"Haskell" the language defined by the Haskell Report is very stable - with one revision in the last decade. "Glasgow Haskell" the dialect of Haskell provided by default by GHC is fast moving but changes to the Glasgow Haskell language are mostly "additions" of new features rather than "removals" or "mutations". GHC can always use standard Haskell when it is invoked with the appropriate flag. The standardized Haskell library i.e the Prelude, is very stable; though the Prelude is very small you will need more libraries to do any real work. The "standard" library of Glasgow Haskell i.e. GHC's base libraries is fairly stable. Though unlike Glasgow Haskell "the language", changes to the base libraries can as easily be removals and mutations and not just additions. Significant breaking changes tend to get parceled up as part of a major release of GHC, so the changes are well publicized. In "user land", commonly used libraries / frameworks can be highly dynamic with breaking changes happening frequently.