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API Reference

Imports are omitted for clarity in the code samples below. However, the relative import locations for each contract are provided in the parentheses to the right of each contract's name.

Core Contracts

These contracts are part of the core public API of fallback().

DefaultServer (src/HttpServer.sol:DefaultServer)

Extend this contract to quickly create a Solidity HTTP server from a WebApp instance. For example:

contract MyServer is DefaultServer {
constructor() DefaultServer(new MyApp()) {

See WebApp below for documentation on debug mode.

HttpServer (src/HttpServer.sol:HttpServer)

Extend this to customize your HTTP server's request handling behavior (e.g. change maximum headers, path length, etc.). For example:

contract CustomServer is HttpServer {
function customHttpMessagesOptions()
returns (HttpMessages.Options memory options)
options.maxRequestHeaders = 1000;
options.maxRequestHeaderLength = 1000;
options.maxPathLength = 1000;
return options;

constructor(WebApp _app) HttpServer(_app, customHttpMessagesOptions()) {}

WebApp (src/WebApp.sol:WebApp)

Extend this contract to define routes in a custom web app. For example:

contract MyApp is WebApp {
constructor() {
routes[HttpConstants.Method.GET]["/github"] = "getGithub";

function getGithub() external pure returns (HttpMessages.Response memory) {
return redirect(302, "");

If debug mode is enabled on WebApp, the error pages will show the full request and response data.

Response Helper Functions

WebApp defines some response helper functions to make constructing responses easier:

  • text(string textString) returns (HttpMessages.Response)
    • Creates a response with Content-Type: text/plain
  • json(string jsonString) returns (HttpMessages.Response)
    • Creates a response with Content-Type: application/json
  • html(string htmlString) returns (HttpMessages.Response)
    • Creates a response with Content-Type: text/html
  • redirect(uint16 statusCode, string location) returns (HttpMessages.Response)
    • Creates a redirect response with the given status code to the given location
  • redirect(string location) returns (HttpMessages.Response)
    • Creates a temporary redirect response (302) to the given location

Error Handlers

  • handleNotFound(HttpMessages.Request) returns (HttpMessages.Response)
    • Override this function to customize the 404 page
  • handleBadRequest(HttpMessages.Request) returns (HttpMessages.Response)
    • Override this function to customize the 400 page
  • handleError(HttpMessages.Request) returns (HttpMessages.Response)
    • Override this function to customize the 500 page

In debug mode, example error pages can be accessed at the paths /__not_found (404), /__bad_request (400), and /__error (500).

To return other HTTP error codes, set the statusCode member on the HttpMessages.Response struct and set the content manually.

H (src/html-dsl/H.sol:H)

A Solidity HTML DSL, inspired by HyperScript and Pug. The basic building block of H is the function H.h, which has three variations:

  • h(string tagName) returns (string)
    • Returns the tag without any children (e.g. h("div") => "<div></div>")
  • h(string tagName, string childrenOrAttributes) returns (string)
    • Returns the tag with the given children (or attributes if the tag is self-closing)
  • h(string tagName, string attributes, string children) returns (string)
    • Returns the tag with the given attributes and children

The list of self-closing tags is taken from the HTML spec and H.h handles self-closing behavior accordingly.

In addition to H.h, functions are present on the H contract for all standard HTML elements. Each standard HTML element has three functions, e.g. for div:

  • div() returns (string)
    • Returns h("div"), i.e. "<div></div>"
  • div(string children) returns (string)
    • Returns h("div", children)
  • div(string attributes, string children) returns (string)
    • Returns h("div", attributes, children)

If the tag is self-closing, it does not have the third function above, and the second function creates the tag with attributes rather than children.

Type Contracts

These contracts should be imported only to access the types defined on them; their actual code should be considered private implementation details.

  • src/http/HttpConstants.sol: Import this contract to access the HttpContants.Method enum
  • src/http/HttpMessages.sol: Import this contract to access the HttpMessages.Options, HttpMessages.Request and HttpMessages.Response structs

Utility Libraries

These contracts are utility libraries that you are free to use but aren't core to fallback() itself.

  • src/strings/*.sol: String casing, comparison, and concatenation utilities
  • src/integers/Integers.sol: Integer utilities