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

This seems like a job for lenses. Especially the Control.Lens.Tuple module together with .= and use:

p1 = do
   a <- use _1
   -- do something --
   _1 .= a'

However, it's usually better if you give the things in your state proper names, e.g.

{-# LANGUAGE TemplateHaskell #-

data Record = MkRecord { _age :: Int
                       , _name :: String
                       , _programmer :: Bool
                       } deriving (Show, Eq)
makeLenses ''Record

That way, you have better names for your field:

p1 = do
   a <- use age
   -- do something --
   age .= a'

Note that this still helps you if you don't want to use lenses, since you can use record syntax to update your data:

 p1 = do
      r <- get
      let a = _age r
      --- do something
      put $ r{_age = a'}

Answer 2

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.

Answer 3

This is a good situation to use records, with the gets and modify functions to manipulate subparts of the state:

data Env = Env
  { envNumber :: Int
  , envText :: String
  }

p1 :: State Env ()
p1 = do
    a <- gets envNumber
    -- ...
    modify $ \r -> r { envNumber = a' }

p2 :: State Env ()
p2 = do
    b <- gets envText
    -- ...
    modify $ \r -> r { envText = b' }

gets turns a pure getter function into a state action:

gets :: (s -> a) -> State s a
envNumber :: Env -> Int
gets envNumber :: State Env Int

And modify turns a pure update function into a state action:

modify :: (s -> s) -> State s ()
(\r -> r { envText = b' }) :: Env -> Env
modify (\r -> ...) :: State Env ()