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 something ...
put (a', b)
p2 :: State (Int, String) ()
p2 = do
(a, b) <- get
... do something else ...
put (a, b')
main = do
... initializing a0 b0 ...
print . flip evalState (a0, b0)
. sequence $ replicate 10 p1 ++ repeat p2
However, as the number of state variable grows, this quickly gets way more verbose than necessary:
p1 :: State (Int, String, Bool, Int, String, Bool) ()
p1 = do
(a, b, c, d, e, f) <- get
... do something ...
put (a, b, c', d, e, f')
p2 :: State (Int, String, Bool, Int, String, Bool) ()
p2 = do
(a, b, c, d, e, f) <- get
... do something ...
put (a', b', c, d, e, f)
main = do
print . flip evalState (a0, b0, c0, d0, e0, f0)
. sequence $ replicate 10 p1 ++ repeat p2
As I was wondering, is there a way of updating only a few state variables without having to refer to all the unused ones? I was thinking something like IORef but for State (in fact there is a package stateref), but I'm not sure if there are already some common idioms that other people have been using.
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'}