1 files changed, 34 insertions, 0 deletions
diff --git a/src/lang/builtin/index.ts b/src/lang/builtin/index.ts
new file mode 100644
index 0000000..373f54a
--- /dev/null
+++ b/src/lang/builtin/index.ts
@@ -0,0 +1,34 @@
+import {
+  AddNumbers,
+  AddStrings,
+  Multiply,
+  ToString,
+  UnarrayIfOnlyHead,
+} from "../util";
+
+export type FnError<T extends string> = `Function execution error: ${T}`;
+
+export type BUILTIN_Arr<Args extends readonly any[]> = Args;
+
+export type BUILTIN_ToString<Args extends readonly any[]> = ToString<
+  UnarrayIfOnlyHead<{
+    [Idx in keyof Args]: ToString<Args[Idx]>;
+  }>
+>;
+
+export type BUILTIN_Add<Args extends readonly any[]> =
+  Args extends readonly string[]
+    ? AddStrings<Args>
+    : Args extends readonly number[]
+    ? AddNumbers<Args>
+    : FnError<`Cannot add operands ${ToString<Args>}`>;
+
+export type BUILTIN_Mul<Args extends readonly any[]> = Args extends [
+  infer A,
+  infer B,
+  infer C
+]
+  ? FnError<`Can only multiply [number, number], but got ${ToString<Args>}`>
+  : Args extends [infer M extends number, infer N extends number]
+  ? Multiply<M, N>
+  : FnError<`Can only multiply [number, number], but got ${ToString<Args>}`>;