GHC has `unsafePerformIO` in order to implement pure abstractions. It's an implementation detail, Haskell proper does not support it, and you wouldn't use it in your code.

> and you wouldn't use it in your code

Speak for yourself. I don't see any value in not using a tool when it's available and seems to do the job:

http://science.raphael.poss.name/haskell-for-ocaml-programme...

Personally I don't believe in 100% pure FP, it's both a pain in the ass and doesn't really give you more than 85% FP already would. Actually, most of the FP world seems to agree: out of many Lisps, Scala, OCaml, SML, F#, Erlang/Elixir etc. only Haskell is that hell-bent on purity. Even things like PureScript are more pragmatic than Haskell in this regard.

Here's a great talk on getting what's best from FP without also taking all the problems: http://www.infoq.com/presentations/functional-pros-cons

> Personally I don't believe in 100% pure FP, it's both a pain in the ass and doesn't really give you more than 85% FP already would.

How so? Having a pure language has some significant benefits, like laziness. Data structures as control flow is awesome.

> Even things like PureScript are more pragmatic than Haskell in this regard

Interesting. In what way?