1 files changed, 23 insertions, 0 deletions
diff --git a/stdlib/nat.ts b/stdlib/nat.ts
new file mode 100644
index 0000000..4334a30
--- /dev/null
+++ b/stdlib/nat.ts
@@ -0,0 +1,23 @@
+export type Nat = Zero | { _prev: Nat }
+export type Zero = { _prev: null }
+export type Succ<N extends Nat> = { _prev: N }
+export type Pred<N extends Nat> = N extends Zero ? Zero : N['_prev']
+
+export type Add<A extends Nat, B extends Nat> =
+  A extends Zero ? B : Add<Pred<A>, Succ<B>>
+
+export type ToNumber<N extends Nat, Acc extends 0[] = []> =
+  N extends Zero ? Acc['length']
+    : ToNumber<Pred<N>, [...Acc, 0]>
+
+export type FromNumber<N extends number, Res extends Nat = Zero, Acc extends 0[] = []> =
+  N extends Acc['length'] ? Res
+    : FromNumber<N, Succ<Res>, [...Acc, 0]>
+
+export type _0 = Zero
+export type _1 = Succ<_0>
+export type _2 = Succ<_1>
+export type _3 = Succ<_2>
+export type _4 = Succ<_3>
+export type _5 = Succ<_4>
+export type _9 = Add<_4, _5>