Leon Smith
My intuition says that none of the side-effects in my implementation are visible from the abstract level of the module. However, the use of unsafePerformIO certainly modifies the behaviour of the module.
Here's a simple test you can apply: Can you rewrite your module in pure Haskell so that it has the same observed behaviour but (probably) different operational behaviour (e.g. space and time usage)? If the answer is yes, then you preserve referential transparency, type safety and other desirable properties (on the assumption that pure Haskell has these properties). If the answer is no, you've probably broken one or more important properties. (There may still be hope but you might have to get very devious (cf. non-determinism in Rittri et al.'s UniqueSupply, Hughes and O'Donnell's non-determism monad and exception handling).) -- Alastair Reid reid@cs.utah.edu http://www.cs.utah.edu/~reid/