In general, you can't do it -- there's no way to perform a write to external logs in the midst of pure code. That would mean it's not pure any more, consequently changing its type signature and forcing you to rewrite a bunch of stuff. The use of the GHCi debugger is highly recommended. The first few times you use the debugger, you'll notice it seems to go backward. That is laziness in action -- it goes immediately to the result of an expression and then walks backward as it is forced to evaluate the intermediate values. Well, okay -- there is one way to do the logging like you want; but it's dangerous. Use `unsafePerformIO` all over the place and pray :) -- Jason Dusek |...GHCi debugger...| http://www.haskell.org/ghc/docs/latest/html/users_guide/ghci-debugger.html