On Mon, Oct 6, 2008 at 9:48 PM, Andrew Coppin
Andrew Coppin wrote:
I have some longwinded code that works, but I'm still thinking about how to do this more elegantly. It looks like what I really need is something like
type M = StateT State (ResultSetT (ErrorT ErrorType Identity))
Is that the correct ordering?
If so, I guess that means I have to somehow construct ResultSetT. Is there an easy way to do that, given that I already have ResultSet? For example, if I put ResultSet into Traversable, would that let me do it?
...and again I'm talking to myself... :-/
So after much experimentation, I have managed to piece together the following facts:
- It appears that the outer-most monad transformer represents the inner-most monad. So "StateT Foo ListT" means a list of stateful computations, while "ListT (StateT Foo)" means a stateful list of computations.
Have you read "Monad Transformers Step by Step" [1] by Martin Grabmueller? It's a fantastic introduction to these beasties, leading the reader through a series of transformations from pure code to using about 4 different monads/transformers for all sorts of extra features. Seriously recommend it. []: http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.pdf Cheers, D