fix: support truthy values

In scheme, everything except false is considered truthy.

src/evaluator/env.rs

@@ -166,12 +166,12 @@ pub fn core_environment() -> Rc<Environment> {
                 let (cond, consequence, alternative) =
                     (args[0].clone(), args[1].clone(), args[2].clone());
 
-                // If the value is anything other than true or nil, then we return the alternative
-                if *cond == Expression::Boolean(true) || *cond == Expression::Nil {
-                    return Ok(consequence);
+                // If the value is anything other than false, then it is truthy
+                if *cond == Expression::Boolean(false) {
+                    return Ok(alternative);
                 };
 
-                Ok(alternative)
+                Ok(consequence)
             }),
         ),
         // Environment Manipulation

src/evaluator/mod.rs

@@ -213,8 +213,8 @@ mod test {
     #[case("(if true true false)", "true")]
     #[case("(if nil true false)", "true")]
     #[case("(if false true false)", "false")]
-    #[case("(if 4 true false)", "false")]
-    #[case("(if \"blue\" true false)", "false")]
+    #[case("(if 4 true false)", "true")]
+    #[case("(if \"blue\" true false)", "true")]
     // Functions
     #[case("((fn* (a) a) 3)", "3")]
     #[case("((fn* (a) (+ a 2)) 1)", "3")]