How do I avoid referring to all state variables when updating only a few?

Question

An idiom I use for composing a couple of procedures (with memory) is as follows:

p1 :: State (Int, String) ()
p1 = do
    (a, b) <- get
    ... do somethi         


        

Answer 1

lens's zoom combinator lifts a computation in a State monad into a computation that runs in a "larger" State monad.

zoom :: Lens' s t -> State t a -> State s a

So, given a "big" state:

data Big = Big {
    _big1 :: Medium,
    _big2 :: Medium
}
data Medium = Medium {
    _medium1 :: Small,
    _medium2 :: Small
}
data Small = Small { _small :: Int }

makeLenses ''Big
makeLenses ''Medium
makeLenses ''Small

you can "zoom in" on a part of the state:

incr :: State Int ()
incr = id += 1

incrSmall :: State Big ()
incrSmall = zoom (big2.medium1.small) incr

Of course, this'll work on big tuples as well as records, using lens's built-in tuple field accessors.

zoom's real type signature is more general than the simple one I quoted above. It uses MonadState constraints to work under a monad transformer stack, rather than in State specifically.