[Haskell-cafe] Retrieving inner state from outside the transformer

Phil phil at beadling.co.uk
Thu Jul 30 20:06:37 EDT 2009


I've hit a brick wall trying to work out, what should be (and probably is!)
a simple problem.

I have a StateT stack (1 State monad, 2 StateT transformers) which works
fine and returns the result of the outer monad.  I thought I understood this
fine, but perhaps not.  My understanding is that the result returned by the
inner-most monad is always 'transformed' by the outer monads and thus the
result you get is that computed in the outer transformer.

The problem I have is now I'd like not only to get the final state of the
outer most transformer, but I'd also like to know the final states of the
the inner StateT and the inner State at the end of the computation (so that
at a later point in time I can reinitialize a similar stack and continue
with the same set of states I finished with).

So I figured I could have a separate (parent) State Monad (not part of this
stack) that would store the final state of the sequence below.  I figured it
couldn't be part of this stack, as one computation on the stack does not
lead to one result in the parent State Monad; it is only the end states of
the sequence I care about.

Anyway, currently I just have the stack evaluated as below.  Is there anyway
from outside of the computation that I can interrogate the states of the
inner layers?  The only way I can see to do this is inside the outer monad
itself.  As I'm not using the result I could use 'lift get' and 'lift lift
get' to make the outer transformer return the two inner states as it's
result.  I could ignore this result for the first (iterations-1) and bind a
final iteration which uses replicateM instead of replicateM_.

This strikes me as pretty horrible tho!

So, in the example below if I want to modify the 'result' function so it
returns no only the outer state, but also the two inners states as a tuple
(Double,Double,Double) is there an easier way of doing this?

result :: RngClass a => NormalClass b => a -> b -> MonteCarloUserData ->
result initRngState initNormState userData = evalState a initRngState

where  a = evalStateT b initNormState

           b = execStateT ( do replicateM_ (iterations userData) (mc
userData)) 0

Any advice greatly appreciated!


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