How has Haskell changed?

The history of the language, including major milestones and design decisions, is described in

• A History of Haskell: being lazy with class.

@INPROCEEDINGS{Hudak07ahistory,
    author = {Paul Hudak and John Hughes and Simon Peyton Jones and Philip Wadler},
    title = {A history of Haskell: Being lazy with class},
    booktitle = {In Proceedings of the 3rd ACM SIGPLAN Conference on History of Programming Languages (HOPL-III},
    year = {2007},
    pages = {1--55},
    publisher = {ACM Press}
}