I'm solving the Project Euler problems in order, using Haskell as the sole programming language, as a way to learn Haskell.
My initial introduction to Haskell was with Learn You a Haskell. It's a good introduction to Haskell in that it serves as an introduction to the language and the standard library while also serving as an argument for the beauty of Haskell, all at once. It's also funny, but it can be a little detached, from the real world so I decided to solve Project Euler problems with it.