In the early years of Haskell there were a number of implementations experimenting with different things, and eventually the surviving ones converged on a stand

In the early years of Haskell there were a number of implementations experimenting with different things, and eventually the surviving ones converged on a standard -- Haskell 98. Since then (especially in the last 5 years), the GHC community has been regularly releasing all sorts of shiny new toys -- obviously introducing features unsupported by other implementations, but also frequently breaking compatibility with their own previous implementations. OS portability has not been much of an issue. Haskell Prime is an effort underway by the GHC guys to not only define a new set of standard libraries, but to make Python3000-style purposefully incompatible changes to the core language. This is hoped to become a new standard, Haskell 2010.