I believe functionality is complete

src/roll/index.ts

@@ -1,13 +1,8 @@
-import { CustomSides, DicePoolOptions } from '../types/options'
-import { DiceNotation } from '../types/primitives'
+import { RollArgument } from '../types/argument'
 import { RollResult } from '../types/results'
 import generateResult from './generate-results'
 import parseArguments from './parse-arguments'
 
-function roll(
-  arg?: number | DicePoolOptions | DiceNotation | CustomSides
-): RollResult {
+export default function roll(arg?: RollArgument): RollResult {
   return generateResult(parseArguments(arg))
 }
-
-export default roll

src/roll/parse-arguments/index.ts

@@ -1,7 +1,7 @@
 import { coreNotationPattern } from '../../constants/regexp'
 import { dieFactory } from '../../Die'
 import { isCustomSides } from '../../Die/guards'
-import { RollArgument } from '../../types/argument'
+import { CoreRollArgument, RollArgument } from '../../types/argument'
 import { DicePoolOptions } from '../../types/options'
 import { DicePoolParameters, RollParameters } from '../../types/parameters'
 import { DiceNotation } from '../../types/primitives'
@@ -20,7 +20,7 @@ export const isDiceNotation = (
   !!coreNotationPattern.test(String(argument))
 
 function parseDiceOptions(
-  options: RollArgument
+  options: CoreRollArgument | undefined
 ): DicePoolOptions<string | number> {
   if (isDicePoolOptions(options)) {
     return options
@@ -36,7 +36,9 @@ function parseDiceOptions(
   }
 }
 
-function parseArgument(argument: RollArgument): RollParameters['dicePools'] {
+function parseArgument(
+  argument: CoreRollArgument | undefined
+): RollParameters['dicePools'] {
   const id = crypto.randomUUID()
   const options = parseDiceOptions(argument)
 
@@ -49,7 +51,7 @@ function parseArgument(argument: RollArgument): RollParameters['dicePools'] {
   }
 }
 
-function parseArguments(argument: RollArgument): RollParameters {
+function parseArguments(argument: RollArgument | undefined): RollParameters {
   const args = [argument].flat()
   const dicePools = args.reduce(
     (acc, arg) => ({ ...acc, ...parseArgument(arg) }),

src/types/argument.ts

@@ -1,10 +1,13 @@
 import { CustomSides, DicePoolOptions } from './options'
 import { DiceNotation } from './primitives'
 
-export type RollArgument =
-  | undefined
+export type CoreRollArgument =
   | string
   | number
-  | DicePoolOptions<number | string>
-  | DiceNotation<number | string>
+  | DicePoolOptions<string>
+  | DicePoolOptions<number>
+  | DiceNotation<number>
+  | DiceNotation<string>
   | CustomSides
+
+export type RollArgument = CoreRollArgument | CoreRollArgument[]

src/types/options.ts

@@ -2,7 +2,7 @@ export type TypeOrArrayOfType<T> = T | T[]
 
 export type CustomSides = (string | number)[]
 
-export interface DicePoolOptions<D = string | number> {
+export interface DicePoolOptions<D extends string | number> {
   quantity?: number
   sides: D extends number ? number : CustomSides
   modifiers?: D extends number ? Modifiers : never

src/types/primitives.ts

@@ -1,9 +1,10 @@
 export type DiceNotationWithNumericSides = `${number}${
   | 'd'
   | 'D'}${number}${string}`
+
 export type DiceNotationWithCustomSides = `${number}${'d' | 'D'}{${string}}`
 
-export type DiceNotation<D = string | number> = D extends number
+export type DiceNotation<D extends string | number> = D extends number
   ? DiceNotationWithNumericSides
   : DiceNotationWithCustomSides