4 files changed, 39 insertions, 2 deletions
diff --git a/src/lang/js-lang/builtin/builtin.ts b/src/lang/js-lang/builtin/builtin.ts
index dde91b6..d54c439 100644
--- a/src/lang/js-lang/builtin/builtin.ts
+++ b/src/lang/js-lang/builtin/builtin.ts
@@ -31,9 +31,24 @@ export const V_BUILTIN_Mul: BUILTIN = (args) => {
   );
 };
 
+export const V_BUILTIN_IfElse: BUILTIN = (args) => {
+  if (args.length !== 3) {
+    throw new Error(`Invalid args for "if": ${JSON.stringify(args)}`);
+  }
+
+  const [cond, trueVal, falseVal] = args;
+
+  if (typeof cond !== "boolean") {
+    throw new Error(`Condition value ${JSON.stringify(cond)} is not a boolean`);
+  }
+
+  return cond ? trueVal : falseVal;
+};
+
 export const nameToBUILTIN: Record<string, BUILTIN> = {
   arr: V_BUILTIN_Arr,
   tostring: V_BUILTIN_ToString,
   add: V_BUILTIN_Add,
   mul: V_BUILTIN_Mul,
+  "?": V_BUILTIN_IfElse,
 };
diff --git a/src/lang/ts-lang/builtin/builtin.ts b/src/lang/ts-lang/builtin/builtin.ts
index a289867..ed6b0a3 100644
--- a/src/lang/ts-lang/builtin/builtin.ts
+++ b/src/lang/ts-lang/builtin/builtin.ts
@@ -31,3 +31,18 @@ export type BUILTIN_Mul<Args extends readonly any[]> = Args extends [
   : Args extends [infer M extends number, infer N extends number]
   ? Multiply<M, N>
   : FnError<`Can only multiply [number, number], but got ${ToString<Args>}`>;
+
+export type BUILTIN_IfElse<Args extends readonly any[]> = Args extends [
+  infer A,
+  infer B,
+  infer C,
+  infer D
+]
+  ? FnError<`Invalid args for "if": ${ToString<Args>}`>
+  : Args extends [infer Cond, infer TrueVal, infer FalseVal]
+  ? Cond extends true
+    ? TrueVal
+    : Cond extends false
+    ? FalseVal
+    : FnError<`Condition value ${ToString<Cond>} is not a boolean`>
+  : FnError<`Invalid args for "if": ${ToString<Args>}`>;
diff --git a/src/lang/ts-lang/core/eval.ts b/src/lang/ts-lang/core/eval.ts
index 5f22299..b2fecaa 100644
--- a/src/lang/ts-lang/core/eval.ts
+++ b/src/lang/ts-lang/core/eval.ts
@@ -1,6 +1,7 @@
 import {
   BUILTIN_Add,
   BUILTIN_Arr,
+  BUILTIN_IfElse,
   BUILTIN_Mul,
   BUILTIN_ToString,
   SBUILTIN_Call,
@@ -26,6 +27,8 @@ export type MapBuiltins<
     ? BUILTIN_Add<Args>
     : Node["name"] extends "mul"
     ? BUILTIN_Mul<Args>
+    : Node["name"] extends "?"
+    ? BUILTIN_IfElse<Args>
     : SENTINEL_NO_BUILTIN
   : never;
 
diff --git a/test/test.ts b/test/test.ts
index 89697fe..672fb4e 100644
--- a/test/test.ts
+++ b/test/test.ts
@@ -1,6 +1,10 @@
 import { createFn } from "../src";
 
-const adder = createFn("fn(x, map(arr(10,20,30), fn(y, add(x, y))))");
+const KS_boolToBin = "fn(b, ?(b, 1, 0))";
 
-const result = adder(5);
+const boolArrToBinaryArr = createFn<[boolean[]]>()(
+  `fn(boolArr, map(boolArr, ${KS_boolToBin}))`
+);
+
+const result = boolArrToBinaryArr([true, false, true]);
 console.log(result);