State Monad ExampleProblem - haskell

Question

I am working on this problem and had previously asked related question. Implementation of State Monad To further refine my code i tried to implement it using only one increment function.
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 }
incr :: String-> State GlobState ()
incr x = do
modify(\g -> g {x =x g + 1})
main:: IO()
main = do
let a1= flip execState newGlobState $ do
incr c1
incr c2
incr c1
print a
But here i am getting an error
`x' is not a (visible) constructor field name
How can i remove this error?

Answer 1

You have hit a weakness in Haskell: records are not first class values!
Indeed, it would be very nice to write as you have done, but it is not possible.
However, you can use different libraries to achieve the desired effect.
This is how it looks if you use
fclabels:
{-# LANGUAGE TemplateHaskell, TypeOperators #-}
module StateExample where
import Control.Monad.State hiding (modify)
import Data.Label (mkLabels)
import Data.Label.Pure ((:->))
import Data.Label.PureM
data GlobState = GlobState { _c1 :: Int , _c2 :: Int , _c3 :: Int } deriving Show
$(mkLabels [''GlobState])
newGlobState:: GlobState
newGlobState = GlobState { _c1 = 0, _c2 = 0, _c3 = 0 }
incr :: (GlobState :-> Int) -> State GlobState ()
incr x = modify x (+1)
main :: IO ()
main = do
let a = flip execState newGlobState $ do
incr c1
incr c2
incr c1
print a
There are some magic parts here. We define the GlobState with the same
record names but prependend with an underscore. Then the function
mkLabels uses TemplateHaskell to define "lenses" for every field
in the record. These lenses will have the same name but without the underscore. The argument (GlobState :-> Int) to incr is such a
lens, and we can use the modify function from Data.Label.PureM
which updates records defined this way inside the state monad. We hide
modify from Control.Monad.State, to avoid collision.
You can look at the other
functions in the
documentation for
PureM
for other functions usable with state monads, as
gets and puts.
If you do not have fclabels installed, but you have the cabal executable from the cabal-install package (which you get if you install the Haskell Platform), you can install fclabels by simply running:
cabal install fclabels
If this is the first time you run cabal, you first need to update the database:
cabal update