docs: update README with an example

packages/io-ts-http/README.md

@@ -2,12 +2,44 @@
 
 Runtime types for (de)serializing HTTP requests from both the client and server side
 
+## Contents
+
+- [@api-ts/io-ts-http](#api-tsio-ts-http)
+  - [Contents](#contents)
+  - [Preface](#preface)
+  - [Introduction](#introduction)
+  - [Overview](#overview)
+  - [Example](#example)
+    - [`apiSpec`](#apispec)
+    - [`httpRoute`](#httproute)
+      - [`path`](#path)
+      - [`method`](#method)
+      - [`request`](#request)
+      - [`response`](#response)
+    - [`httpRequest`](#httprequest)
+      - [`params`](#params)
+      - [`query`](#query)
+      - [`headers`](#headers)
+      - [`body`](#body)
+      - [Decoding an `httpRequest`](#decoding-an-httprequest)
+  - [Documentation](#documentation)
+
+## Preface
+
 This package extends [io-ts](https://github.com/gcanti/io-ts) with functionality useful
-for typing http requests. Start there for base knowledge required to use this package.
+for typing HTTP requests. Start there for base knowledge required to use this package.
+
+## Introduction
+
+io-ts-http is the definition language for api-ts specifications, which define the API
+contract for a web sever to an arbitrary degree of precision. Web servers can use the
+io-ts-http spec to parse HTTP requests at runtime, and encode HTTP responses. Clients
+can use the io-ts-http spec to enforce API compatibility at compile time, and to encode
+requests to the server and decode responses.
 
 ## Overview
 
-The primary function in this library is `httpRequest`, which is used to build codecs
+The primary function in this library is `httpRequest`. You can use this to build codecs
 which can parse a generic HTTP request into a more refined type. The generic HTTP
 request should conform to the following interface:
 
@@ -19,6 +51,9 @@ interface GenericHttpRequest {
   query: {
     [K: string]: string | string[];
   };
+  headers: {
+    [K: string]: string;
+  };
   body?: unknown;
 }
 ```
@@ -81,6 +116,264 @@ possible to encode the API contract (or at least as much of it that is possible
 express in the type system) and therefore someone calling the API can be confident that
 the server will correctly interpret a request if the arguments typecheck.
 
+## Example
+
+Let's define the api-ts spec for a hypothetical `message-user` service. The conventional
+top-level export is an
+[`apiSpec`](https://github.com/BitGo/api-ts/blob/master/packages/io-ts-http/docs/apiSpec.md)
+value; for example:
+
+### `apiSpec`
+
+```typescript
+import { apiSpec } from '@api-ts/io-ts-http';
+
+import { GetMessage, CreateMessage } from './routes/message';
+import { GetUser, CreateUser, UpdateUser, DeleteUser } from './routes/user';
+
+/**
+ * message-user service
+ *
+ * @version 1.0.0
+ */
+export const API = apiSpec({
+  'api.v1.message': {
+    get: GetMessage,
+    post: CreateMessage,
+  },
+  'api.v1.user': {
+    get: GetUser,
+    post: CreateUser,
+    put: UpdateUser,
+    delete: DeleteUser,
+  },
+});
+```
+
+The `apiSpec` is imported, along with some named `httpRoute`s (`{Get|Create}Message`,
+and `{Get|Create|Update|Delete}User`) [which we'll discuss below](#httproute).
+
+> Currently, if you add the `@version` JSDoc tag to the exported API spec, it will be
+> used as the API `version` when generating an OpenAPI schema. Support for other tags
+> may be added in the future.
+
+The top-level export for `message-user-types` is `API`, which we define as an `apiSpec`
+with two endpoints `api/v1/message` and `api/v1/user`. The `api/v1/message` endpoint
+responds to `GET` and `POST` verbs while the second reponds to `GET`, `POST`, `PUT`, and
+`DELETE` verbs using `httpRoute`s defined in `./routes/message`. The following are the
+`httpRoute`s defined in `./routes/message`.
+
+### `httpRoute`
+
+```typescript
+import * as t from 'io-ts';
+import { httpRoute, httpRequest } from '@api-ts/io-ts-http';
+
+export const GetMessage = httpRoute({
+  path: '/message/{id}',
+  method: 'GET',
+  request: httpRequest({
+    params: {
+      id: t.string,
+    },
+  }),
+  response: {
+    200: t.type({
+      id: t.string,
+      message: t.string,
+    }),
+    404: t.type({
+      error: t.string,
+    }),
+  },
+});
+
+export const CreateMessage = httpRoute({
+  path: '/message',
+  method: 'POST',
+  request: httpRequest({
+    body: {
+      message: t.string,
+    },
+  }),
+  response: {
+    200: t.type({
+      id: t.string,
+      message: t.string,
+    }),
+    404: t.type({
+      error: t.string,
+    }),
+  },
+});
+```
+
+The first import is the `io-ts` package. It's usually imported `as t` for use in
+describing the types of data properties. Again, review
+[io-ts](https://github.com/gcanti/io-ts) documentation for more context on how to use it
+and this package.
+
+Then `httpRoute` and `httpRequest` are imported. We'll review the
+[`httpRequest`](#httprequest) below, but first, let's review the `GetMessage`
+`httpRoute`.
+
+```typescript
+export const GetMessage = httpRoute({
+  path: '/message/{id}',
+  method: 'GET',
+  request: httpRequest({
+    params: {
+      id: t.string,
+    },
+  }),
+  response: {
+    200: t.type({
+      id: t.string,
+      message: t.string,
+    }),
+    404: t.type({
+      error: t.string,
+    }),
+  },
+});
+```
+
+[`httpRoute`](https://github.com/BitGo/api-ts/blob/master/packages/io-ts-http/docs/httpRoute.md)s
+`apiSpec`s use
+[`httpRoute`](https://github.com/BitGo/api-ts/blob/master/packages/io-ts-http/docs/httpRoute.md)s
+to define the `path`, `method`, `request` and `response` of a route.
+
+#### `path`
+
+The route's `path` along with possible path variables. You should surround variables
+with brackets like `{name}`, and are to the `request` codec under the `params` property.
+
+#### `method`
+
+The route's `method` is the
+[HTTP request method](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods) to use
+for that route. In our `GetMessage` example, the `method` is `GET`, while in our
+`PostMessage` example, the `method` is `POST`.
+
+#### `request`
+
+The route's `request` is the output of the `httpRequest` function. This will be
+described below.
+
+#### `response`
+
+The route's `response` describes the possible
+[HTTP responses](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status) the route can
+produce. The key-value pairs of the `response` object are an HTTP status code followed
+by the `io-ts` type of the response body. In our `GetMessage` example, a `200` status
+response yields a payload of a JSON object with two properties, `message` which is a
+`string` and `id` which is also a `string`, and a `404` yeilds a payload of a JSON
+object with a single property `error` which is a `String`.
+
+### `httpRequest`
+
+Use `httpRequest` to build the expected type that you pass in a request to the route. In
+our example `GetMessage`
+
+```typescript
+export const GetMessage = httpRoute({
+  path: '/message/{id}',
+  method: 'GET',
+  request: httpRequest({
+    params: {
+      id: t.string,
+    },
+  }),
+  // ...
+});
+```
+
+`httpRequest`s have a total of 4 optional properties: `params` (shown in the example),
+`query`, `headers`, and `body`.
+
+#### `params`
+
+`params` is an object representing the types of path parameters in a URL. Any URL
+parameters in the `path` property of an `httpRoute` must be accounted for in the
+`params` property of the `httpRequest`. Our request has a single URL parameter it is
+expecting, `id`. This is the type of this parameter is captured in the `params` object
+of our `httpRequest`.
+
+#### `query`
+
+`query` is the object representing the values passed in via query parameters at the end
+of a URL. The following example uses a new route, `GetMessages`, to our API that
+searches messages related to a specific `author`:
+
+```typescript
+export const GetMessages = httpRoute({
+  path: '/messages',
+  method: 'GET',
+  request: httpRequest({
+    query: {
+      author: t.string,
+    },
+  }),
+  // ...
+});
+```
+
+#### `headers`
+
+`headers` is an object representing the types of the
+[HTTP headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers) passed in with
+a request.
+
+#### `body`
+
+`body` is an object representing the type of the
+[HTTP body](https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages#body) of a
+request. Often this is a JSON object. The `CreateMessage` `httpRoute` in our example
+uses the `body` property:
+
+```typescript
+export const CreateMessage = httpRoute({
+  path: '/message',
+  method: 'POST',
+  request: httpRequest({
+    body: {
+      message: t.string,
+    },
+  }),
+  // ...
+});
+```
+
+#### Decoding an `httpRequest`
+
+When you decode `httpRequests` using `io-ts` helpers, the properties of the request are
+flattened like this:
+
+```typescript
+import { DateFromString, NumberFromString } from 'io-ts-types';
+
+// build an httpRequest with one parameter id and a body with content and a timestamp
+const Request = httpRequest({
+  params: {
+    id: NumberFromString,
+  },
+  body: {
+    content: t.string,
+    timestamp: DateFromISOString,
+  },
+});
+
+// use io-ts to get the type of the Request
+type Request = t.TypeOf<typeof Request>;
+
+// same as
+type Request = {
+  id: number;
+  content: string;
+  timestamp: Date;
+};
+```
+
 ## Documentation
 
 - [API Reference](https://github.com/BitGo/api-ts/blob/master/packages/io-ts-http/docs/apiReference.md)