It is absolutely necessary, for the peace and safety of mankind, that some of earth's dark, dead corners and unplumbed depths be let alone; lest sleeping abnormalities wake to resurgent life, and blasphemously surviving nightmares squirm and splash out of their black lairs to newer and wider conquests.
prospect is a library that provides primitives for exploring functions, and by
extension, monads. As such, it allows for a best-attempt static analysis of free
monads. Such power, however, does not come for free; using
prospect is an
implicit promise with the Eldrich horrors that you'll tread lightly. Feckless
wanderers into these depths will be rewarded with naught but terror, madness,
and runtime crashes.
The library provides a function,
prospect :: Free f a -> (Maybe a, [f ()]),
which can probe the depths of a free monad, finding as many
f constructors as
it can before the monad branches dynamically.
Be careful when inspecting the
f ()s, if any of them depend on variables bound
in the monad, they will leak exceptions when you are least expecting them. It's
a good idea to run your
f ()s through
ensure :: Alternative m => a -> m a
after you've scrutinized their constructors.
prospect can be used to perform a best-effort static analysis of a free monad:
data Pattern a = Cont (Bool -> a) | Action Int a deriving (Functor, Generic1) cont :: MonadFree Pattern m => m Bool cont = liftF $ Cont id action :: MonadFree Pattern m => Int -> m () action i = liftF $ Action i () success :: (Maybe String, [Pattern ()] success = prospect $ do a <- cont action 1 pure "success" -- success = (Just "success", [Cont (const ()), Action 1 ()]) failure :: (Maybe String, [Pattern ()] failure = prospect $ do a <- cont action 1 if a -- static analysis ends here, as it would require branching on the -- result of a monadic action then action 2 else action 3 action 4 pure "failure" -- failure = (Nothing, [Cont (const ()), Action 1 ()])
In these examples, we can continue analyzing a
Free Pattern monad until
the result of its
Cont continuation is forced.