Allow adding zero regardless of units (#310)

core/src/num/unit.rs

@@ -184,6 +184,9 @@ impl Value {
 	}
 
 	pub(crate) fn add<I: Interrupt>(self, rhs: Self, int: &I) -> FResult<Self> {
+		if rhs.is_zero(int)? {
+			return Ok(self);
+		}
 		let scale_factor = Unit::compute_scale_factor(&rhs.unit, &self.unit, int)?;
 		let scaled = Exact::new(rhs.value, rhs.exact)
 			.mul(&scale_factor.scale_1.apply(Dist::from), int)?
@@ -241,19 +244,7 @@ impl Value {
 	}
 
 	pub(crate) fn sub<I: Interrupt>(self, rhs: Self, int: &I) -> FResult<Self> {
-		let scale_factor = Unit::compute_scale_factor(&rhs.unit, &self.unit, int)?;
-		let scaled = Exact::new(rhs.value, rhs.exact)
-			.mul(&scale_factor.scale_1.apply(Dist::from), int)?
-			.div(&scale_factor.scale_2.apply(Dist::from), int)?;
-		let value = Exact::new(self.value, self.exact).add(&-scaled, int)?;
-		Ok(Self {
-			value: value.value,
-			unit: self.unit,
-			exact: self.exact && rhs.exact && value.exact,
-			base: self.base,
-			format: self.format,
-			simplifiable: self.simplifiable,
-		})
+		self.add(-rhs, int)
 	}
 
 	pub(crate) fn div<I: Interrupt>(self, rhs: Self, int: &I) -> FResult<Self> {

core/tests/integration_tests.rs

@@ -6033,3 +6033,11 @@ fn test_words() {
 	test_eval_simple("1000000000000000000000 to words", "one sextillion");
 	test_eval_simple("1000000000000000000000000 to words", "one septillion");
 }
+
+#[test]
+fn test_plus_zero_ignore_units() {
+	test_eval("4m + 0", "4 m");
+	test_eval("4m + 0kg", "4 m");
+	test_eval("4m + (sin pi) kg", "4 m");
+	expect_error("4m + (sin (pi/2)) kg", Some("cannot convert from kg to m: units 'kilogram' and 'meter' are incompatible"));
+}