        (20.0, 68.0),
+        (100.0, 212.0),
+        (400.0, 752.0),
+        (1000.0, 1832.0),
+    ];
+
     #[test]
-    fn test_from() {
+    fn celsius_to_fahrenheit() {
-        let g = GnomeCoin::from(FairyCredit(12));
+        for (celsius, fahrenheit) in CASES {
-        assert_eq!(g, GnomeCoin(1200));
+            let Fahrenheit(actual) = Celsius(celsius).into();
+            assert_eq!(actual.round(), fahrenheit);
+        }
     }
 
     #[test]
-    fn test_into() {
+    fn fahrenheit_to_celsius() {
-        let g: GnomeCoin = FairyCredit(9).into();
+        for (celsius, fahrenheit) in CASES {
-        assert_eq!(g, GnomeCoin(900));
+            let Celsius(actual) = Fahrenheit(fahrenheit).into();
+            assert_eq!(actual.round(), celsius);
+        }
     }
 }

rustlings-macros/info.toml

@@ -1171,9 +1171,8 @@ Use the `as` operator to cast one of the operands in the last line of the
 name = "conversions2"
 dir = "23_conversions"
 hint = """
-Implement From<FairyCredit> for GnomeCoin. Check the documentation of `From` to
+For the conversion from Fahrenheit to Celsius, you have to determine the formula
-learn about its required items:
+yourself. Don't forget the order of operations!"""
-https://doc.rust-lang.org/std/convert/trait.From.html"""
 
 [[exercises]]
 name = "conversions3"

solutions/23_conversions/conversions2.rs

@@ -3,52 +3,55 @@
 // 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
+// Representing units of measurements with separate types is a common practice.
-// world-renowned chocolatier. The truffles are priced in GnomeCoin though, and
+// It avoids accidentally mixing up values of different units of measurement.
-// 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 Celsius(f64);
-struct FairyCredit(u32);
 
-#[derive(Debug, PartialEq)]
+struct Fahrenheit(f64);
-struct GnomeCoin(u64);
 
-impl From<FairyCredit> for GnomeCoin {
+impl From<Celsius> for Fahrenheit {
-    fn from(value: FairyCredit) -> Self {
+    fn from(Celsius(celsius): Celsius) -> Self {
-        Self(value.0 as u64 * 100)
+        Fahrenheit(celsius * 1.8 + 32.0)
     }
 }
 
-// Note that we shouldn't provide the opposite conversion: from GnomeCoin to
+impl From<Fahrenheit> for Celsius {
-// FairyCredits. That's because less than 100 GnomeCoins cannot be represented
+    fn from(Fahrenheit(fahrenheit): Fahrenheit) -> Self {
-// as FairyCredits, which would make the conversion lossy. The `From` trait is
+        Celsius((fahrenheit - 32.0) / 1.8)
-// only appropriate for infallible and lossless conversions.
+    }
+}
 
 fn main() {
-    // Use the `from` function.
+    // You can optionally experiment here.
-    let g1 = GnomeCoin::from(FairyCredit(12));
-    println!("{g1:?}");
-
-    // Since `From` is implemented for GnomeCoin, we are able to use `Into`.
-    let g2: GnomeCoin = FairyCredit(9).into();
-    println!("{g2:?}");
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    const CASES: [(f64, f64); 6] = [
+        (-50.0, -58.0),
+        (0.0, 32.0),
+        (20.0, 68.0),
+        (100.0, 212.0),
+        (400.0, 752.0),
+        (1000.0, 1832.0),
+    ];
+
     #[test]
-    fn test_from() {
+    fn celsius_to_fahrenheit() {
-        let g = GnomeCoin::from(FairyCredit(12));
+        for (celsius, fahrenheit) in CASES {
-        assert_eq!(g, GnomeCoin(1200));
+            let Fahrenheit(actual) = Celsius(celsius).into();
+            assert_eq!(actual.round(), fahrenheit);
+        }
     }
 
     #[test]
-    fn test_into() {
+    fn fahrenheit_to_celsius() {
-        let g: GnomeCoin = FairyCredit(9).into();
+        for (celsius, fahrenheit) in CASES {
-        assert_eq!(g, GnomeCoin(900));
+            let Celsius(actual) = Fahrenheit(fahrenheit).into();
+            assert_eq!(actual.round(), celsius);
+        }
     }
 }