[Haskell-cafe] How to obtain unique integers

I'm implementing a variant of Prolog in Haskell (yes, I know others already exist...) and am looking to solve the following issues: (1) I need to translate anonymous variables "_" into unique variable names. Data.Unique does not give me a printable identifier. Is there another package that does this? (2) Is there a cabal package similar to the Atom.hs module form ginsu, that associates strings with unique hashes? The alternative of course is to maintain state variables and pass them along to display and parsing functions, but that makes it difficult to make the terms instances of Read/Show without reference to a global variable anyhow. Thanks, Rob