This is said a lot and technically valid in a sense, but I think over-pedantic.

Some monads, IO, ST, State, do support mutation in the internal language. Then their implementation, their representation in the meta language, Haskell, can be "pure or whatever who cares".

    http://www.reddit.com/r/haskell/comments/30l46z/haskell_for_all_algebraic_side_effects/cptlduv