On Wed, Mar 01, 2006 at 11:53:42AM +0000, Simon Marlow wrote:
something along these lines is likely to be quite straightforward to implement, won't require any changes to the type system, and gives you a useful form of implicit parameters without any of the drawbacks.
The main difference from implicit parameters would be that thread-local variables would be restricted to the IO monad.
These two paragraphs sound _heavily_ contradictory to me. The point of implicit parameters ("fluids" or just "parameters" in Scheme) is that they provide a controlled form of dynamic scoping without introducing any stateful mess. Implicit parameters are useful in plain purely functional code just to make certain values customizable without forcing them to be propagated explicitely everywhere even though default values are ok most of the time. Restricting them to the IO monad would severely undermine their purpose. Now, I wonder whether we really really really need to track implicit parameters in the type system. After all, exceptions, too, introduce a certain amount of impurity yet they work just fine in pure code. Couldn't the same kind of semantic trickery that was used in the imprecise exceptions paper also be applied to Scheme-style parameter objects? Lauri