Add an "if" function

If the first argument is true, returns the second argument, otherwise, return the third

src/function/builtin.rs

@@ -121,6 +121,15 @@ pub fn builtin_function(identifier: &str) -> Option<Function> {
                 Ok(Value::Float(max_float))
             }
         })),
+        "if" => Some(Function::new(|argument| {
+            if let [condition, if_true, if_false] = &argument.as_fixed_len_tuple(3)?[..] {
+                return Ok(if condition.as_boolean()? { if_true } else { if_false }.clone())
+            }
+            Err(EvalexprError::type_error(
+                argument.clone(),
+                vec![ValueType::Boolean, ValueType::Empty, ValueType::Empty],
+            ))
+        })),
         "len" => Some(Function::new(|argument| {
             if let Ok(subject) = argument.as_string() {
                 Ok(Value::from(subject.len() as i64))

src/lib.rs

@@ -327,6 +327,7 @@
 //! | `floor`              | 1               | Numeric                | Returns the largest integer less than or equal to a number |
 //! | `round`              | 1               | Numeric                | Returns the nearest integer to a number. Rounds half-way cases away from 0.0 |
 //! | `ceil`               | 1               | Numeric                | Returns the smallest integer greater than or equal to a number |
+//! | `if`                 | 3               | Boolean, Any, Any      | If the first argument is true, returns the second argument, otherwise, return the third  |
 //! | `math::ln`           | 1               | Numeric                | Returns the natural logarithm of the number |
 //! | `math::log`          | 2               | Numeric, Numeric       | Returns the logarithm of the number with respect to an arbitrary base |
 //! | `math::log2`         | 1               | Numeric                | Returns the base 2 logarithm of the number |

tests/integration.rs

@@ -408,6 +408,9 @@ fn test_builtin_functions() {
     assert_eq!(eval("shl(-6, 5)"), Ok(Value::Int(-192)));
     assert_eq!(eval("shr(5, 1)"), Ok(Value::Int(2)));
     assert_eq!(eval("shr(-6, 5)"), Ok(Value::Int(-1)));
+    assert_eq!(eval("if(true, -6, 5)"), Ok(Value::Int(-6)));
+    assert_eq!(eval("if(false, -6, 5)"), Ok(Value::Int(5)));
+    assert_eq!(eval("if(2-1==1, \"good\", 0)"), Ok(Value::String(String::from("good"))));
 }
 
 #[test]