Developing Web Applications with Haskell and Yesod

Appendix C. Web Application Interface

It is a problem almost every language used for web development has dealt with: the low-level interface between the web server and the application. The earliest example of a solution is the venerable and battle-worn Common Gateway Interface (CGI), providing a language-agnostic interface using only standard input, standard output, and environment variables.

Back when Perl was becoming the de facto web programming language, a major shortcoming of CGI became apparent: the process needed to be started anew for each request. When dealing with an interpreted language and application requiring database connection, this overhead became unbearable. FastCGI (and later SCGI) arose as a successor to CGI, but it seems that much of the programming world went in a different direction.

Each language began creating its own standard for interfacing with servers: mod_perl, mod_python, mod_php, mod_ruby. Within the same language, multiple interfaces arose. In some cases, we even had interfaces on top of interfaces. And all of this led to much duplicated effort: a Python application designed to work with FastCGI wouldn’t work with mod_python—mod_python only exists for certain web servers—and these programming language specific web server extensions need to be written for each programming language.

Haskell has its own history. We originally had the cgi package, which provided a monadic interface. The fastcgi package then provided the same interface. Meanwhile, it seemed ...