Underscores and request parsing

Continuing with our theme of how to get help from your compiler, we have guides for:

• The beloved GHC feature called typed holes.
• Compiler warnings related to discarded values.
• All the uses of the underscore character.

We’ve also released lesson 9 in the Web Servers course: Parsing requests.Because parsing is a large topic, we previously published the Parsing page to give an introductory overview of some Haskell parsing libraries to supplement this lesson. This is a lengthy one. The first half gives background about parsing concepts, an introduction to some of the details of using the Attoparsec library, and discussion of the Alternative typeclass that so often comes up alongside parsers. Then we start writing the HTTP request parser, with a continued emphasis that transcribing the specification into Haskell is a methodical task for which careful reading is more important than cleverness.