The most proximate question here, with respect to purity, is "can you do O(1) mutation (outside of the narrow case of replacing a thunk with its result)?" The

The most proximate question here, with respect to purity, is "can you do O(1) mutation (outside of the narrow case of replacing a thunk with its result)?" The theory is far more interesting when we say "no", but the answer in the case of Haskell is "yes".