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'}