Question

I am working on a simple example to implement the state monad in Haskell. I have made a data type GlobState using record syntax. Functions incr1, incr2 and incr3 are there to increment the respective record elements. The code i have written is:

module StateExample where
import Control.Monad.State

data GlobState = GlobState { c1 :: Int, c2:: Int, c3:: Int} deriving (Show)

newGlobState:: GlobState
newGlobState = GlobState { c1=0,c2=0,c3=0 }

--incr1 :: State GlobState ()
incr1 = do
    gets c1
    modify (+1)

--incr2 :: State GlobState ()
incr2 = do
    gets c2
    modify(+1)

incr3 = do
    gets c3
    modify(+1)

main =  do
    let ((),a1) = flip runState newGlobState $ do
        x<- incr1
        y<- incr2 x
        z<- incr1 y
        return z
    print a1

This program is not giving the desired output. And i am not sure about the signature of increment functions or if gets is doing what i intend to do. Please help.

How can i make changes to the code so that instead of defining the different increment functions, i need to make only one increment function. i intend to do like:

incr :: String-> State GlobState ()
incr x = do
modify(\g -> g {x =x g + 1})

Answer 1

Your types are right, at least.

incr1 :: State GlobState ()
incr1 = modify (\state -> state {c1 = 1 + c1 state})

Your modify (+1) would be fine if you were working in State Int (), say, but we’re not. You seem to think that gets focusses the following lines onto a particular field of the state — but it doesn’t.

incr2 and incr3 need to be changed similarly.

As to main:

1. You need to indent your code properly. You appear to want the print a1 to be part of the outer do block, but not part of the inner do block. Without indentation, the compiler cannot figure that out.
2. You are (attempting to) manually thread the state, but the whole point about State is that its monadic/applicative plumbing does that for you.

main = do let ((), a1) = flip runState newGlobState $ do incr1
                                                         incr2
                                                         incr1
          print a1