Developing Web Applications with Haskell and Yesod

Chapter 1. Introduction

Since web programming began, people have been trying to make the development process a more pleasant one. As a community, we have continually pushed new techniques to try and solve some of the lingering difficulties of security threats, the stateless nature of HTTP, the multiple languages (HTML, CSS, JavaScript) necessary to create a powerful web application, and more.

Yesod attempts to ease the web development process by playing to the strengths of the Haskell programming language. Haskell’s strong compile-time guarantees of correctness not only encompass types; referential transparency ensures that we don’t have any unintended side effects. Pattern matching on algebraic data types can help guarantee we’ve accounted for every possible case. By building upon Haskell, entire classes of bugs disappear.

Unfortunately, using Haskell isn’t enough. The Web, by its very nature, is not type safe. Even the simplest case of distinguishing between an integer and string is impossible: all data on the Web is transferred as raw bytes, evading our best efforts at type safety. Every app writer is left with the task of validating all input. I call this problem the boundary issue: as much as your application is type safe on the inside, every boundary with the outside world still needs to be sanitized.

Type Safety

This is where Yesod comes in. By using high-level declarative techniques, you can specify the exact input types you are expecting. And the process works the other way ...