In general, T * T -> T is not valid, but f64 * T is.

Author: Jonathan Pallant

src/measurement.rs

@@ -106,16 +106,6 @@ macro_rules! implement_measurement {
             }
         }
 
-        // Multiplying a `$t` by another `$t` returns the product of those
-        // measurements.
-        impl ::std::ops::Mul<$t> for $t {
-            type Output = Self;
-
-            fn mul(self, rhs: Self) -> Self {
-                Self::from_base_units(self.get_base_units() * rhs.get_base_units())
-            }
-        }
-
         // Multiplying a `$t` by a factor increases (or decreases) that
         // measurement a number of times.
         impl ::std::ops::Mul<f64> for $t {
@@ -126,6 +116,15 @@ macro_rules! implement_measurement {
             }
         }
 
+        // Multiplying by a factor is commutative
+        impl ::std::ops::Mul<$t> for f64 {
+            type Output = $t;
+
+            fn mul(self, rhs: $t) -> $t {
+                rhs * self
+            }
+        }
+
         impl ::std::cmp::Eq for $t { }
         impl ::std::cmp::PartialEq for $t {
             fn eq(&self, other: &Self) -> bool {

src/temperature.rs

@@ -138,6 +138,14 @@ impl ::std::ops::Add<TemperatureDelta> for Temperature {
     }
 }
 
+impl ::std::ops::Add<Temperature> for TemperatureDelta {
+    type Output = Temperature;
+
+    fn add(self, other: Temperature) -> Temperature {
+        other + self
+    }
+}
+
 impl ::std::ops::Sub<TemperatureDelta> for Temperature {
     type Output = Temperature;
 
@@ -167,18 +175,5 @@ impl ::std::cmp::PartialOrd for Temperature {
     }
 }
 
-impl ::std::cmp::Eq for TemperatureDelta {}
-impl ::std::cmp::PartialEq for TemperatureDelta {
-    fn eq(&self, other: &Self) -> bool {
-        self.kelvin_degrees == other.kelvin_degrees
-    }
-}
-
-impl ::std::cmp::PartialOrd for TemperatureDelta {
-    fn partial_cmp(&self, other: &Self) -> Option<::std::cmp::Ordering> {
-        self.kelvin_degrees.partial_cmp(&other.kelvin_degrees)
-    }
-}
-
 implement_display!(Temperature);
-implement_display!(TemperatureDelta);
+implement_measurement!(TemperatureDelta);

src/tests/length_tests.rs

@@ -153,7 +153,9 @@ fn add() {
     let a = Length::from_meters(2.0);
     let b = Length::from_meters(4.0);
     let c = a + b;
+    let d = b + a;
     assert_almost_eq(c.as_meters(), 6.0);
+    assert_eq!(c, d);
 }
 
 #[test]
@@ -166,22 +168,21 @@ fn sub() {
 
 #[test]
 fn mul() {
-    let a = Length::from_meters(2.0);
-    let b = Length::from_meters(4.0);
-    let c = a * b;
-    let d = b * 2.0;
-    assert_almost_eq(c.as_meters(), 8.0);
-    assert_almost_eq(d.as_meters(), 8.0);
+    let a = Length::from_meters(3.0);
+    let b = a * 2.0;
+    let c = 2.0 * a;
+    assert_almost_eq(b.as_meters(), 6.0);
+    assert_eq!(b, c);
 }
 
 #[test]
 fn div() {
     let a = Length::from_meters(2.0);
     let b = Length::from_meters(4.0);
-    let c = a * b;
-    let d = a * 2.0;
-    assert_almost_eq(c.as_meters(), 8.0);
-    assert_almost_eq(d.as_meters(), 4.0);
+    let c = a / b;
+    let d = a / 2.0;
+    assert_almost_eq(c, 0.5);
+    assert_almost_eq(d.as_meters(), 1.0);
 }
 
 #[test]

src/tests/temperature_tests.rs

@@ -40,7 +40,19 @@ fn add() {
     let a = Temperature::from_kelvin(2.0);
     let b = TemperatureDelta::from_kelvin(4.0);
     let c = a + b;
+    let d = b + a;
     assert_almost_eq(c.as_kelvin(), 6.0);
+    assert_eq!(c, d);
+}
+
+#[test]
+fn add2() {
+    let a = TemperatureDelta::from_kelvin(2.0);
+    let b = TemperatureDelta::from_kelvin(4.0);
+    let c = a + b;
+    let d = b + a;
+    assert_almost_eq(c.as_kelvin(), 6.0);
+    assert_eq!(c, d);
 }
 
 #[test]
@@ -59,6 +71,23 @@ fn sub2() {
     assert_almost_eq(c.as_kelvin(), 25.0);
 }
 
+#[test]
+fn sub3() {
+    let a = TemperatureDelta::from_fahrenheit(180.0);
+    let b = TemperatureDelta::from_celsius(75.0);
+    let c = a - b;
+    assert_almost_eq(c.as_kelvin(), 25.0);
+}
+
+#[test]
+fn mul() {
+    let a = TemperatureDelta::from_celsius(5.0);
+    let b = a * 2.0;
+    let c = 2.0 * a;
+    assert_almost_eq(b.as_celsius(), 10.0);
+    assert_eq!(b, c);
+}
+
 #[test]
 fn eq() {
     let a = Temperature::from_kelvin(2.0);

src/tests/volume_tests.rs

@@ -215,7 +215,9 @@ fn add() {
     let a = Volume::from_litres(2.0);
     let b = Volume::from_litres(4.0);
     let c = a + b;
+    let d = b + a;
     assert_almost_eq(c.as_litres(), 6.0);
+    assert_eq!(c, d);
 }
 
 #[test]
@@ -228,12 +230,11 @@ fn sub() {
 
 #[test]
 fn mul() {
-    let a = Volume::from_litres(2.0);
-    let b = Volume::from_litres(4.0);
-    let c = a * b;
-    let d = a * 2.0;
-    assert_almost_eq(c.as_litres(), 8.0);
-    assert_almost_eq(d.as_litres(), 4.0);
+    let a = Volume::from_litres(3.0);
+    let b = a * 2.0;
+    let c = 2.0 * a;
+    assert_almost_eq(b.as_litres(), 6.0);
+    assert_eq!(b, c);
 }
 
 #[test]

src/tests/weight_tests.rs

@@ -171,7 +171,9 @@ fn add() {
     let a = Weight::from_kilograms(2.0);
     let b = Weight::from_kilograms(4.0);
     let c = a + b;
+    let d = b + a;
     assert_almost_eq(c.as_kilograms(), 6.0);
+    assert_eq!(c, d);
 }
 
 #[test]
@@ -184,12 +186,11 @@ fn sub() {
 
 #[test]
 fn mul() {
-    let a = Weight::from_kilograms(2.0);
-    let b = Weight::from_kilograms(4.0);
-    let c = a * b;
-    let d = a * 2.0;
-    assert_almost_eq(c.as_kilograms(), 8.0);
-    assert_almost_eq(d.as_kilograms(), 4.0);
+    let a = Weight::from_kilograms(3.0);
+    let b = a * 2.0;
+    let c = 2.0 * a;
+    assert_almost_eq(b.as_kilograms(), 6.0);
+    assert_eq!(b, c);
 }
 
 #[test]