处理Either和ST monads

Question

Let's say I have the following functions:

checkA :: a -> Either err b
checkA = undefined

checkB :: b -> ST s (Either err c)
checkB = undefined

check :: a -> ST s (Either err c)
check a = either (return . Left) checkB (checkA a)

Is there any way to write check such that it doesn't require using return . Left return . Left ? Usually I'd do something like >>= , but in this case the return of checkB is wrapped inside another state monad, so it doesn't work. The other constraint is that checkB should only run if checkA a evaluates to Right , and should just fail with the error on Left

To generalize, are there any standard approaches to using nested monads?

Answer 1

Here's one way to do it with ExceptT :

checkA :: a -> Either err b
checkA = undefined

checkB :: b -> ExceptT err (ST s) c
checkB = undefined

check :: a -> ExceptT err (ST s) c
check a = except (checkA a) >>= checkB
-- or
check = except . checkA >=> checkB

except turns Either err b into Monad m => ExceptT err mb , and then you can do everything else in the ExceptT err (ST s) monad.

As a general rule, ExceptT is a great way to work with monadic actions that may fail when you usually want to bail on failure. The main exception is when the underlying monad is IO , in which case it's more common to use the built-in exception features from Control.Exception .

Of course, if you only need one monadic bind, ExceptT seems a bit like overkill, but once you need more it definitely makes sense.