You have to figure out here how you can use the function which is the argument
to map.
It seems you want to end up with a function of type a -> Evaluator b, to be
used as an argument to mapM.
One idea is to add a new constructor Var to represent variables, and something
like
evalVar :: Obs b -> a -> Evaluator b
which handles Var by "return"ing the second argument, and everything else as in
evalObs.
Then you can evaluate Map like
evalObs (Map f) as = evalObs as >>= mapM (evalVar (f Var))
Except, this won't quit work because the type for evalVar is wrong.
That can be fixed by adding an extra parameter for the variable type:
data Obs varType a where
...
Var :: Obs v v
Map :: (Obs a a -> Obs a b) -> Obs v [a] -> Obs v [b]
or something like that.
Brandon
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