feat: have a special type for nexttoken

src/parsing.ts

@@ -1,5 +1,5 @@
 /** A collection of parsing codecs */
-import { array, Codec } from 'purify-ts/Codec'
+import { array, Codec, string } from 'purify-ts/Codec'
 import { Left, Right } from 'purify-ts/Either'
 
 export const ensureArray = <T>(codec: Codec<T>): Codec<T[]> => {
@@ -62,3 +62,14 @@ export const serviceStatus = Codec.custom<ServiceStatus>({
   encode: (x) => x,
   schema: () => ({ type: 'string', enum: ['GREEN', 'YELLOW', 'RED'] }),
 })
+
+export class NextToken<T> {
+  constructor(private action: T, public token: string) {}
+}
+
+export const nextToken = <T extends string>(action: T) =>
+  Codec.custom<NextToken<T>>({
+    decode: (x) => string.decode(x).map((string_) => new NextToken(action, string_)),
+    encode: (x) => x,
+    schema: () => ({ type: 'string' }),
+  })

src/sections/sellers.ts

@@ -2,12 +2,18 @@ import { Codec, GetInterface, optional, string } from 'purify-ts'
 
 import { ParsingError } from '../error'
 import { HttpClient, RequestMeta, Resource } from '../http'
-import { ensureArray, mwsBoolean, serviceStatus } from '../parsing'
+import {
+  ensureArray,
+  mwsBoolean,
+  NextToken,
+  nextToken as nextTokenCodec,
+  serviceStatus,
+} from '../parsing'
 
 const SELLERS_API_VERSION = '2011-07-01'
 
 const MarketplaceParticipations = Codec.interface({
-  NextToken: optional(string),
+  NextToken: optional(nextTokenCodec('ListMarketplaceParticipations')),
   ListParticipations: Codec.interface({
     Participation: ensureArray(
       Codec.interface({
@@ -80,13 +86,13 @@ export class Sellers {
   }
 
   async listMarketplaceParticipationsByNextToken(
-    nextToken: string,
+    nextToken: NextToken<'ListMarketplaceParticipations'>,
   ): Promise<[MarketplaceParticipations, RequestMeta]> {
     const [response, meta] = await this.httpClient.request('POST', {
       resource: Resource.Sellers,
       version: SELLERS_API_VERSION,
       action: 'ListMarketplaceParticipationsByNextToken',
-      parameters: { NextToken: nextToken },
+      parameters: { NextToken: nextToken.token },
     })
 
     return MarketplaceParticipationsByNextTokenResponse.decode(response).caseOf({

test/unit/parsing.test.ts

@@ -1,6 +1,13 @@
 import { number, Right } from 'purify-ts'
 
-import { ensureArray, mwsBoolean, ServiceStatus, serviceStatus } from '../../src/parsing'
+import {
+  ensureArray,
+  mwsBoolean,
+  NextToken,
+  nextToken,
+  ServiceStatus,
+  serviceStatus,
+} from '../../src/parsing'
 
 describe('ensureArray', () => {
   it('acts like an idenity if the value to be decoded is already an array', () => {
@@ -61,3 +68,11 @@ describe('serviceStatus', () => {
     expect(serviceStatus.decode('Green')).toMatchSnapshot()
   })
 })
+
+describe('nextToken', () => {
+  it('is a string parser that constructs a NextToken object', () => {
+    expect.assertions(1)
+
+    expect(nextToken('Action').decode('123')).toStrictEqual(Right(new NextToken('Action', '123')))
+  })
+})