Type magic

src/dice.ts

@@ -0,0 +1,10 @@
+import { type Add } from './math';
+
+export type D<T extends number> = number extends T ? number :_D<T, []>;
+type _D<T extends number, R extends number[]> = R['length'] extends T ?
+  R[number] :
+  _D<T, [...R, Add<R['length'], 1>]>;
+
+export interface Dice<T extends number> {
+  roll(): D<T>
+}

src/math.ts

@@ -0,0 +1,26 @@
+export type Add<N extends number, AmountToAdd extends number> = _Add<N, AmountToAdd, [], []>;
+type _Add<
+  N extends number,
+  AmountToAdd extends number,
+  LeftAmount extends unknown[],
+  RightAmount extends unknown[]
+  > =
+  LeftAmount['length'] extends N ?
+    RightAmount['length'] extends AmountToAdd ?
+      [...LeftAmount, ...RightAmount]['length'] :
+      _Add<N, AmountToAdd, LeftAmount, [unknown, ...RightAmount]> :
+    _Add<N, AmountToAdd, [unknown, ...LeftAmount], []>
+
+export type Sub<N extends number, AmountToSub extends number> = _Sub<N, AmountToSub, [], [], []>;
+type _Sub<
+  N extends number,
+  AmountToSub extends number,
+  Take extends unknown[],
+  Left extends unknown[],
+  All extends unknown[],
+  > =
+  All['length'] extends N ?
+    Left['length'] :
+    Take['length'] extends AmountToSub ?
+      _Sub<N, AmountToSub, Take, [unknown, ...Left], [unknown, ...All]>:
+      _Sub<N, AmountToSub, [unknown, ...Take], [], [unknown, ...All]>;