Editing GHC/FAQ (section)

=== When is it safe to use [[unsafe functions]] such as <hask>unsafePerformIO</hask>? ===

We'll give two answers to this question, each of which may be helpful. These criteria are not rigorous in any real sense (you'd need a formal semantics for Haskell in order to give a proper answer to this question), but should give you a feel for the kind of things you can and cannot do with unsafePerformIO.

* It is safe to implement a function or API using unsafePerformIO if you could imagine also implementing the same function or API in Haskell without using unsafePerformIO (forget about efficiency, just consider the semantics).
* In pure Haskell, the value of a function depends only on the values of its arguments (and free variables, if it has any). If you can implement the function using unsafePerformIO and still retain this invariant, then you're probably using unsafePerformIO in a safe way. Note that you need only consider the observable values of the arguments and result.

For more information, see [http://www.haskell.org/pipermail/glasgow-haskell-users/2002-July/003681.html this thread].