[Haskell-cafe] Monad transformers [Stacking monads]

Andrew Coppin andrewcoppin at btinternet.com
Sun Oct 5 12:32:29 EDT 2008

David Menendez wrote:
> So it might be possible to rewrite your code along these lines:
>     type M = StateT State []
>     run :: Foo -> M ()
>     runOr :: Foo -> Foo -> M ()
>     runOr x y = mplus (run x) (run y)
>     runAnd :: Foo -> Foo -> M ()
>     runAnd x y = run x >> run y
> The type "StateT State [] alpha" is isomorphic to "State -> [(alpha,
> State)]", which means that each of the computations in mplus gets its
> own copy of the state.
> There are a few ways to add exceptions to this, depending on how you
> want the exceptions to interact with the non-determinism.
> 2. StateT State (NondetT (Either ErrorType)) alpha

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 

  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?

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