Seems a fundamentally pure concept to me. Writing down facts that never change in append-only fashion.

What do you mean by a 'fundamentally impure concept'?

Not the OP but maybe he means there are side effects. Of course Haskell can side effects that is one of the big misconceptions.

Impure as in "has side-effects". An append-only state that is used for synchronisation between different machines by definition has side-effects. Of course, you can have side-effects in Haskell, I just thought it was an interesting choice.

Haskell takes a stand against implicit side-effects, letting you see what a function is capable of doing in its type. It's really weird to suggest that performing side-effects in such a language is strange or awkward or cute or whatever. Doing those things is the whole point of writing programs.