Add support for the IF special form

bootstrap/pkg/core/eval.go

@@ -161,9 +161,9 @@ tco:
 				ret, err = Fn(rt, scope, list.Rest().(*List))
 				break tco // return
 
-			// case "if":
-			// 	ret, err = If(rt, scope, list.Rest().(*List))
-			// 	break tco // return
+			case "if":
+				ret, err = If(rt, scope, list.Rest().(*List))
+				break tco // return
 
 			// list evaluation rules:
 			// * The first element of the list has to be an expression which is callable

bootstrap/pkg/core/sforms.go

@@ -79,3 +79,29 @@ func Fn(rt *Runtime, scope IScope, args *List) (IExpr, IError) {
 
 	return MakeFunction(scope, params, body), nil
 }
+
+// If defines a conditional expression which evaluates the first
+// element of the given `args` as the conditional and if the result
+// is truthy evaluates and returns the second arg otherwise does the
+// same for the third arg.
+func If(rt *Runtime, scope IScope, args *List) (IExpr, IError) {
+
+	if args.Count() != 3 {
+		return nil, MakeError(rt, "'if' needs exactly 3 aruments")
+	}
+
+	pred, err := EvalForms(rt, scope, args.First())
+	result := pred.GetType()
+
+	if err != nil {
+		return nil, err
+	}
+
+	if result != ast.False && result != ast.Nil {
+		// Truthy clause
+		return EvalForms(rt, scope, args.Rest().First())
+	}
+
+	// Falsy clause
+	return EvalForms(rt, scope, args.Rest().Rest().First())
+}