feat: add abs function

src/nodes/eval.rs

@@ -153,6 +153,8 @@ impl Function {
             Self::ArcTangent => Float::atan_mut(n),
             Self::Secant => Float::sec_mut(n),
             Self::CoSecant => Float::csc_mut(n),
+
+            Self::Abs => Float::abs_mut(n),
         }
     }
 }

src/nodes/mod.rs

@@ -86,4 +86,6 @@ pub enum Function {
     ArcTangent,
     Secant,
     CoSecant,
+
+    Abs,
 }

src/nodes/parse.rs

@@ -50,6 +50,9 @@ impl FromStr for Function {
             "atan" | "arctan" => Ok(Self::ArcTangent),
             "sec" => Ok(Self::Secant),
             "csc" => Ok(Self::CoSecant),
+
+            "abs" => Ok(Self::Abs),
+
             _ => Err(s.to_owned()),
         }
     }

src/nodes/tests.rs

@@ -63,3 +63,8 @@ fn operator_after_parenthesis() {
     assert_eq!(eval("(0.2)20^2"), 80.);
     assert_eq!(eval("(0.2)20^2/2 - (0.2)10^2/2"), 30.);
 }
+
+#[test]
+fn abs() {
+    assert_eq!(eval("abs(-1)"), 1.);
+}