Can't match expected type and stuck on assignment about State Monads

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For an assignment for FP we have to write a function that runs a state monadic computation given an initial state, and then returns the computed value and the number of operations counted.

Counts looks like this:

data Counts = Counts {
    binds   :: Int,
    returns :: Int,
    gets    :: Int,
    puts    :: Int
} deriving (Eq, Show)

Where oneBind = Counts 1 0 0 0 (for example). There was also an mempty and <*> defined, but I wasn't able to use "mempty" instead of "Counts 0 0 0 0" with initCounts.

The States are defined as:

newtype State' s a = State' { runState' :: (s, Counts) -> (a, s, Counts) }

So far this is what I have got, but I've been stuck at about the same level for a few hours now.

run :: State' s a -> s -> (a, Counts)
run s ns = do
    initState <- return ns
    initCounts <- return (Counts 0 0 0 0)
    newState <- return (runState' s (initState, initCounts))
    newCounts <- return (runState' (retCounts newState) (newState, initCounts))
    let st = let (a,_,_) = newState
             in a
    let count = let (c,_,_) = newCounts
                in c
    return (count)


retCounts :: State' s a -> State' s Counts
retCounts st = State' (\ (s, count) -> (calcCounts st, s, count))

calcCounts :: State' s a -> Counts
calcCounts st = undefined

I assume I have to use pattern matching in calcCounts to somehow actually count all the operators/functions, but right now I'm getting a type matching error:

Assignment4.hs:236:47:
Couldn't match expected type ‘State' (a, s, Counts) a0’
            with actual type ‘(a, s, Counts)’
Relevant bindings include
  newState :: (a, s, Counts) (bound at Assignment4.hs:235:5)
  initState :: s (bound at Assignment4.hs:233:5)
  ns :: s (bound at Assignment4.hs:232:7)
  s :: State' s a (bound at Assignment4.hs:232:5)
  run :: State' s a -> s -> (a, Counts)
    (bound at Assignment4.hs:232:1)
In the first argument of ‘retCounts’, namely ‘newState’
In the first argument of ‘runState'’, namely ‘(retCounts newState)’

If I could get any help on how I could solve this type error and some hints to go from here, it would be highly appreciated.

PS: I realize it might be a good idea to rename calcCounts to something like calcFunctions

asked 1 min ago

Celestral

17

Can't match expected type and stuck on assignment about State Monads